1. Power Revisited

2.  Power is the rate at which work is done. P = W t  Since work = a change in energy, power is the rate at which energy (changes) is transferred. P = ΔPE t Units for power are Watts (W) Recall:

3. P = ΔPE t P = I V P = I 2 R P = V 2 R 4 Ways to Calculate Power

4. 1. V = Δ PE So, QV = Δ PE Q Substitue P = Δ PE P = QV and I = Q t t t So P = I V Derivations of last 3 formulas:

5. Combining P = IV and V = IR Substitute IR in for V P = I(IR) P = I 2 R 2.

6. 3. Rearrange V = IR to solve for I I = V Substitute into P = IV R P = V (V) R P = V 2 R

7. A 40 Watt light bulb is connected to a 120 V outlet. a. What is the current in the wire? b. What is the resistance of the filament? Example:

8. Go to Ms. Roth’s Website and answer questions #1-5 on the Power Problems wkst.

9.  Electric (or power) companies measure the energy consumed (not power). They do not measure energy in Joules. They use kilowatt-hours (kWh) P = ΔPE so P*t = ΔPE t 1kW * 1h = 1kWh The Electric Bill

10. How much does it cost to operate a 100W light bulb for 30 minutes if electrical energy costs $0.12 / kWh? Example:

11. How much does it cost a different light bulb with resistance 240 Ω and a current of 0.5A through it to run for the 30 min and same cost per kWh? Example 2:

12. Ms. Roth's electric meter with only one light on

13. Ms. Roth's electric meter with all lights, fans, tv's on.

14. Ms. Roth's electric bill

15. Average usage per month