1. Ch 19 Current and Potential Difference

2. Current is rate of charge movement: I = Δq/Δt. The unit of current is the ampere, or amp. 1A = 1C/s

3. The current in a light bulb is 0.835 A. How long does it take for a total charge of 1.67 C to pass a point in the wire?

4. Although electrons are the charged particles moving in most cases, conventional current is described for most flows of electricity.

5. Conventional current is the current of positive charge that would have the same effect as the actual charge that is flowing.

6. When a circuit is completed, an electric field is produced in the conductor. This happens at near the speed of light.

7. The electrons themselves move much slower. Collisions with material in the conductor cause the electrons to follow a zig-zag path.

8. The individual electrons move slowly along the conductor due to this zig- zag path in the opposite direction of the electric field. Their velocity is known as the drift velocity.

9. The magnitude of this drift velocity is usually very small. In a typical copper wire with a current of 10 Amps, the electrons take about an hour to move 1 meter.The electric field; however, moves close to the speed of light.

10. A battery (cell) converts chemical energy to electrical energy. A potential difference is produced between the two terminals. This produces an electric field in a conductor that connects the two terminals.

11. Generators convert mechanical energy to electrical energy. They produce the average potential difference of 120 volts in a wall outlet.

12. The two types of current are alternating current (AC) and direct current (DC).

13. In DC the current always flows the same direction. In AC the direction of current flows changes rapidly. If the changes were too slow, you would notice lights flickering, etc.

14. To prevent this, AC oscillates 60 times per second (60 Hz) in the U.S. Batteries produce DC; generators can produce AC or DC.

15. Current in a circuit is determined by the potential difference. It is also determined by the resistance. Resistance is the opposition to the motion of charge.

16. The SI unit of resistance is the ohm (Ω). One ohm = 1 volt/1 ampere.

17. Ohm’s law is V = IR. It holds true for a wide range of materials and voltages, but is not true for all materials.

18. In a non-ohmic material, the slope of a graph of current vs. potential difference will not be a straight line.

19. A diode is a semiconducting device that is non-ohmic.

20. We will assume that all resistors follow Ohm’s law.

21. Some factors that affect the resistance of conductors are:

22. length - longer conductors have greater resistance, cross-sectional area - greater area produces less resistance, material - better conductors have less resistance, temperature - higher temperatures increase resistance.

23. By V = IR, changing the resistance of a circuit changes the current. Changing the voltage also changes the current, but this is not practical in household circuits.

24. The resistance of a steam iron is 19.0 Ω. What is the current in the iron when it is connected across a potential difference of 120 V?

25. Superconductors have no resistance below a critical temperature. By V = IR, if R is zero, a current can exist without a potential difference. These have been observed to exist for years!

26. Unfortunately, these critical temperatures are typically very low, many below 10 Kelvins. Work is proceeding to find room temperature superconductors.