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Electrodynamics Electrostatics the study of electrical the study of electrical charges in motion charges at rest Two opposite types of charge exist, named positive and negative by Benjamin Franklin. Charge is a property of matter. Charged particles exist in atoms. Electrons are responsible for negative charge; protons for positive charge; neutrons have no charge. Small amounts of ordinary matter contain incredible amounts of subatomic particles! Conductor link Insulator link material that allows charges to move about easily material through which charges will not easily move Basic Law of Electrostatics Like charges repel; unlike charges attract Link charging a rod and electroscope positively and negatively by conduction and induction When charging by conduction, the rod touches the electroscope. The electroscope gets the same charge as the rod. When charging by induction, the rod does not touch the electroscope. The electroscope gets the opposite charge of the rod. The direction of the electric field at any point is defined to be the same direction as the direction of force on a positive test charge placed in the region at that point. Field lines point away from positive and toward negative charges. Click here to view a simulation showing the magnitude and direction of the electric force on a test charge when placed near other charges. Click here to view a simulation of a charged particle moving through a region occupied by other charges. Electric Potential Difference the change in electric potential energy per unit charge V = W/Q The SI unit of electric potential difference is the VOLT, named in honor of Alessandro Volta. One VOLT is the electric potential difference between two points when one Joule of work is done in moving one Coulomb of charge between the points. the flow of charged particles; can be positive or negative, but usually negative (electrons) through a conducting metal Electric current is measured in Amperes, in honor of Andre Marie Ampere. One Ampere is the flow of one Coulomb of charge per second. 1 Amp = 1 Coulomb per second = 1 C/s I = Q/t Ammeter a device that measures current Voltmeter a device that measures electric potential difference Resistance determines the amount of current flow = the ratio of potential difference to current R= V The SI unit of resistance is the I Ohm, W, named in honor of Georg Simon Ohm. One Ohm of resistance is the resistance such that one Volt of potential difference is needed to obtain a current of one Amp. The resistance of a circuit element depends on: 1. the length of the conductor as length increases, resistance increases proportionally 2. the cross-sectional area of the conductor as area increases, resistance decreases proportionally 3. the resistivity of the conductor as resistivity increases, resistance increases proportionally Ohm’s Law The ratio of potential difference to current is constant. If R = V/I is a constant value for a given resistor, then that resistor is said to obey Ohm’s Law. Click here and here to link to pages describing resistor color codes. Many circuit elements do not obey Ohm’s Law. Resistors that get hot, like light bulbs and heating elements, do not keep a constant resistance. Resistance generally increases as objects become hotter. Click here and here to run simulations of Ohm’s Law. Series Resistor Circuits 1. total resistance is the sum of the separate resistors RT = R1 + R2 + R3 + ... 2. current is the same through each resistor IT = I1 = I2 = I3 = ... 3. total potential difference is the sum of each VT = V1 + V2 + V3 + ... In other words, in a series circuit, resistance and voltage add, but current stays the same. R, W E = 12 V R1 R3 R2 R1 8.0 R2 2.0 R3 5.0 RT = VT = IT = PT = V, V I, A P, W R, W E = 12 V R1 R3 R2 V, V I, A P, W R1 8.0 6.4 0.80 5.1 R2 2.0 1.6 0.80 1.3 R3 5.0 4.0 0.80 3.2 RT = 15 Ω VT = 12 V IT = 0.80 A PT = 9.6 W Parallel 1. reciprocal of the total resistance is the sum of the reciprocals of the separate resistors 1/RT = 1/R1 + 1/R2 +1/R3 + ... 2. total current is the sum of the current through each resistor IT = I1 + I2 + I3 + ... 3. potential difference is the same across each resistor VT = V1 = V2 = V3 = ... In other words, in a parallel circuit, resistance adds as reciprocals, voltage stays the same, and current splits. R, W E = 12 V R1 R1 12 R2 8.0 R3 12 R2 R3 RT = VT = IT = PT = V, V I, A P, W R, W E = 12 V R1 V, V I, A R1 12 12 R2 8.0 12 1.5 18 R3 12 12 1.0 12 1.0 12 R2 R3 P, W RT = 3.42 Ω VT = 12 V IT = 3.50 A PT = 42 W The site linked here allows you to build and test your own series, parallel, and/or combination circuits. For a complete interactive tutorial on electricity and magnetism, go here.