1. Current, Potential, and Resistance

2. What is charge?
Short answer: no one really knows.

3. What is charge?
+ -
But we do know that there are two types of charge.

4. What is charge?
+ -
And we are quite adept at using these charges to do work for us.

5. Electric Current
In order to get charge to work for us, charge must move from one place to another.

6. Electric Current
The rate at which charge moves through a cross section of wire is called electric current.

7. Electric Current
The symbol for electric current is I and the SI unit is the ampere (or amp, A.) 1 A = 1 C/s

8. Electric Current
I = Q / t

9. Example
 A famous strike just before the Apollo 15 launch in 1971 was measured at 100,000 A. If the strike took 0.20 s to discharge, how much charge was contained in the strike? How many electrons does this represent?
Most lightning strikes have a current between 5000 A and 20,000 A .

10. But how do we get charge to move?
A charge in an electric field will have potential energy, just like mass in a gravitational field will have potential energy.

11. Electric Potential Energy
To calculate this PE, we can use the following formula:
PE = -qEd
*what is qE?

12. Electric Potential
Often we want to know the potential in an electric field independent of the charge in the field. To do this, we can define what is known as electric potential, V.

13. Electric Potential
V = PEelectric / q

14. V = PEelectric / q or V = -Ed
Potential Difference
V = PEelectric / q or V = -Ed

15. Example
If a charge moves 3.5 cm in an electric field of 150 N/C, the potential energy decreases by 5.0 x J. a) What is the charge on the moving particle? b) What is the potential difference between the two locations?

16. Not all conductors were created equal.
Conductors are materials that allow charge to move within them. However, some conductors allow charges to move through them more easily than others.

17. Resistance
The opposition to the motion of charge through a conductor is called resistance. The SI unit for resistance is the Ohm (Ω).

18. Resistance
What factors do you think would affect resistance?

19. Resistance
What factors do you think would affect resistance?

20. Resistance
What factors do you think would affect resistance?

21. Resistance
What factors do you think would affect resistance?

22. Resistance
What factors do you think would affect resistance?

23. Resistance
What factors do you think would affect resistance?

24. Resistance
All materials have some resistance. Can you think of circumstances where resistance has negative consequences? Can you think of circumstances where resistance has benefits?

25. Good or Bad Resistance? Good or Bad resistance?

26. Resistance
Resistors are used in electronic devices to regulate the amount of current and voltage going through a circuit.

29. Superconductors Check these out:

30. Resistance
Resistance can be calculated by: ∆V R = I

31. Resistance ∆V R = I
This shows that resistance is the ratio of the voltage (push behind charge) and the current (the flow of charge).

32. Resistance ∆V R = I
The relationship is called Ohm’s Law is more commonly written as…

33. Example
How much current is drawn by a television with a resistance of 35Ω that is connected across a potential difference of 120 V?

34. Homework
p. 599/practice A, p.609/practice C and p.615/practice D