1. ELECTRIC CURRENT 2 Ohm’s law shows the relationship between current, potential, and voltage. We need a few more rules to make predictions about current flow through circuits.

2. Rule 1: Conservation of Charge The number of charges flowing into a point is the same as the number of charges flowing out of a point

3. 5 amps 3 amps ?

4. 5 amps 3 amps 2 amps

5. Rule 2: Conservation of Energy The total drop in potential energy of a circuit is equal to the total voltage of the circuit

6. SERIES CIRCUITS

7. The voltage drops across each resistor, but the total voltage drop is still 60 volts.

8. So, V 1 + V 2 = 60 volts… V1V1 V2V2 And V=IR R1R1 R2R2

9. V1V1 V2V2 R1R1 R2R2 So, IR 1 + IR 2 = IR total But the current is the same through R 1 and R 2.

10. V1V1 V2V2 R1R1 R2R2 IR total = I (R 1 +_R 2 ) R total = R 1 +_R 2 or

11. Summary: For series circuits the total resistance is equal to the sum of all the resistors in series. Functionally, this is the same as increasing the length of a resistor. As L increases, so does R.

12. From the Reference Tables

13. V1V1 V2V2 R1R1 R2R2 So, back to our problem! R 1 + R 2 = 30 Ω = total resistance Therefore the total current = V/R = 2 amps 2 A

14. V 1 = 40 volts Remember, IR 1 + IR 2 = IR total So: (2 amps x 20 ohms) = 40 volts and (2 amps x 10 ohms) = 20 volts V 2 = 20 volts

15. PARALLEL CIRCUITS

16. The voltage drop across R 1 and R 2 is the same and R1R1 R2R2 ItIt I1I1 I2I2 I t = I 1 + I 2

17. R1R1 R2R2 ItIt I1I1 I2I2

18. R1R1 R2R2 ItIt I1I1 I2I2 So:

19. Summary: For parallel circuits the reciprocal of the total resistance is equal to the sum of the reciprocal of each resistance. Functionally, this is the same as increasing the area of a resistor. As A increases, R decreases.

20. From the Reference Tables

21. R1R1 R2R2 ItIt I1I1 I2I2 Back to our problem

22. R1R1 R2R2 ItIt I1I1 I2I2 Notice that the total resistance is less that either one

23. R1R1 R2R2 ItIt I1I1 I2I2 We can now calculate the total current…

24. R1R1 R2R2 I t = 0.6 amps I1I1 I2I2 …and the current through each resistor.

25. Finally, R1R1 R2R2 I t = 0.6 amps I 2 = 0.3 amps