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The rate at which we use the energy (energy divided by time) is called POWER.
The unit for power is the watt (W).
P
The formula for power = current times voltage,
OR: using symbols, P = IV.
The magic triangle is shown at the right.
I
V
The following problems use the formula.
REMEMBER: You must show work for credit (Equation, Setup, Answer , Units)
1. An electric hair dryer has 10 amperes of current going through it. If the voltage in the house is 120 V,
what is the power used by the hair dryer?
2. A boom box uses 12 watts of power. If it runs on a 9 V battery, what is the current going through the
radio?
3. A toaster has 20 ohms of resistance and is in a circuit with 120 V of potential difference.
a. What is the current in the circuit? (Hint: use Ohm’s Law to find the current)
b. What is the power it uses?
We have fuses or circuit breakers in our house to protect against too much current passing through the wires.
Most circuits have a limit of 20 amperes of current. If this is exceeded, the circuit is broken.
4. Dizzy Dorey has all of the following appliances turned on in the same circuit. She has:
 4 light bulbs that are 60 watts each
 a 960-watt toaster
 a 300-watt television.
a. What is the total power (add up the total watts) of these appliances?
b. How much current will these appliances use if the potential difference (voltage) in her house is 120 V?
c. Dizzy then turns on her 1200-watt hair dryer. How much current does this appliance use? (Same
Voltage = 120v)
d. Will she blow a fuse if she uses all these appliances at the same time?
Finding the cost of running an appliance:
1. If electricity cost 6 cents a kWhr, how much does it cost to run a 2000-watt heater for 10 hours? Follow
these steps:
a. How many Whrs does the heater use?
b. Divide Whrs by 1000 to get kWhr.
c. Take kWhr x cost to determine the answer – put a box around answer.
2. If an electric dryer on a 240 V circuit used 15 amperes of current, how much will it cost to dry clothes if it
runs for 5 hours at 6 cents a kWhr? Steps:
a. Calculate power.
b. Find Whrs.
c. Find kWhrs.
a. Take kWhr x cost to determine the answer.
b. What is the resistance of the dryer?