• Electrostatics • Electric Current • Magnetism• Electromagnetism
Electrostatics• It is believed that Thales of Miletus, one of the wise men of ancient Greece, discovered the phenomenon called static electricity. He observed at around 600 B.C. that amber, a fossilized resin attracted tiny particles of wood when rubbed with wool.
Electrostatics• During the later part of the 15th century, Sir William Gilbert, an English scientist, madea number of experiments to investigate theProperties of loadstones and amber. These startedthe first systematic study of static electricity. In hisbook “De Magnete”,published in 1600, he reportedhis findings and conclusions. He introduced theword “electric” to designate materials whichbehave like amber.
• Electric Charge- is a basic property of certainelementary particles of which all matter iscomposed. There are two kinds of electriccharge, positive charge and negative charge.• Electron - negatively charged particles of an atom.• Proton - positively charged particles of an atom.
Modern plastics can now be used to obtainpositive and negative charges. Polythene stripcan acquire a negative charge when it is rubbedwith a duster. On the other hand, acellulose acetate strip can acquire positivecharge using the same duster. Like charges repel and unlike charges attract
The Electron Theory When two substances are rubbedtogether, the electrons are the ones transferredfrom one substance to the other. Substancesbecame positive when some electrons areremoved from them and become negative whensome electrons are added to them.
A device to detect the presence of electriccharges can facilitate the study of electrostatics.The electroscope is a simple instrument whichcan do this.
The electroscope consists of a light strip ofmetal foil or paper hinged to the metal rods orstem. This metal rod is connected to the metalcap or disk and the whole assembly is mountedin a plastic or glass container. This separation of charges when there is a charged material nearby is called electrostatic induction.
Conductors and Insulators Conductors allow current to move through it .This indicates that electricity is not possible withoutthem. On the other hand, insulators are also valuable inthe sense that they stop charges from flowing from oneconductor to another. Most materials are good conductors becausethey contain large numbers free electrons per unitvolume. Insulators contain practically no freeelectrons. The electrons in them are tightly boundand normally charge does not flow.
Electrostatic Generator• There are two main types of electrostatic generators which are traditional items in a physics laboratory. They are the electrophorus and the Van de Graaf generator.
Electrostatic GeneratorsThe electrophorus is a simple electrostatic generatorinvented by Alessandro Volta.It has a non-conducting base and a metal plate withan insulated handle. The base acquires a negativecharge on being rubbed with fur. When the metal plate is placed on the base, it is stronglycharged by induction. Then grounding will leave theplate charged positively.
Electrostatic Generators• The Van De Graaf generator is a machine used to charge objects easily. It is capable of producing very high voltage in the order of 10 MV with respect to the ground.
Coulomb’s Law• Charles Coulomb – a French Physicist, made the first quantitative investigation of electric forces with the use of a torsion balance which he invented.• Coulomb’s Law – states that the force exerted by one charged object on another is proportional to the product of the magnitude of the charges and is inversely proportional to the square of the distance between them.
Sample Problem1. Find the electric force between twoelectrons which are separated by a distance of1mm. The charge of an electron is1.6 x 10 ⁻¹⁹C.
• Electric Field – is a region around a charged object where any charge placed in it experiences a force which is electrical in nature.• Electric Field Intensity – is defined as the force per unit charge acting on a charged object placed at that point in the field. The electric field intensity at a point varies inversely as the square of the distance “r” of the point from Q and directly as the magnitude of the charge.
Sample Problem2. How much is the electric field intensityat a point 1m away from a charge of10 x 10⁻⁶C?
Electric Lines of Force An Electric field can be describedvisually by drawing lines of electric force.These lines will represent the direction ofmotion of a small “free” positive chargeplaced in the field. The density of the linesis used to indicate the intensity of the field.If the charge is positive, the lines of electricforce are directed away from it. The lines ofelectric force is directed towards the chargeif it is negative.
Electric Potential and Electric Potential Difference• Electric potential – work done in bringing charge from a point of zero potential to that point.• Electric Potential Difference – work done in transferring a charge from one point to another.• Electron volt – is the energy acquired by an electron that has been accelerated through a potential difference of 1 volt.
Electric Current• Electric current – rate of flow of electric charges.• Ampere – unit of electric current in the MKS system. This is equal to a flow of one coulomb of charge per second.• Andre Marie Ampere – a french mathematician and physicist who formulated some fundamental laws of electricity and magnetism.
Sample Problem3. How much current is produced if3.75 x 10²⁰ electrons pass a given point in aconductor in one minute? The charge of anelectron is 1.6 x 10⁻¹⁹C.
• Sources of EMF – is any device that will transform non-electrical energy into electrical energy.• EMF – electromotive force difference in potential between the electrodes of a source in an open circuit.• Cell – an electron pump• Battery – a device for storing and generating an electric current by chemical reaction. This is a series of cells.
Electric Circuit• An electric circuit is the path through which an electric current flows.
Series Circuit• If we connect 2 or more lamps as shown in the figure, all the electrons that go through 1 lamp must also go to the other lamps.
Parallel Circuit• If we connect a cell to 2 or more lamps, as shown in this figure, we say that the lamps are connected in parallel. As the electrons reach point N, they divide between the branches of the circuit until they reach point M where they join together again.
Electric Circuit An instrument which measures the currentin a circuit is called an ammeter. It is alwaysconnected in series in the circuit. On the otherhand, an instrument which measures potentialdifference is called a Voltmeter.
Ohm’s Law and Resistance In 1826, George Simon Ohm, a German physicist,discovered that the current (I) through a metal wire isproportional to the potential difference (V) across it providedthat the temperature is kept constant.This relationship is called Ohm’s Law.
This means that the Current (I) isproportional to the voltage or potentialdifference (V). In equation form it is V/I = R, where theconstant R stands for the resistance of thewire. We define resistance of a wire as the ratioof potential difference to the current. Thisdefinition is actually the Ohm’s Law.
Sample Problem4. A potential difference of 12V is appliedacross an electric circuit. a.) What is the current in the circuit if theresistance is 30 Ohm’s? b.) If the resistance in the circuit isdoubled, what is the current?
Factors Affecting ResistanceThe resistance of a conducting wire thatobeys Ohm’s Law depends upon 3 factors:1. The material of which it is made2. The length of the wire3. The cross-sectional area of the wireIt was found that the longer the wire the smaller thecurrent across it.On the other hand, when the cross-sectional area of awire is increased, the resistance it offers to the flow ofelectric charges becomes smaller.
Sample Problem5. What length of resistance wire ofresistivity 100 x 10 ⁻⁸ ohm-m, and of cross-section 2.5 x 10 ⁻⁷ m² would be needed tomake a resistor 57.6 ohms?
Superconductors In 1911, an astonishing discovery wasmade by M. Kamerlingh Onnes who won aNobel Prize for it. He found that whenmercury is cooled at 4.2°C above absolutezero, its resistance suddenly disappearsentirely. The material exhibits the property ofsuperconductivity.
• Superconductors- are materials that lose all resistance when cooled to temperatures near absolute zero. Current once started, flows in them forever. This indicates that there is no need to replace energy that is lost to resistance if superconductors are used. So superconductors may become the key to tremendous savings of energy and money.
Resistors A resistor is a device used tointroduce resistance into an electricalcircuit. Resistors are sometimes madeof a length of nichrome wire. Resistorscan be used to reduce the current in acircuit.
Resistors in Series When resistors are in series, the combinedresistance is the sum of the individualresistances in the combination and istherefore greater than any individualresistance. Rт= R₁+R₂+R₃
Sample Problem6. A potential difference of 6 V is applied to 2resistors of 8 Ω and 4 Ω connected in series.Find: a.) The combined resistance of the 2resistors. b.) The current flowing in the circuit c.) The potential difference across the8 Ω resistor.
Resistors in Parallel When resistors are connected inparallel, the reciprocal of thecombined resistance is equal to thesum of the reciprocals of theindividual resistances. 1/R= 1/R₁ + 1/R₂ + 1/R₃
Sample Problem7. A potential difference of 12V is applied to 2 resistors (3Ω and 6Ω) connected in parallel.Calculate: a.) The combined resistance of the 2 resistors. b.) The current flowing in the main circuit c.) The current in the 6Ω resistor
• Power – is defined as the rate of doing work, or the rate at which energy is produced. In simple terms, power is the amount of work done in a unit of time. Power = Work/ time P= V I P= I² R P= V²/ R Power is measured in joules per second which is called watt.
Sample Problem8. A burning lamp is marked 22W, 220V a.) How much current flows through the lamp? b.) What is its resistance?
Capacitance• Capacitance- ratio of electric charge to the potential difference. Q= CV C = Q/V = 1C/1V = 1 farad• Capacitor- a device that stores charges.
Sample Problem9. A parallel-plate capacitor of 10 microfaradcapacitance is charged to a potentialdifference of 5V. How much charge is storedon each plate?
Magnetism The term Magnetism comes from theregion of Magnesia, an island in the AegeanSea where certain stones were found by theGreeks more than 2000 years ago. Thestones called “ lodestones” had the unusualproperty of attracting pieces of iron. Magnets were first fashioned intocompasses and used for navigation by theChinese in the 12th century.
Magnetism If the north pole of one magnet isbrought near the north pole of anothermagnet, they repel. The same is true of asouth pole near another south pole. Ifopposite poles are brought near eachother, attraction occurs. Hence, theseobservations can be stated as follows:unlike poles attract and like poles repel.
Magnetism Magnetic poles behave similarly to electriccharges in this aspect of attraction andrepulsion. But they are very much different inthat while an electron and a proton can existseparately, a north pole and a south polecannot. They always come in pairs.
Coulomb’s Law of Magnetism Coulomb’s law of magnetism statesthat the force between 2 magnetic polesis directly proportional to the product oftheir pole strengths and inverselyproportional to the square of the distancebetween them.
If a pole of a magnet is brought to touchone end of the first clip, the second clip can bepicked up by the first. This means that the clipwhich is in contact with the magnet, becomesa magnet itself. We call this phenomenon asinduced magnetism.
Magnetic Fields A magnetic field is a region or space inwhich a magnet pole placed in it experiences aforce which is magnetic in nature. An electricfield and a magnetic field have similarcharacteristics but they are not equivalent.
Magnetic Field The path taken by an imaginary “free”N- Pole in a magnetic field is called a magneticline of force or a line of flux.
These flux lines in a magnetic field arecollectively called magnetic fluxrepresented by the Greek letter phi Ø.The unit of magnetic flux in MKS Systemis the Weber. One weber is equal to 10⁸lines of flux.
Theory of Magnetism Since electron is a charged particle, thistheory implies that magnetism is a property of acharge in motion. Two types of electron motionare regarded as important in this concept ofmagnetism. First, the revolution of the electron about thenucleus of an atom imparts magnetic property tothe structure of the atom. The second type of motion is the spinning ofthe electron on its own axis.
How Magnets are Made • Stroking Method • Electrical Method • By hammering • By heating
Destroying Magnetism• By hammering• By heating• By Alternating Current Method
Magnetic effect of an Electric Current In 1813, a Danish Physicist Hans ChristianOersted predicted that a relationshipbetween electricity and magnetism would befound. In 1819, seven years later, Oerstedhimself discovered that electricity has amagnetic effect. He found that a smallcompass needle is deflected when broughtnear a conductor carrying an electric current.This was the first evidence of the suspectedrelationship between electricity andmagnetism.
Forces Between Current-carrying Wires After Oersted’s discovery of the magneticeffect of an electric current, a French physicistAndre Marie Ampere determined the shape ofthe magnetic field about a conductor carryinga current. Earlier he had discovered that forcesexist between two parallel conductors in acircuit. If the current in the parallel conductorsare in the same direction, the two conductorsattract one another. The force between thetwo conductors are repulsive if the currentsare in opposite directions.
Magnetic field around a long straight current carrying conductor
As to the determination of the direction ofthe magnetic field around the conductor, wecan make use of the right-hand rule forstraight conductors.
The magnitude of the magnetic field B alsocalled as magnetic flux density or magneticinduction, at any point in the magnetic field ofa current- carrying conductor is directlyproportional to the current in the conductorand inversely proportional to the radialdistance, r , of the point from the conductor.
Sample Problem10. A long straight wire carries a current of12 A. how much is the magnetic flux density ata point 0.1 meter from the conductor?
Uses of Electromagnets• In electric Bell and electric Buzzer• In Electromagnetic Relays• In Telephone Earpiece and in Radio Earphone
Force on a Current-carrying Conductor in a Magnetic field The force on the wire depends upon the strengths of the 2 magnetic fields. Thus, a force may be increased by increasing the current in the wire and by increasing the strength of the uniform field.
Fleming’s Left-hand Rule states that if the thumb, forefinger and the middle finger of the left hand are held at right angles to each other, then the forefinger points in the direction of the magnetic field, the center finger points in the direction of the conventional current and the thumb points in the direction of the force on the wire.
Electromagnetic Induction In 1831, Michael Faraday discovered how tomake electricity using magnetism. He found out thatan electromotive force is set up in a conductorlocated in a magnetic field, when the magnetic flux iscut by the conductor. Joseph Henry, at about thesame time made a similar discovery.Electromagnetic Induction- current is induced in thewire, if the wire is moved to cut across lines of force
Fleming’s Right-hand Rule states that if your right-hand is held in afist and thumb, the forefinger and the middlefinger are spread out at right angles to eachother, the forefinger points in the direction ofthe magnetic field, the thumb points in thedirection of the movement of the wire and themiddle finger points in the direction of theinduced current.