This document discusses electron emission from thermionic emitters. It describes the process of thermionic emission where heating a metal provides electrons with enough energy to overcome the surface barrier and be emitted. Common thermionic emitters mentioned are tungsten, thoriated tungsten, and oxide coated cathodes. Thoriated tungsten has a lower work function than pure tungsten, allowing emission at lower temperatures. Oxide coated cathodes operate at even lower temperatures but cannot withstand high voltages. The Richardson-Dushman equation governs thermionic emission current density as a function of temperature and work function.
This document discusses key concepts related to gravity including:
1) Gravity is defined as the force that attracts objects towards the center of Earth or other celestial bodies. Newton's law of gravitation states that gravitational force is directly proportional to the product of masses and inversely proportional to the square of the distance between objects.
2) Density is defined as mass per unit volume. Specific gravity is the ratio of a substance's density to that of water.
3) Archimedes' principle states that the buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object. This principle is applied in nursing for examples like hydrometers and urinometers.
Bio physics, Unit -1. P.B. B.Sc NursingRakesh Gomes
This document discusses key concepts in biophysics and its importance for nursing. It defines biophysics as the application of physical principles to study biological processes. Biophysics is classified based on the scale being studied, from molecular to environmental levels, and the techniques used like imaging and medical applications. Fundamental concepts in physics like units of measurement, mass, length, and time are explained. The importance of biophysics for nursing is that it provides practical knowledge of physics principles underlying nursing procedures and equipment. It also helps nurses understand biological phenomena by applying concepts in areas like mechanics, electricity, heat, and molecular/atomic physics.
Unconsciousness is an abnormal state where a patient is unaware of their surroundings. It can be momentary or last for months. Common causes include head injuries, low blood sugar, drug overdoses, or lack of oxygen. The first steps in treatment are the ABCs - maintaining airway, breathing, and circulation. A brief examination and history should be done to investigate the cause and plan further treatment. Potential complications include coma, brain damage, broken ribs from CPR, or choking.
Application of pressure in nursing ( biophysics)Sujata Sahu
Pressure is defined as force per unit area. It is measured in units like pascals and mmHg. Hydrostatic pressure allows for even transmission of pressure in fluids according to Pascal's law. This principle is applied in devices like hydraulic jacks and fluid-filled mattresses for pressure redistribution. Osmotic pressure depends on solute concentration and allows for fluid movement across membranes. Atmospheric pressure is caused by air weight and measured using barometers. It is applied in suction devices. Central venous, arterial, intraocular and intracranial pressures are clinically measured using manometers, transducers and tonometers.
Paediatric nursing involves providing specialized care to children from conception through adolescence. It aims to promote children's growth, development and well-being. Key principles include treating each child as a unique individual, supporting their family, and delivering developmentally-appropriate care. Current trends emphasize family-centered care, shorter hospital stays, and expanded nursing roles in areas like primary care, education and research. Paediatric nursing also addresses important ethical, legal and social issues related to children's health and rights.
Pediatrics is the branch of medicine concerned with the health of children from birth through adolescence. It aims to ensure the healthy growth and development of children as well as prevent, diagnose, and treat illnesses. The field has grown significantly throughout history as more emphasis has been placed on children's health, welfare programs have been established, medical science has advanced, and societies have changed in their view of protecting younger generations. Modern pediatric nursing focuses on advocacy, communication, education, and collaborative care for the child and family.
Unconsciousness is an abnormal state resulting from disturbance of sensory perception such that the patient is unaware of their surroundings. It can be momentary or last for months. Levels of unconsciousness include excitatory, somnolent, and stuporous. Causes include structural brain lesions, metabolic disorders, and psychogenic factors. Diagnostic assessments include Glasgow Coma Scale, CT, MRI, lumbar puncture, and blood tests. Management focuses on ABCs - airway, breathing, circulation, oxygen, IV access, stabilizing the cervical spine, checking blood glucose, controlling seizures, and considering IV treatments like glucose, thiamine, naloxone or flumazenil. Further examination, history,
The document summarizes various electronic medical equipment the author observed during a hospital visit, including cardiac monitors, ultrasound machines, blood glucose monitors, defibrillators, blood gas analyzers, digital thermometers, IR thermometers, brain wave machines, and sphygmomanometers. For each type of equipment, the document provides details on its use, components, measurements, and specifications. It concludes that electronics have significantly advanced the medical field by discussing how various technologies are used in patient care and treatment.
This document discusses key concepts related to gravity including:
1) Gravity is defined as the force that attracts objects towards the center of Earth or other celestial bodies. Newton's law of gravitation states that gravitational force is directly proportional to the product of masses and inversely proportional to the square of the distance between objects.
2) Density is defined as mass per unit volume. Specific gravity is the ratio of a substance's density to that of water.
3) Archimedes' principle states that the buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object. This principle is applied in nursing for examples like hydrometers and urinometers.
Bio physics, Unit -1. P.B. B.Sc NursingRakesh Gomes
This document discusses key concepts in biophysics and its importance for nursing. It defines biophysics as the application of physical principles to study biological processes. Biophysics is classified based on the scale being studied, from molecular to environmental levels, and the techniques used like imaging and medical applications. Fundamental concepts in physics like units of measurement, mass, length, and time are explained. The importance of biophysics for nursing is that it provides practical knowledge of physics principles underlying nursing procedures and equipment. It also helps nurses understand biological phenomena by applying concepts in areas like mechanics, electricity, heat, and molecular/atomic physics.
Unconsciousness is an abnormal state where a patient is unaware of their surroundings. It can be momentary or last for months. Common causes include head injuries, low blood sugar, drug overdoses, or lack of oxygen. The first steps in treatment are the ABCs - maintaining airway, breathing, and circulation. A brief examination and history should be done to investigate the cause and plan further treatment. Potential complications include coma, brain damage, broken ribs from CPR, or choking.
Application of pressure in nursing ( biophysics)Sujata Sahu
Pressure is defined as force per unit area. It is measured in units like pascals and mmHg. Hydrostatic pressure allows for even transmission of pressure in fluids according to Pascal's law. This principle is applied in devices like hydraulic jacks and fluid-filled mattresses for pressure redistribution. Osmotic pressure depends on solute concentration and allows for fluid movement across membranes. Atmospheric pressure is caused by air weight and measured using barometers. It is applied in suction devices. Central venous, arterial, intraocular and intracranial pressures are clinically measured using manometers, transducers and tonometers.
Paediatric nursing involves providing specialized care to children from conception through adolescence. It aims to promote children's growth, development and well-being. Key principles include treating each child as a unique individual, supporting their family, and delivering developmentally-appropriate care. Current trends emphasize family-centered care, shorter hospital stays, and expanded nursing roles in areas like primary care, education and research. Paediatric nursing also addresses important ethical, legal and social issues related to children's health and rights.
Pediatrics is the branch of medicine concerned with the health of children from birth through adolescence. It aims to ensure the healthy growth and development of children as well as prevent, diagnose, and treat illnesses. The field has grown significantly throughout history as more emphasis has been placed on children's health, welfare programs have been established, medical science has advanced, and societies have changed in their view of protecting younger generations. Modern pediatric nursing focuses on advocacy, communication, education, and collaborative care for the child and family.
Unconsciousness is an abnormal state resulting from disturbance of sensory perception such that the patient is unaware of their surroundings. It can be momentary or last for months. Levels of unconsciousness include excitatory, somnolent, and stuporous. Causes include structural brain lesions, metabolic disorders, and psychogenic factors. Diagnostic assessments include Glasgow Coma Scale, CT, MRI, lumbar puncture, and blood tests. Management focuses on ABCs - airway, breathing, circulation, oxygen, IV access, stabilizing the cervical spine, checking blood glucose, controlling seizures, and considering IV treatments like glucose, thiamine, naloxone or flumazenil. Further examination, history,
The document summarizes various electronic medical equipment the author observed during a hospital visit, including cardiac monitors, ultrasound machines, blood glucose monitors, defibrillators, blood gas analyzers, digital thermometers, IR thermometers, brain wave machines, and sphygmomanometers. For each type of equipment, the document provides details on its use, components, measurements, and specifications. It concludes that electronics have significantly advanced the medical field by discussing how various technologies are used in patient care and treatment.
Role of Child Health Nurse in caring of Hospital ChildAlka Singh
Subject : Child Health Nursing. Topic : Role Of Child Health Nurse In Child care at Hospital, Nursing Diagnosis, Various Measures to make hospital Child Friendly, Nurses Role in Care Of Toddlers, Infants, School Children, Adolescent.
The document discusses the hospital environment for sick children and its impact. It notes that the hospital environment can be stressful for children and their families. It presents different strategies for preparing the hospital environment based on a child's age and developmental stage, from infancy to adolescence. The document also outlines common reactions children may have to hospitalization at different ages and the role of nurses in helping children and families cope with the hospital experience.
This document discusses different types of restraints used for infants and children in medical settings. It defines restraints as devices that limit freedom of movement. Common purposes of restraints include immobilizing children during procedures to prevent injury and ensure safety. The main types of restraints discussed are mummy restraints, jacket restraints, elbow restraints, extremity restraints, and mittens. Each type is described along with its purpose and application procedure. Potential side effects are also outlined. Proper nursing management of restraints is emphasized, including frequent checks, explanation to families, stimulation of children, and changing positions periodically.
The center of gravity, also known as the center of mass, is an imaginary point where the entire weight of a body can be considered to be concentrated. It represents the balance point of an object, such that if supported only at this point, the object would remain in equilibrium in any position. For a uniform body, the center of gravity coincides with the geometric center. The center of mass is useful for calculations in mechanics as it allows simplifying problems involving distributed masses into a single point of concentration. It provides a reference point for analyzing systems like planetary bodies and rigid structures.
This document provides information on breastfeeding techniques and positions for new mothers. It discusses:
- The health benefits of breastfeeding for both babies and mothers.
- Different breastfeeding positions like side-lying, football hold, cradle hold, and cross-cradle that can be used depending on factors like a c-section birth or the size of the baby.
- The importance of proper latching on and signs of successful breastfeeding like adequate wet diapers and weight gain in infants.
- Advantages of breastmilk for babies' nutrition, immunity and brain development as well as reduced disease risk for mothers.
Children may sometimes need to be restrained for medical procedures or examinations. The document discusses various restraint techniques and important safety considerations for restraining children. Key restraint methods include mummy restraints, elbow/knee restraints, abdominal restraints, extremity restraints using clove hitch knots, finger restraints, crib-net restraints, jacket restraints, safety belts, and splints. Proper padding and loose restraints are important to avoid complications like restricted circulation or pressure sores.
This document discusses immunization and vaccination. It begins with the objectives of studying the history of immunization and describing different types of vaccines and schedules. It then discusses key topics like the beginning of immunization with Edward Jenner, introduction to vaccination, types of vaccines including live attenuated, inactivated, toxoid, and subunit vaccines. It provides details on common vaccines, administration techniques, storage requirements, and the importance of maintaining the cold chain to ensure vaccine efficacy. Nursing responsibilities in recording immunizations and ensuring proper vaccine handling and administration are also summarized.
A stoma is an opening that is created to allow stool or urine to pass out of the body.
INDICATIONS FOR OSTOMY
SITES OF STOMA
SELECTION OF APPROPRIATE STOMA POUCH
STEPS TO CHANGE POUCH
IRRIGATION
COMPLICATIONS
NURSING MANAGEMENT
Nursing management of low birth weight(lbw) babiesRose Vadakkut
This document provides information on the management of low birth weight babies. It defines different categories of low birth weight, describes optimal care at birth including warming and feeding practices. It outlines monitoring requirements and discusses positioning, thermal comfort, oxygen therapy, phototherapy and infection control. The document also covers nutrition, stimulation, immunization and family support needs for low birth weight infants.
This document discusses stress and adaptation. It defines stress as the body's response to changes in its normal balanced state. There are three stages of the general adaptation syndrome: alarm reaction, resistance, and exhaustion. Stress can be positive or negative depending on its duration and intensity. Psychological adaptation involves mechanisms like anxiety and coping strategies to deal with stressors and maintain homeostasis. Nursing interventions aim to reduce anxiety and promote adaptive responses to stressors.
Pediatrics is the branch of medicine concerned with the care of children from conception to adolescence. It aims to promote health and prevent illness through measures like immunizations, nutrition education, and early screening. The concept of child care has evolved from a focus on mortality and illness to considering the whole child and family environment. The pediatric nurse's role is also expanding, with emphasis on health promotion, family-centered care in various settings, and specialized roles like practitioner and educator.
This document provides an overview of preventive pediatrics. It defines preventive pediatrics as preventing disease and promoting children's physical, mental, and social well-being to achieve positive health. It discusses antenatal, postnatal, and social preventive pediatrics. It also covers various child health programs like immunization, breastfeeding, ICDS, and the roles of organizations like WHO, UNICEF, and nurses in preventive pediatrics.
Trends influencing nursing practice and educationMahmoud Shaqria
Trends influencing nursing practice and education include rapidly expanding knowledge, globalization, and an interdisciplinary approach to community care. Nursing programs must adapt to address issues like the nursing shortage, evolving licensure and regulations, advancing nursing research, and changing demographics. Additionally, ethics, disasters, violence, and the financial challenges of managed care are shaping nursing education and practice.
The document discusses the Under Five Clinic program established by the Department of Health to address high mortality and morbidity among children under five years old. The program aims to provide comprehensive healthcare services including monitoring growth and development, identifying hindering factors, and offering preventive, promotive, curative and referral services through trained nurses at sub-centers and primary health centers. Key services include regular height and weight monitoring, immunizations, vitamin supplementation, deworming, health education, and treatment of common illnesses like diarrhea and respiratory infections. The goal is to provide affordable healthcare to improve nutrition, development and overall health of young children.
immunization of children is essential to prevent childhood illness, morbidity and mortality. immunization or vaccination is the way of protecting child from infectious diseases.
The document discusses current trends in pediatric nursing, including: 1) Family centered care which involves enabling and empowering families in health care decisions. 2) High technology care which uses advanced diagnostic technology. 3) Evidence based practice where nurses make decisions based on the best available evidence. 4) Primary nursing which provides 24-hour accountability by one nurse for a small group of patients. 5) Case management which is used in outpatient settings to assign a case manager to patients or groups of patients.
The document outlines the organization and components of a neonatal intensive care unit (NICU). It discusses the history of NICUs, physical facility requirements including adequate space, equipment, and staffing. Key aspects that are emphasized include maintaining appropriate environmental conditions, developing different levels of care (I, II, III), and facilitating family involvement to create a gentle environment that supports recovery and development of infants. The overall aim is to reduce mortality and morbidity of at-risk newborns through specialized intensive care.
Health talk on immunization ( presentation, health talk, lesson plan )SADDAM HUSSAIN
A health talk topic on immunization for nursing student .
also usefull for reading and improve knowledge
community health nursing , msc nursing , bsc nursing.
Term Paper - Field Assisted Thermionic Emission, Field Emission, and Applicat...Adeagbo Bamise
This document summarizes different types of electron emission from heated metals, including thermionic emission, field emission, and field-assisted thermionic emission (Schottky emission). Thermionic emission occurs when thermal energy from heating overcomes the work function of a metal, allowing electrons to escape. Field emission occurs at room temperature when a strong electric field lowers the potential barrier for electrons. Schottky emission applies an electric field to enhance thermionic emission and lower the barrier at lower temperatures than normal thermionic emission. These emission types find applications in devices like vacuum tubes.
lecture classical and Quantum Free electron theory (FERMI GAS) (23-24).pdfLobnaSharaf
The electron theory of solids explains properties through electronic structure. It applies to metals and nonmetals. The theory developed in three stages:
1. Classical free electron theory treated electrons as free gas particles. It could not explain many properties.
2. Quantum free electron theory incorporated quantum mechanics. Electrons occupy discrete energy levels according to Fermi-Dirac statistics.
3. Band theory views electrons moving in periodic potentials of atom arrays. It explains conductivity, effective mass, and the origin of band gaps.
Role of Child Health Nurse in caring of Hospital ChildAlka Singh
Subject : Child Health Nursing. Topic : Role Of Child Health Nurse In Child care at Hospital, Nursing Diagnosis, Various Measures to make hospital Child Friendly, Nurses Role in Care Of Toddlers, Infants, School Children, Adolescent.
The document discusses the hospital environment for sick children and its impact. It notes that the hospital environment can be stressful for children and their families. It presents different strategies for preparing the hospital environment based on a child's age and developmental stage, from infancy to adolescence. The document also outlines common reactions children may have to hospitalization at different ages and the role of nurses in helping children and families cope with the hospital experience.
This document discusses different types of restraints used for infants and children in medical settings. It defines restraints as devices that limit freedom of movement. Common purposes of restraints include immobilizing children during procedures to prevent injury and ensure safety. The main types of restraints discussed are mummy restraints, jacket restraints, elbow restraints, extremity restraints, and mittens. Each type is described along with its purpose and application procedure. Potential side effects are also outlined. Proper nursing management of restraints is emphasized, including frequent checks, explanation to families, stimulation of children, and changing positions periodically.
The center of gravity, also known as the center of mass, is an imaginary point where the entire weight of a body can be considered to be concentrated. It represents the balance point of an object, such that if supported only at this point, the object would remain in equilibrium in any position. For a uniform body, the center of gravity coincides with the geometric center. The center of mass is useful for calculations in mechanics as it allows simplifying problems involving distributed masses into a single point of concentration. It provides a reference point for analyzing systems like planetary bodies and rigid structures.
This document provides information on breastfeeding techniques and positions for new mothers. It discusses:
- The health benefits of breastfeeding for both babies and mothers.
- Different breastfeeding positions like side-lying, football hold, cradle hold, and cross-cradle that can be used depending on factors like a c-section birth or the size of the baby.
- The importance of proper latching on and signs of successful breastfeeding like adequate wet diapers and weight gain in infants.
- Advantages of breastmilk for babies' nutrition, immunity and brain development as well as reduced disease risk for mothers.
Children may sometimes need to be restrained for medical procedures or examinations. The document discusses various restraint techniques and important safety considerations for restraining children. Key restraint methods include mummy restraints, elbow/knee restraints, abdominal restraints, extremity restraints using clove hitch knots, finger restraints, crib-net restraints, jacket restraints, safety belts, and splints. Proper padding and loose restraints are important to avoid complications like restricted circulation or pressure sores.
This document discusses immunization and vaccination. It begins with the objectives of studying the history of immunization and describing different types of vaccines and schedules. It then discusses key topics like the beginning of immunization with Edward Jenner, introduction to vaccination, types of vaccines including live attenuated, inactivated, toxoid, and subunit vaccines. It provides details on common vaccines, administration techniques, storage requirements, and the importance of maintaining the cold chain to ensure vaccine efficacy. Nursing responsibilities in recording immunizations and ensuring proper vaccine handling and administration are also summarized.
A stoma is an opening that is created to allow stool or urine to pass out of the body.
INDICATIONS FOR OSTOMY
SITES OF STOMA
SELECTION OF APPROPRIATE STOMA POUCH
STEPS TO CHANGE POUCH
IRRIGATION
COMPLICATIONS
NURSING MANAGEMENT
Nursing management of low birth weight(lbw) babiesRose Vadakkut
This document provides information on the management of low birth weight babies. It defines different categories of low birth weight, describes optimal care at birth including warming and feeding practices. It outlines monitoring requirements and discusses positioning, thermal comfort, oxygen therapy, phototherapy and infection control. The document also covers nutrition, stimulation, immunization and family support needs for low birth weight infants.
This document discusses stress and adaptation. It defines stress as the body's response to changes in its normal balanced state. There are three stages of the general adaptation syndrome: alarm reaction, resistance, and exhaustion. Stress can be positive or negative depending on its duration and intensity. Psychological adaptation involves mechanisms like anxiety and coping strategies to deal with stressors and maintain homeostasis. Nursing interventions aim to reduce anxiety and promote adaptive responses to stressors.
Pediatrics is the branch of medicine concerned with the care of children from conception to adolescence. It aims to promote health and prevent illness through measures like immunizations, nutrition education, and early screening. The concept of child care has evolved from a focus on mortality and illness to considering the whole child and family environment. The pediatric nurse's role is also expanding, with emphasis on health promotion, family-centered care in various settings, and specialized roles like practitioner and educator.
This document provides an overview of preventive pediatrics. It defines preventive pediatrics as preventing disease and promoting children's physical, mental, and social well-being to achieve positive health. It discusses antenatal, postnatal, and social preventive pediatrics. It also covers various child health programs like immunization, breastfeeding, ICDS, and the roles of organizations like WHO, UNICEF, and nurses in preventive pediatrics.
Trends influencing nursing practice and educationMahmoud Shaqria
Trends influencing nursing practice and education include rapidly expanding knowledge, globalization, and an interdisciplinary approach to community care. Nursing programs must adapt to address issues like the nursing shortage, evolving licensure and regulations, advancing nursing research, and changing demographics. Additionally, ethics, disasters, violence, and the financial challenges of managed care are shaping nursing education and practice.
The document discusses the Under Five Clinic program established by the Department of Health to address high mortality and morbidity among children under five years old. The program aims to provide comprehensive healthcare services including monitoring growth and development, identifying hindering factors, and offering preventive, promotive, curative and referral services through trained nurses at sub-centers and primary health centers. Key services include regular height and weight monitoring, immunizations, vitamin supplementation, deworming, health education, and treatment of common illnesses like diarrhea and respiratory infections. The goal is to provide affordable healthcare to improve nutrition, development and overall health of young children.
immunization of children is essential to prevent childhood illness, morbidity and mortality. immunization or vaccination is the way of protecting child from infectious diseases.
The document discusses current trends in pediatric nursing, including: 1) Family centered care which involves enabling and empowering families in health care decisions. 2) High technology care which uses advanced diagnostic technology. 3) Evidence based practice where nurses make decisions based on the best available evidence. 4) Primary nursing which provides 24-hour accountability by one nurse for a small group of patients. 5) Case management which is used in outpatient settings to assign a case manager to patients or groups of patients.
The document outlines the organization and components of a neonatal intensive care unit (NICU). It discusses the history of NICUs, physical facility requirements including adequate space, equipment, and staffing. Key aspects that are emphasized include maintaining appropriate environmental conditions, developing different levels of care (I, II, III), and facilitating family involvement to create a gentle environment that supports recovery and development of infants. The overall aim is to reduce mortality and morbidity of at-risk newborns through specialized intensive care.
Health talk on immunization ( presentation, health talk, lesson plan )SADDAM HUSSAIN
A health talk topic on immunization for nursing student .
also usefull for reading and improve knowledge
community health nursing , msc nursing , bsc nursing.
Term Paper - Field Assisted Thermionic Emission, Field Emission, and Applicat...Adeagbo Bamise
This document summarizes different types of electron emission from heated metals, including thermionic emission, field emission, and field-assisted thermionic emission (Schottky emission). Thermionic emission occurs when thermal energy from heating overcomes the work function of a metal, allowing electrons to escape. Field emission occurs at room temperature when a strong electric field lowers the potential barrier for electrons. Schottky emission applies an electric field to enhance thermionic emission and lower the barrier at lower temperatures than normal thermionic emission. These emission types find applications in devices like vacuum tubes.
lecture classical and Quantum Free electron theory (FERMI GAS) (23-24).pdfLobnaSharaf
The electron theory of solids explains properties through electronic structure. It applies to metals and nonmetals. The theory developed in three stages:
1. Classical free electron theory treated electrons as free gas particles. It could not explain many properties.
2. Quantum free electron theory incorporated quantum mechanics. Electrons occupy discrete energy levels according to Fermi-Dirac statistics.
3. Band theory views electrons moving in periodic potentials of atom arrays. It explains conductivity, effective mass, and the origin of band gaps.
The document discusses the principles and physics of welding. It covers topics such as fusion welding processes, characteristics of heat sources like welding arcs, arc structures, and potential drop characteristics. The key points are:
1) In fusion welding, material around the joint is melted to join two parts together. Important factors include the heat source, arc characteristics, filler material deposition, and heat flow.
2) A welding arc is a sustained electrical discharge through an ionized gas that produces heat. It is maintained by thermionic emission and ionization between the electrodes.
3) The voltage drop across a welding arc depends on factors like the electrode material, spacing and current. There is an optimal arc length that produces maximum power
This document summarizes key concepts in high voltage engineering related to insulation breakdown in gaseous dielectrics. It discusses:
1) Different ionization processes that can occur when electrons collide with gas molecules, including simple collision ionization, excitation, double electron impact ionization, and photoionization.
2) Townsend breakdown process where free electrons cause an "electron avalanche" through successive ionizing collisions, leading to exponential growth in the number of electrons.
3) Mathematical analysis of Townsend breakdown using ionization coefficients to describe the number of electrons/ions produced per unit length.
This document summarizes key concepts in high voltage engineering related to insulation breakdown in gaseous dielectrics. It discusses:
1) Various ionization processes that can occur when electrons collide with gas molecules, including simple collision ionization, excitation, double electron impact ionization, and photoionization.
2) Townsend breakdown process where free electrons cause an "electron avalanche" through successive ionizing collisions, leading to exponential growth in the number of electrons.
3) Townsend's first and second ionization coefficients which describe the number of electrons produced per unit length and number of electrons released from the cathode per positive ion, respectively.
Electron Beam Machining (Modern ManufacturingProcess)Dinesh Panchal
The document summarizes electron beam machining (EBM). EBM uses a focused beam of high-energy electrons to melt and vaporize metal, allowing for precise machining. There are two types - thermal EBM uses the beam's heat to selectively vaporize material, while non-thermal EBM causes surface chemical reactions. The document discusses the generation and control of electron beams, the physical processes involved in thermal EBM, and a phenomenological theory of non-thermal EBM film growth proposed by Christly.
20200829-XII-Physics-Dual Nature of Radiation and Matter-1 of 7-Ppt.pptxManishMishra398080
This document provides an overview of electron emission and the photoelectric effect. It discusses four methods of electron emission: thermionic emission through heating, field emission using electric fields, photoelectric emission using light, and secondary emission from electron bombardment. For photoelectric emission, it describes Lenard's experimental setup and findings that photocurrent increases with light intensity but stopping potential is independent of intensity. Key topics covered include work function, the photoelectric effect in different metals, and how collector potential and light intensity affect photocurrent when frequency is fixed.
Ph8253 physics for electronics engineeringSindiaIsac
1) The document discusses conducting materials and their properties. It describes how metals have high electrical conductivity due to free electrons. Current density is defined as the current per unit area.
2) Conducting materials are classified as zero resistive, low resistive, or high resistive based on their conductivity. Zero resistive materials conduct electricity with almost zero resistance below a transition temperature. Low and high resistive materials are used for conductors and resistors.
3) The classical free electron theory and quantum free electron theory are discussed as ways to explain electrical conductivity in metals based on their electronic structure and behavior of free electrons.
1) The document reports on an experiment investigating the photoelectric effect. It describes how shining light on certain metals, like potassium, causes electrons to be ejected from the metal surface.
2) Classical electromagnetic theory could not explain observations like the instantaneous emission of electrons and the fact that kinetic energy of emitted electrons depends on frequency but not intensity of light.
3) Einstein explained the photoelectric effect using a quantum theory where light is described as discrete packets of energy called photons. When a photon strikes an electron and transfers its entire energy, the electron can escape from the metal if the photon energy exceeds the metal's work function.
Atomic Structure from A level chemistry.saqibnaveed9
This document provides an overview of atomic structure and the discovery of subatomic particles like electrons. It discusses J.J. Thomson's cathode ray tube experiments in the late 1800s that led to the discovery of electrons and determination of their charge to mass ratio (e/m). It also describes Millikan's oil drop experiment from 1909 that precisely measured the charge of individual electrons as 1.6022×10-19 coulombs and calculated the mass of an electron as 9.1095×10-31 kg.
The document summarizes various cathode processes and decay processes involved in electrical breakdown in gases. It describes four main ways electrons can be emitted from a cathode surface: 1) photoelectric emission through photon bombardment, 2) electron emission through positive ion or excited atom impact, 3) thermionic emission through heating the cathode, and 4) field emission through very strong electric fields lowering the work function. It also discusses two major decay processes: 1) deionization through recombination of positive and negative ions, and 2) deionization through attachment of free electrons to form stable negative ions, especially in electronegative gases.
This document outlines topics related to semiconductor physics and optoelectronics physics, including:
1. Free electron theory of metals, Bloch's theorem, energy band diagrams, direct and indirect bandgaps, density of states, and the types of electronic materials including metals, semiconductors and insulators.
2. Lasers, which use stimulated emission of radiation to produce an intense, coherent beam of light. Key concepts covered include spontaneous emission, stimulated absorption, population inversion, and semiconductor lasers.
3. Photodetectors and noise sources, with reference made to the Fermi Golden Rule. The document provides an overview of key concepts that will be covered in more depth within these physics courses.
The document discusses electrical breakdown in gases. It explains that gases are commonly used as dielectric mediums in electrical apparatus due to their insulating properties. However, when high voltages are applied, electrical breakdown can occur through ionization. The Townsend theory and streamer theory are presented as explanations for the breakdown mechanism under different conditions. Collision processes, mobility of ions and electrons, diffusion, and mean free path are also discussed. The document further explains the ionization process and Townsend's criteria for electrical breakdown in gases.
I gave this presentation in my university as part of the High Voltage Engineering course. The contents of this presentation are as follows:
1) Ionization Process
2) Gases as Insulating Medium
3) Secondary Ionization
4) Cathode Process
5) Types of Cathode Processes
1. The document discusses the wave and particle nature of light and provides evidence from phenomena such as interference, diffraction for the wave nature and the photoelectric effect and Compton effect for the particle nature.
2. It then describes the photoelectric effect in detail, explaining terms like threshold frequency, work function, and how Einstein's photoelectric equation explained the instantaneous emission of electrons.
3. Applications of the photoelectric effect include its use in cameras for light meters and in security systems.
The document describes J.J. Thomson's 1897 experiment to determine the specific charge (e/m ratio) of electrons using a cathode ray tube. Thomson observed that cathode rays were deflected by electric and magnetic fields, allowing him to calculate e/m. He developed a formula relating the electric and magnetic fields to electron beam deflection. His finding that e/m was constant supported the then-novel idea that cathode rays consisted of fundamental particles, which he named "corpuscles" but are now called electrons. This experiment provided early evidence challenging the belief that atoms were indivisible.
1) The document discusses energy band theory and how it relates to the electrical properties of semiconductors, insulators, and metals. It explains that semiconductors have a small forbidden band gap between the valence and conduction bands, allowing thermal or electromagnetic excitation of electrons.
2) The concept of effective mass is introduced, where electrons in a crystal lattice behave as if they have a different mass than free electrons due to the crystal potential. Effective mass depends on the curvature of electron energy-momentum diagrams.
3) Direct and indirect band gap materials are distinguished based on whether the minimum of the conduction band and maximum of the valence band occur at the same or different crystal momentum values.
Study Some Parameters of Electrical Discharge in N2 and CO2 Without and With ...IOSRJECE
:We study the breakdown voltage under low pressure for N2, CO2 gases of with a magnetic field to the electrode of iron and aluminum with diameter (8.8cm) cm and distance separation between them is (3cm). by using Passion curve, we measur less effort collapsed, and we notice that less effort is linked to the collapse of a function held cities and when the magnetic field will be reduced to shed breakdown voltage. Since the breakdown voltage for CO2 is greater than breakdown voltage N2. Through curved Passion was calculated (훾) and when to shed the magnetic field will increase in value
The document discusses various topics in physics including:
1) Light behaves as both a particle (photon) and wave exhibiting properties like interference and diffraction. Photoelectric effect shows light has quantized particle nature.
2) Bohr model of the atom successfully explained hydrogen spectrum but failed for other atoms. Modern quantum theory with energy levels provides more accurate description.
3) Lasers operate on principle of stimulated emission, producing coherent, collimated light through population inversion in gain medium.
1) The document discusses the electronic structure of atoms, beginning with a description of the electromagnetic spectrum and wave-particle duality of light. 2) It then covers early atomic models including Planck's quantum theory, Bohr's model of the atom, and de Broglie's proposal that electrons exhibit wave-like properties. 3) The document concludes by mentioning the development of quantum mechanics and Heisenberg's uncertainty principle.
Similar to Principles of Electronics chapter - II (20)
Understanding Inductive Bias in Machine LearningSUTEJAS
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Literature Review Basics and Understanding Reference Management.pptxDr Ramhari Poudyal
Three-day training on academic research focuses on analytical tools at United Technical College, supported by the University Grant Commission, Nepal. 24-26 May 2024
1. 28 Principles of Electronics
2.1 Electron Emission
2.2 Types of Electron Emission
2.3 Thermionic Emission
2.4 Thermionic Emitter
2.5 Commonly Used Thermionic Emitters
2.6 Cathode Construction
2.7 Field Emission
2.8 Secondary Emission
2.9 Photo Electric Emission
Electron Emission
2
INTRINTRINTRINTRINTRODUCTIONODUCTIONODUCTIONODUCTIONODUCTION
T
he reader is familiar with the current conduction (i.e. flow of electrons) through a conduc-
tor. The valence electrons of the conductor atoms are loosely bound to the atomic nuclei. At
room temperature, the thermal energy in the conductor is adequate to break the bonds of the
valence electrons and leave them unattached to any one nucleus. These unbound electrons move at
random within the conductor and are known as
free electrons. If an electric field is applied across
the conductor, these free electrons move through
the conductor in an orderly manner, thus consti-
tuting electric current. This is how these free elec-
trons move through the conductor or electric cur-
rent flows through a wire.
Many electronic devices depend for their op-
eration on the movement of electrons in an evacu-
ated space. For this purpose, the free electrons
must be ejected from the surface of metallic con- Electron Emission
2. Electron Emission 29
ductor by supplying sufficient energy from some external source. This is known as electron emission.
The emitted electrons can be made to move in vacuum under the influence of an electric field, thus
constituting electric current in vacuum. In this chapter, we shall confine our attention to the various
aspects of electron emission.
2.1 Electron Emission
The liberation of electrons from the surface of a substance is known as electron emission.
For electron emission, metals are used because they
have many free electrons. If a piece of metal is
investigated at room temperature, the random motion
of free electrons is as shown in Fig. 2.1. However, these
electrons are free only to the extent that they may transfer
from one atom to another within the metal but they
cannot leave the metal surface to provide electron
emission. It is because the free electrons that start at
the surface of metal find behind them positive nuclei
pulling them back and none pulling forward. Thus at
the surface of a metal, a free electron encounters forces
that prevent it to leave the metal. In other words, the
metallic surface offers a barrier to free electrons and is
known as surface barrier.
However, if sufficient external energy is given to the free electron, its kinetic energy is increased
and thus electron will cross over the surface barrier to leave the metal. This additional energy required
by an electron to overcome the surface barrier of the metal is called work function of the metal.
The amount of additional energy required to emit an electron from a metallic surface is known
as work function of that metal.
Thus, if the total energy required to liberate an electron from a metal is 4 eV* and the energy
already possessed by the electron is 0.5 eV, then additional energy required (i.e., work function) is
4.0− 0.5 = 3.5 eV. The work function of pure metals varies roughly from 2 to 6 eV. It depends upon
the nature of metal, its purity and the conditions of its surface. It may be noted that it is desirable that
metal used for electron emission should have low work function so that a small amount of energy is
required to cause emission of electrons.
2.2 Types of Electron Emission
The electron emission from the surface of a metal is possible only if sufficient additional energy
(equal to the work function of the metal) is supplied from some external source. This external energy
may come from a variety of sources such as heat energy, energy stored in electric field, light energy or
kinetic energy of the electric charges bombarding the metal surface. Accordingly, there are following
four principal methods of obtaining electron emission from the surface of a metal :
* Work function is the additional energy required for the liberation of electrons. Therefore, it should have
the conventional unit of energy i.e. joules. But this unit is very large for computing electronics work.
Therefore, in practice, a smaller unit called electron volt (abbreviated as eV) is used.
One electron-volt is the amount of energy acquired by an electron when it is accelerated through a poten-
tial difference of 1 V.
Thus, if an electron moves from a point of 0 potential to a point of +10V, then amount of energy acquired
by the electron is 10 eV.
Since charge on an electron = 1.602 × 10
−19
C and voltage = 1 V,
∴ 1 electron-volt = Q V = (1.602 × 10
−19
) × 1 J
or 1 eV = 1.602 × 10
−19
J
Fig. 2.1
○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
3. 30 Principles of Electronics
(i) Thermionic emission. In this method, the metal is heated to sufficient temperature (about
2500ºC) to enable the free electrons to leave the metal surface. The number of electrons emitted
depends upon the temperature. The higher the temperature, the greater is the emission of electrons.
This type of emission is employed in vacuum tubes.
(ii) Field emission. In this method, a strong electric field (i.e. a high positive voltage) is
applied at the metal surface which pulls the free electrons out of metal because of the attraction of
positive field. The stronger the electric field, the greater is the electron emission.
(iii) Photo-electric emission. In this method, the energy of light falling upon the metal surface
is transferred to the free electrons within the metal to enable them to leave the surface. The greater the
intensity (i.e. brightness) of light beam falling on the metal surface, the greater is the photo-electric
emission.
(iv) Secondary emission. In this method, a high velocity beam of electrons strikes the metal
surface and causes the free electrons of the metal to be knocked out from the surface.
2.3 Thermionic Emission
The process of electron emission from a metal surface by supplying thermal energy to it is known as
thermionic emission.
At ordinary temperatures, the energy possessed by free electrons in the metal is inadequate to
cause them to escape from the surface. When heat is applied to the metal, some of heat energy is
converted into kinetic energy, causing accelerated motion of free electrons. When the temperature
rises sufficiently, these electrons acquire additional energy equal to the work function of the metal.
Consequently, they overcome the restraining surface barrier and leave the metal surface.
Metals with lower work function will require less additional energy and, therefore, will emit
electrons at lower temperatures. The commonly used materials for electron emission are tungsten,
thoriated tungsten and metallic oxides of barium and strontium. It may be added here that high
temperatures are necessary to cause thermionic emission. For example, pure tungsten must be heated
to about 2300ºC to get electron emission. However, oxide coated emitters need only 750ºC to cause
thermionic emission.
Richardson-Dushman equation. The amount of thermionic emission increases rapidly as the
emitter temperature is raised. The emission current density is given by Richardson-Dushman equa-
tion given below :
Js = A T
2 b
Te
−
amp/m
2
...(i)
where Js = emission current density i.e. current per square metre of the
emitting surface
T = absolute temperature of emitter in K
A = constant, depending upon the type of emitter and is measured
in amp/m2
/K2
b = a constant for the emitter
e = natural logarithmic base
The value of b is constant for a metal and is given by :
b =
e
k
φ
where φ = work function of emitter
e = electron charge = 1.602 × 10−19
coulomb
k = Boltzmann’s constant = 1.38 × 10
−23
J/K
∴ b =
19
23
1.602 10
1.38 10
−
−
φ× ×
×
= 11600 φ K
4. Electron Emission 31
Putting the value of b in exp. (i), we get,
Js =
11600
2 TAT e
φ
−
...(ii)
The following points may be noted from eqn. (ii) :
(i) The emission is markedly affected by temperature changes. Doubling the temperature of an
emitter may increase electron emission by more than 10
7
times. For instance, emission from pure
tungsten metal is about 10− 6
ampere per sq. cm. at 1300ºC but rises to enormous value of about 100
amperes when temperature is raised to 2900ºC.
(ii) Small changes in the work function of the emitter can produce enormous effects on emis-
sion. Halving the work function has exactly the same effect as doubling the temperature.
Example 2.1. A tungsten filament consists of a cylindrical cathode 5 cm long and 0.01 cm in
diameter. If the operating temperature is 2500 K, find the emission current. Given that
A = 60.2 × 104
A/m2
/ K2
, φ = 4.517 eV.
Solution. A = 60.2 × 10
4
amp/m
2
/K
2
, T = 2500 K, φ = 4.517 eV
∴ b = 11600 φ K = 11600 × 4.517 K = 52400 K
Using Richardson-Dushman equation, emission current density is given by :
Js = 2
b
TAT e
−
amp/m
2
= 60.2 × 10
4
× (2500)
2
×
52400
2500
(2.718)
−
= 0.3 × 10
4
amp/m
2
Surface area of cathode, a = π d l = 3.146 × 0.01 × 5 = 0.157 cm2
= 0.157 × 10−4
m2
∴ Emission current = Js × a = (0.3 × 10
4
) × (0.157 × 10
−4
) = 0.047 A
Example 2.2. A tungsten wire of unknown composition emits 0.1 amp/cm
2
at a temperature of
1900 K. Find the work function of tungsten filament. Determine whether the tungsten is pure or
contaminated with substance of lower work function. Given that A = 60.2 amp/cm2
/K2
.
Solution. Js = 0.1 amp/cm
2
; A = 60.2 amp/cm
2
/K
2
; T = 1900 K
Let φ electron-volt be the work function of the filament.
∴ b = 11600 φ K
Using Richardson-Dushman equation, emission current density is given by :
Js = 2
b
TA T e
−
amp/cm2
or 0.1 = 60.2 × (1900)
2
×
11600
1900
e
φ
−
or
11600
1900
e
φ
− = 2
0.1
60.2 (1900)× = 4.6 × 10−10
or e
−6.1 φ
= 4.6 × 10
−10
or − 6.1 φ loge e = loge 4.6 − 10 loge 10
or − 6.1 φ = 1.526 − 23.02
∴ φ =
1.526 23.02
6.1
−
− = 3.56 eV
Since the work function of pure tungsten is 4.52 eV, the sample must be contaminated. Thoriated
tungsten has a work function ranging from 2.63 eV to 4.52 eV, depending upon the percentage of
metallic thorium. Therefore, the sample is most likely to be thoriated tungsten.
5. 32 Principles of Electronics
Thermionic Emitter
2.4 Thermionic Emitter
The substance used for electron emission is known as an emitter or cathode.
The cathode is heated in an evacuated space to emit electrons. If the cathode
were heated to the required temperature in open air, it would burn up because of
the presence of oxygen in the air. A cathode should have the following properties:
(i) Low work function. The substance selected as cathode should have
low work function so that electron emission takes place by applying small amount
of heat energy i.e. at low temperatures.
(ii) High melting point. As electron emission takes place at very high
temperatures (>1500ºC), therefore, the substance used as a cathode should have
high melting point. For a material such as copper, which has the advantage of a
low work function, it is seen that it cannot be used as a cathode because it melts
at 810ºC. Consequently, it will vaporise before it begins to emit electrons.
(iii) High mechanical strength. The emitter should have high mechanical
strength to withstand the bombardment of positive ions. In any vacuum tube, no matter how careful
the evacuation, there are always present some gas molecules which may form ions by impact with
electrons when current flows. Under the influence of electric field, the positive ions strike the cath-
ode. If high voltages are used, the cathode is subjected to considerable bombardment and may be dam-
aged.
2.5 Commonly Used Thermionic Emitters
The high temperatures needed for satisfactory thermionic emission in vacuum tubes limit the
number of suitable emitters to such substances as tungsten, thoriated tungsten and certain oxide
coated metals.
(i) Tungsten. It was the earliest material used as a
cathode and has a slightly higher work function (4.52 eV).
The important factors in its favour are : high melting point
(3650 K), greater mechanical strength and longer life. The
disadvantages are : high operating temperature (2500 K),
high work function and low emission efficiency. Therefore,
it is used in applications involving voltages exceeding 5 kV
e.g. in X-ray tubes.
(ii) Thoriated tungsten. A mixture of two metals may
have a lower work function than either of the pure metals
alone. Thus, a tungsten emitter with a small quantity of
thorium has a work function of 2.63 eV, compared with 3.4
eV for thorium and 4.52 eV for tungsten. At the same time, thoriated tungsten provides thermionic
emission at lower temperature (1700ºC) with consequent reduction in the heating power required.
In the manufacture of this type of cathode, tungsten filament is impregnated with thorium oxide
and heated to a very high temperature (1850ºC to 2500ºC). The thorium oxide is reduced to metallic
thorium and coats the filament surface with a thin layer of thorium. Thoriated tungsten cathodes are
used for intermediate power tubes at voltages between 500 to 5000 volts.
(iii) Oxide-coated cathode. The cathode of this *type consists of a nickel ribbon coated with
Thoriated Tungsten
* Oxides of any alkaline-earth metal (e.g. calcium, strontium, barium etc.) have very good emission
characteristics. In the manufacture of this type of emitter, the base metal (e.g. nickel) is first coated with a
mixture of strontium and barium carbonates. It is then heated to a high temperature in vacuum glass tube
until the carbonates decompose into oxides. By proper heating, a layer of oxides of barium and strontium
is coated over the cathode surface to give oxide-coated emitter.
○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
6. Electron Emission 33
barium and strontium oxides. The oxide-coated cathode has low work function (1.1 eV), operates at
comparatively low temperature (750ºC) and has high emission efficiency. However, the principal
limitation of oxide-coated cathode is that it cannot withstand high voltages. Therefore, it is mostly
used in receiving tubes or where voltages involved do not exceed 1000 volts.
S.No. Emitter Work Function Operating Emission
temperature efficiency
1 Tungsten 4.52 eV 2327ºC 4 mA/watt
2 Thoriated tungsten 2.63 eV 1700ºC 60 mA/watt
3 Oxide-coated 1.1 eV 750ºC 200 mA/watt
2.6 Cathode Construction
As cathode is sealed in vacuum, therefore, the most convenient way to heat it is electrically. On this
basis, the thermionic cathodes are divided into two types viz directly heated cathode and indirectly
heated cathode.
(i) Directly heated cathode. In this type, the cathode consists of oxide-coated
nickel ribbon, called the *filament. The heating current is directly passed through this ribbon which
emits the electrons. Fig. 2.2 (i) shows the structure of directly heated cathode whereas Fig. 2.2 (ii)
shows its symbol.
Fig. 2.2
The directly heated cathode is more efficient in converting heating power into thermionic emission.
Therefore, it is generally used in power tubes that need large amounts of emission and in small tubes
operated from batteries where efficiency and quick heating are important. The principal limitation of
this type of cathode is that any variation in heater voltage affects the electron emission and thus
produces hum in the circuit.
(ii) Indirectly heated cathode. In this type, the cathode consists of a thin metal sleeve coated
with barium and strontium oxides. A filament or heater is enclosed within the sleeve and insulated
from it. There is no electrical connection between the heater and the cathode. The heating current is
passed through the heater and the cathode is heated indirectly through heat transfer from the heater
element. Fig. 2.3 (i) shows the structure of indirectly heated cathode whereas Fig. 2.3 (ii) shows its
symbol.
* Filament. The term filament (literally means a thin wire) denotes the element through which the cathode
heating current flows. In case of directly heated, cathode is itself the filament. If indirectly heated, heater
is the filament.
○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
7. 34 Principles of Electronics
Fig. 2.3
Indirectly heated cathode has many advantages. As
cathode is completely separated from the heating circuit,
therefore, it can be readily connected to any desired
potential as needed, independent of the heater potential.
Furthermore, because of relatively large mass of cylindrical
cathode, it takes time to heat or cool and as such does not
introduce hum due to heater voltage fluctuations. Finally,
a.c. can be used in the heater circuit to simplify the power
requirements. Almost all modern receiving tubes use this
type of cathode.
2.7 Field Emission
The process of electron emission by the application of
strong electric field at the surface of a metal is known as
field emission.
When a metal surface is placed close to a high voltage
conductor which is positive w.r.t. the metal surface, the
electric field exerts attractive force on the free electrons
in the metal. If the positive potential is great enough, it
succeeds in overcoming the restraining forces of the metal
surface and the free electrons will be emitted from the
metal surface as shown in Fig. 2.4.
Very intense electric field is required to produce field emission. Usually, a voltage of the order
of a million volts per centimetre distance between the emitting surface and the positive conductor is
necessary to cause field emission. Field emission can be obtained at temperatures much lower (e.g.
room temperature) than required for thermionic emission and, therefore, it is also sometimes called
cold cathode emission or auto- electronic emission.
2.8 Secondary Emission
Electron emission from a metallic surface by the bombardment of high-speed electrons or other
particles is known as secondary emission.
Fig. 2.4
8. Electron Emission 35
Fig. 2.6
* An interesting aspect of secondary emission is that a high-speed bombarding electron may liberate as many
as 10 “secondary electrons”. This amounts to a multiplication of electron flow by a ratio as great as 10 and
is utilised in current multiplier devices.
○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
When high-speed electrons
suddenly strike a metallic surface,
they may give some or all of their
kinetic energy to the free electrons
in the metal. If the energy of the
striking electrons is sufficient, it
may cause free electrons to escape
from the metal surface. This
phenomenon is called secondary
emission. The electrons that strike
the metal are called primary
electrons while the emitted
electrons are known as secondary
electrons. The intensity of
secondary emission depends upon
the emitter material, mass and
energy of the bombarding
particles.
The principle of secondary
emission is illustrated in Fig. 2.5.
An evacuated glass envelope con-
tains an emitting surface E, the collecting anode
A and a source of primary electrons S. The an-
ode is maintained at positive potential w.r.t. the
emitting surface by battery B. When the primary
electrons strike the emitting surface E, they knock
out secondary electrons which are attracted to
the anode and constitute a flow of current. This
current may be measured by connecting a sensi-
tive galvanometer G in the anode circuit.
The effects of secondary emission are very
undesirable in many electronic devices. For
example, in a tetrode valve, secondary emission
is responsible for the negative resistance. In
some electronic devices, however, secondary
emission effects are utilised e.g. *electron
multiplier, cathode ray tube etc.
2.9 Photo Electric Emission
Electron emission from a metallic surface by the
application of light is known as photo electric
emission.
When a beam of light strikes the surface of
certain metals (e.g. potassium, sodium, cesium),
the energy of photons of light is transferred to the free electrons within the metal. If the energy of the
Fig. 2.5
9. 36 Principles of Electronics
striking photons is greater than the work
function of the metal, then free electrons
will be knocked out from the surface of
the metal. The emitted electrons are
known as photo electrons and the
phenomenon is known as photoelectric
emission. The amount of photoelectric
emission depends upon the intensity of
light falling upon the emitter and
frequency of radiations. The greater the
intensity and frequency of radiations, the
greater is the photo electric emission.
Photo-electric emission is utilised in photo
tubes which form the basis of television
and sound films.
Fig. 2.6 illustrates the phenomenon
of photoelectric emission. The emitter E and anode A are enclosed in an evacuated glass envelope G.
A battery B maintains the anode at positive potential w.r.t. emitter. When light of suitable intensity
and frequency falls on the emitter, electrons are ejected from its surface. These electrons are attracted
by the positive anode to constitute current in the circuit. It may be noted that current will exist in the
circuit so long as illumination is maintained.
MULTIPLE-CHOICE QUESTIONS
Photo Tube
1. Work function of metals is generally mea-
sured in ..............
(i) joules (ii) electron-volt
(iii) watt-hour (iv) watt
2. The operating temperature of an oxide-
coated emitter is about ..............
(i) 750ºC (ii) 1200ºC
(iii) 2300ºC (iv) 3650ºC
3. .............. is used in high voltage (> 10 kV)
applications.
(i) tungsten emitter
(ii) oxide-coated emitter
(iii) thoriated-tungsten emitter
(iv) none of the above
4. A desirable characteristic of an emitter is
that it should have .............. work function.
(i) large (ii) very large
(iii) small (iv) none of the above
5. The thermionic emitter that has the highest
operating temperature is ..............
(i) oxide-coated (ii) thoriated-tungsten
(iii) tungsten (iv) none of the above
6. If the temperature of an emitter is increased
two times, the electron emission is ..............
(i) increased two times
(ii) increased four times
(iii) increased several million times
(iv) none of the above
7. In X-ray tubes, .............. emitter is used.
(i) thoriated tungsten
(ii) tungsten
(iii) oxide-coated
(iv) none of the above
8. The life of an oxide-coated emitter is about
..............
(i) 500 hours (ii) 1000 hours
(iii) 200 hours (iv) 10,000 hours
9. The electrons emitted by a thermionic emit-
ter are called ..............
(i) free electrons
(ii) loose electrons
(iii) thermionic electrons
(iv) bound electrons
10. Electron Emission 37
10. The work function of an oxide-coated emit-
ter is about ..............
(i) 1.1 eV (ii) 4 eV
(iii) 2.63 eV (iv) 4.52 eV
11. The warm-up time of a directly heated cath-
ode is .............. that of indirectly heated cath-
ode.
(i) more than (ii) less than
(iii) same as (iv) data incomplete
12. The most commonly used emitter in the tubes
of a radio receiver is ..............
(i) tungsten (ii) thoriated-tungsten
(iii) oxide-coated (iv) none of the above
13. Field emission is utilised in ..............
(i) vacuum tubes
(ii) TV picture tubes
(iii) gas-filled tubes
(iv) mercury pool devices
14. Oxide-coated emitters have electron emis-
sion of .............. per watt of heating power.
(i) 5-10 mA (ii) 40-90 mA
(iii) 50-100 mA (iv) 150-1000 mA
15. The oxide-coated cathodes can be used for
voltages upto ..............
(i) 1000 V (ii) 3000 V
(iii) 4000 V (iv) 10,000 V
Answers to Multiple-Choice Questions
1. (ii) 2. (i) 3. (i) 4. (iii) 5. (iii)
6. (iii) 7. (ii) 8. (iv) 9. (iii) 10. (i)
11. (ii) 12. (iii) 13. (iv) 14. (iv) 15. (i)
Chapter Review Topics
1. What is electron emission ? Explain the terms : surface barrier and work function.
2. What general conditions must be satisfied before an electron can escape from the surface of a
material ?
3. Name and explain briefly four practical ways by which electron emission can occur.
4. What are the materials used for thermionic emitters ? Compare the relative merits of each.
5. Discuss briefly construction and relative advantages of directly and indirectly heated cathodes.
Problems
1. An oxide-coated emitter has a surface area of 0.157 cm
2
. If the operating temperature is 110 K, find
the emission current. Given A = 100 A/m2
/K2
, work function = 1.04 eV. [0.0352 A]
2. A tungsten filament of unknown composition emits 1000 A/m
2
at an operating temperature of
1900 K. Find the work function of tungsten filament. Given A = 60.2 × 10
4
A/m
2
/ K
2
. [3.44 eV]
3. Calculate the total emission available from barium-strontium oxide emitter, 10 cm long and 0.01 cm
in diameter, operated at 1900 K. Given that A = 10−12
Amp/cm2
/K2
and b = 12,000. [0.345 A]
Discussion Questions
1. Why does electron emission not occur at room temperature ?
2. Why are high temperatures necessary for thermionic emission ?
3. Why are electron emitters heated electrically ?
4. Why are thermionic emitters heated in vacuum ?
5. Why are tungsten and thoriated tungsten cathodes always of directly heated type ?
6. Why cannot oxide-coated cathodes be used for voltages exceeding 1000 volts?
7. Why do directly heated cathodes introduce hum in the circuit ?
8. Why are directly heated cathodes used in high power applications ?