1. 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 10/30/2015APHY1011

2. 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 ! 10/30/2015APHY1012

3. Electric Circuits   Ohm’s Law and Resistance Unit: ohms (Ω) = volts/amps 10/30/2015APHY1013

4. 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. 10/30/2015APHY1014

5. Electric Circuits 10/30/2015APHY1015

6. 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 10/30/2015APHY1016

7. Electric Circuits   Series circuits 10/30/2015APHY1017

8. Electric Circuits   Series circuits 1. The current is the same everywhere in the circuit. I total = I 1 = I 2 = … 2. The total resistance of the circuit is R total = R 1 + R 2 + … 3. The sum of the voltages across each device equals the source voltage ΔV source = ΔV 1 + ΔV 2 + … 10/30/2015APHY1018

9. Electric Circuits   Parallel circuits 10/30/2015APHY1019

10. Electric Circuits   Parallel circuits 1. The total current in the circuit equals the sum of the current in each device. I total = I 1 + I 2 + … 2. The total resistance of the circuit is 3. The voltage across each device is the same. ΔV source = ΔV 1 = ΔV 2 = … 10/30/2015APHY10110

11. 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 10/30/2015APHY10111

12. 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 circuitFusesbreakers 10/30/2015APHY10112