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Electric Circuits Electric Current The amount of charge passing through a surface per second. • Unit: ampere or amp (A) = coulombs/seconds The “conventional” current (I) is defined by the motion of positive charges → Benjamin Franklin 5/23/2017 APHY101 1 Electric Circuits Drift Speed The average speed of charges that make a current • Usually ~ 0.1 mm/s The electric field is created throughout the wire so that all charges respond almost instantaneously The charges collide with atoms in the wire and proceed with a random walk • The atoms get hotter due to these collisions → light ! 5/23/2017 APHY101 2 Electric Circuits Ohm’s Law and Resistance • Unit: ohms (Ω) = volts/amps 5/23/2017 APHY101 3 Electric Circuits Resistivity What determines the value of R for a resistor? • Type of material, length and thickness • Temperature? Conductors, insulators, heating units, lights, etc. 5/23/2017 APHY101 4 Electric Circuits 5/23/2017 APHY101 5 Electric Circuits Energy and Electric Potential in a Circuit The PE of a charge is unchanged after traveling around a circuit PE is gained at the voltage source (ie. battery) and lost at the resistor (ie. light bulb) The sum of the ΔV around the circuit equals zero • Voltage “drops” across a resistor 5/23/2017 APHY101 6 Electric Circuits Series circuits 5/23/2017 APHY101 7 Electric Circuits Series circuits 1. The current is the same everywhere in the circuit. Itotal = I1 = I2 = … 2. The total resistance of the circuit is Rtotal = R1 + R2 + … 3. The sum of the voltages across each device equals the source voltage ΔVsource = ΔV1 + ΔV2 + … 5/23/2017 APHY101 8 Electric Circuits Parallel circuits 5/23/2017 APHY101 9 Electric Circuits Parallel circuits 1. The total current in the circuit equals the sum of the current in each device. Itotal = I1 + I2 + … 2. The total resistance of the circuit is 3. The voltage across each device is the same. ΔVsource = ΔV1 = ΔV2 = … 5/23/2017 APHY101 10 Electric Circuits Electric Power Energy transfer per unit time • Unit: watts or joules/second Since it takes work (W) to move a charge (q) through a potential difference (ΔV) then we can say that 5/23/2017 APHY101 11 Electric Circuits Electric Power Overloading occurs when the circuit resistance drops to where the circuit carries too much current. Fuses or breakers are used to control the amount of current in a circuit 5/23/2017 APHY101 12