1. Electric Power

2. Power: is the rate at which the work is done
Electric Power is the rate at which energy converted or is used.
The watt (W) is a unit of power.
Since energy is measured in joules, power is measured in joules per second.
One joule per second is equal to one watt.

3. Power
Voltage (volts)
P = VI
Power (watts)
Current (amps)

4. Calculate power
A light bulb with a resistance of 1.5Ω is connected to a 1.5- volt battery in the circuit shown at right.
Calculate the power used by the light bulb.
1) You are asked to find the power used by the light bulb.
2) You are given the voltage of the battery and the bulb’s resistance.
3) Use Ohm’s law, I = V/R, to calculate the current; then use the power equation, P=VI, to calculate the power.
4) Solve: I = 1.5V ÷ 1.5Ω = 1A
P = 1.5V × 1A = 1.5 W; the bulb uses 1.5 watts of electric power.

5. A kilowatt (kW) is equal to 1,000 watts.
The other common unit of power often seen on electric motors is the horsepower.
One horsepower is 746 watts.

6. Paying for electricity
Electric companies charge for the number of kilowatt-hours used during a set period of time, often a month.
One kilowatt-hour (kWh) means that a kilowatt of power has been used for one hour.
Since power multiplied by time is energy, a kilowatt-hour is a unit of energy.
One kilowatt-hour is 3.6 x 106 joules.

7. Calculate power
1) You are asked to find the cost of using the coffee maker.
2) You are given the power in watts and the time.
3) Use the power formula P = VI and the fact that 1 kWh = 1kW x 1h.
4) Solve: Find the number of kilowatts of power that the coffee maker uses.
1,050 W × 1 kW/1,000 W = 1.05 kW
Find the kilowatt-hours used by the coffee maker each month.
1.05 kW × 1 hr/day x 30 days/month = 31.5 kWh per month.
Find the cost of using the coffee maker.
31.5 kWh/month × $0.14/kWh = $4.41 per month.
Your electric company charges 14 cents per kilowatt-hour. Your coffee maker has a power rating of 1,050 watts.
How much does it cost to use the coffee maker one hour per day for a month?