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# Unit 1 Static Electricity

## on Mar 02, 2011

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• In this Van de Graaff generator, an electric motor pulls a latex belt over a felt covered pulley. Electrons are transferred from the felt to the belt as the belt pulls away from the pulley. The electrons ride the belt to the upper dome, where a pick-up wire transfers them from the belt to the dome. Each electron carries a negative charge and, since like charges repel, the electrons on the dome attempt to get as far away from each other as possible. At some point, too many electrons are placed on the dome. When this occurs the electrons attempt to reach the earth by leaping through the air to a nearby object, creating a large spark.

## Unit 1 Static ElectricityPresentation Transcript

• SCIENCE 10F electricity the nature of
• Unit 1 – The Nature of Electricity What is a model?
• What is Static Electricity?
• Static electricity is the situation where electrical charges build up on the surface of a material. It is called “static” because there is no current flowing as in AC or DC electricity. Static electricity is usually caused when materials are rubbed together. The result is that objects may be attracted to each other or may even cause a spark to jump from one object to the other.
Unit 1 – The Nature of Electricity
•
• Static Electricity
• Many different materials can be electrified by rubbing.
• An object can be charged by rubbing and charge gives rise to forces of attraction and repulsion.
• Given the observations of attraction and repulsion, we can say there exist two kinds of charge.
• Charges that are like (that is, created in similar circumstances) repel , and charges which are unlike attract .
Unit 1 – The Nature of Electricity
• Two Fluid Model of Static Electricity
• Charles Dufray, French scientist in 1700’s.
• 2 fluids, vitreous and resinous.
Unit 1 – The Nature of Electricity
• Two Fluid Model of Static Electricity
• The rod at the right has an equal amount of vitreous and resinous fluid. For this reason it is electrically neutral.
• The rod at the right has more resinous than vitreous fluid. The object would be considered resinous in this case.
Unit 1 – The Nature of Electricity
• Two Fluid Model of Static Electricity
• The rod at the right has more vitreous than resinous fluid and would be called vitreous.
Unit 1 – The Nature of Electricity
• One Fluid Model of Static Electricity
• Benjamin Franklin proposed a one fluid model.
• He used the terms positive and negative, stating that there was only one fluid and its positive or negative nature depended on the excess or deficiency of the fluid.
• The object to the right
• is electrically neutral.
• It is completely filled
• with electric fluid. This
• is the object's natural state.
Unit 1 – The Nature of Electricity
• One Fluid Model of Static Electricity
• The object to the right has a deficiency of electric fluid and is considered to be electrically negative. The electrical fluid is shown inside the irregular line.
• The positive object has an excess of electric fluid. The electrical fluid is shown inside the irregular line that is now larger than the object.
Unit 1 – The Nature of Electricity
• Static Electricity
• Two-fluid model (late 1700s) of electricity
• Charles Dufay (French physicist) explained two kinds of charges as caused by varying amounts of two fluids called vitreous fluid and resinous fluid
• an electrically neutral object had equal amounts of both fluids
• One-fluid model (1752) of electricity by Benjamin Franklin
• explained a charged object as having an excess or deficiency of electrical fluid
• a neutral object would have the proper amount of electrical fluid
• first person to use the terms negative and positive in regards to charged object
Unit 1 – The Nature of Electricity
• Static Electricity - Video Unit 1 – The Nature of Electricity
• Particle Model of Electricity Electron Theory
• Scientists made important discoveries about the structure of the atom in the late 1800s and the early 1900s. Initially, it seemed the atom was one indivisible particle, but further experimenting showed that there were subatomic (smaller than an atom) particles. Subatomic particles make up the atom. They include protons, neutrons, and electrons .
Unit 1 – The Nature of Electricity
• Particle Model of Electricity Electron Theory
• The electron was found to be very small, have very little mass, and was negatively charged.
• It was demonstrated that the nucleus contained small positively charged objects called protons . A proton had the same amount of charge as an electron but it was opposite to the electron or positive. The proton was approximately 2000 times as dense as an electron.
• The neutron was a neutral particle with no charge and was found in nucleus of an atom.
Unit 1 – The nature of Electricity
• Particle Model of Electricity Electron Theory
• Atoms ordinarily contain the same number of electrons and protons. The equal numbers of positive and negative charges in the atom mean that it is electrostatically neutral
Unit 1 – The nature of Electricity
• Particle Model of Electricity Electron Theory Unit 1 – The nature of Electricity
• Particle Model of Electricity Electron Theory
• There are strong forces that hold the protons in the nucleus. Electrons, on the other hand, are more loosely held and travel around the nucleus, so they are more easily removed.
• If an atom gains an electric charge, it is because it either gains or loses electrons. Protons will not move from the nucleus.
• If an electron is added to the atom, it now contains one more electron than protons and has a net negative charge. If an electron is removed, it now has one less electron than protons and has a net positive charge. The negative and positive charges result from the addition or removal of electrons only.
Unit 1 – The nature of Electricity
• Particle Model of Electricity Electron Theory
• Electrons are so easily transferred to or from some atoms that friction provides enough energy to cause the transfer to take place. For this reason, you rub a plastic straw with wool to create an electrostatic charge in the straw. The wool transfers electrons to the plastic straw and it becomes negatively charged. The wool, because it has lost electrons, becomes positively charged.
Unit 1 – The nature of Electricity
• Particle Model of Electricity Electron Theory Unit 1 – The nature of Electricity
• Static Electricity Unit 1 – The nature of Electricity
• Van de Graaff Generator
• A Van de Graaff generator is a device used to create a high voltage static charge.
Unit 1 – The nature of Electricity
• Conductors & Insulators
• Two types of material are identified based on how well they transmit electric charge. One type of materials allows the electric charge to easily pass through. These substances are called conductors . The other type of materials does not easily allow the electric charge to pass through. These materials are called insulators .
Unit 1 – The nature of Electricity
• 3 Electrical States
• Objects can exist in three states electrically.
• An object can be neutral , in which case the positive and negative charges on the object are balanced (the net charge is zero).
• An object can be positively charged , which means that there are more positive charges on the object than negative (the net charge is positive).
• If the object is negatively charged , it means that there are more negative charges than positive on the object(the net charge is negative).
Unit 1 – The nature of Electricity
• Charging By Contact
• You have observed objects that can be charged by contact. A plastic straw or a piece of vinyl in contact (rubbed) with wool receives an electric charge. A glass rod in contact (rubbed) with silk in contact (rubbed) with cotton creates a different charge.
• Certain materials such as wool and vinyl and some materials such as glass contain negative particles that are loosely held. When these substances are in contact with (rubbed by) another material, they are able to give away negatively charged particles.
• The particle model for electric charge states that only negative charges move in solids.
Unit 1 – The nature of Electricity
• Charging By Contact
• Remember that plastic, vinyl, glass, and rubber are all insulators. This means the charge produced on the objects does not travel anywhere but remains at the spot where it is rubbed by a cloth. As a result the objects have localized charges that do not move. This means you can hold the objects without their losing the electric charge.
Unit 1 – The nature of Electricity
• Charging By Contact Charging by Using Charge Transfer
• You have seen that electric charge can be moved from one object to another through friction. The particle model adequately explains how this transfer takes place. Another way that electric charge can travel from one object to another is by touching a charged object and another, usually neutral, object.
• The charge moves between the charged object and the neutral object. This method of charge transfer takes place by contact or conduction.
Unit 1 – The nature of Electricity
• Charging By Conduction Unit 1 – The nature of Electricity
• Charging By Induction
• When charging by conduction takes place a charge separation occurs. A charge separation means there is more of one type of charge than another on an object. Charge transfer can take place using two methods. One method is called conduction. The second method is called induction; here the charged object is brought close to, but not allowed to touch the neutral object. Induction is a method of transferring charge without having the charged object touch the object receiving the charge.
Unit 1 – The nature of Electricity
• Charging By Induction
• In the induction process of charging:
• a charged object is brought near to but not touching another object
• the presence of the charged object above the other object, induces electrons within the neutral object to move accordingly.
• with the charged object still held above the neutral object, it (the neutral object) is touched.
• at this point electrons will flow between the neutral object and the ground, giving the once neutral object an overall charge.
• when the charged object is pulled away, the once neutral object has a net charge.
Unit 1 – The nature of Electricity
• Charging By Induction Unit 1 – The nature of Electricity
• Charging By Conduction & Induction Unit 1 – The nature of Electricity
• Triboelectric Series
• The Triboelectric Series lists materials according to how likely they are to let go of electrons or to take on electrons from other materials. Most of the materials in the Triboelectric Series are complex compounds and the release or attraction of electrons has to do with their molecular structure or geometry.
• Dry human skin and rabbit fur have the greatest tendency to give up electrons when rubbed on something and become positively ( + ) charged. Teflon and vinyl have the greatest tendency to become negatively charged ( - ) when rubbed. If you want to create static electricity, rubbing fur on teflon should give the best results.
•
•        What are materials in the Triboelectric Series?
•        How can the list of materials be used to create static electricity?
Unit 1 – The nature of Electricity
• Electrostatic Tools
• What is an electroscope ?
• How does an electroscope work?
• What 3 things can an electroscope measure?
• What is an electrophorus ?
• How does an electrophorus work?
• What is an electrophorus used for?
Unit 1 – The nature of Electricity
• Electrostatic Tools – The Electrosocope
• An electroscope functions as a tool for determining whether or not an electrical charge is present in an object.
• An electroscope is able to perform three kinds of measurement to:
• detect the presence of an electrostatic charge on an object
• determine the type of charge on an object
• determine the amount of charge on an object
Unit 1 – The nature of Electricity
• Electrostatic Tools – The Electrosocope
• A standard electroscope is shown in the diagram below. It has parts that are conductors - the metal sphere, the metal rod and the gold foil leaves. These conducting parts allow the negative charges to move freely throughout the electroscope. There are also some insulating parts - the rubber stopper and the glass flask. The insulating parts keep the negative charges inside the electroscope.
Unit 1 – The nature of Electricity
• Electrostatic Tools – The Electrosocope Unit 1 – The nature of Electricity
• Electrostatic Tools – The Electrophorus
• The electrophorus is a device that can be used to create a large charge quickly using charge transfer by electrostatic induction. The electrophorus can be used to create a large charge over and over again with no diminishing of the net charge because the charge transfer occurs by induction and the charge always comes from the ground, not from the foam plate.
Unit 1 – The nature of Electricity
• Electrostatic Tools – The Electrophorus Unit 1 – The nature of Electricity
• Electrostatic Tools – The Electrophorus Unit 1 – The nature of Electricity
• Electrostatic Tools – The Leyden Jar
• The Leyden jar is a simple, yet powerful device for storing large quantities of electrical charge.
• The diagram below shows a commercial Leyden jar. This Leyden jar looks the same as it did 250 years ago.
Unit 1 – The nature of Electricity Metal ball Metal rod Rubber or cork stopper Glass bottle Small chain Metal conductor on the inside and the outside of the glass bottle
• The Leyden Jar
• The Leyden jar below has been charged negatively by contact (conduction) . The metal ball, rod, and conductor on the inside of the Leyden jar are negatively charged. Since the metal conductor on the outside of the glass jar is grounded, the negative charges are repelled to ground by the negative charge on the inside of the jar. The outside of the jar becomes positively charged by induction.
• The negative charge on the inside and the positive charge on the outside of the Leyden jar attract each other and hold each other on the respective surfaces. In this way the Leyden jar can store charge for long periods of time.
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