5.73: Electric Power and Electrical Energy Use
Miranda has to use a hair dryer in order to dry her long hair quickly enough to get to school on time. She has a powerful hair dryer that dries her hair faster than a less powerful hair dryer would. That’s because a more powerful hair dryer changes electric current to thermal energy more quickly.
Electric Power
The rate at which a device changes electric current to another form of energy is called electric power. The SI unit for power—including electric power—is the watt. A watt equals 1 joule of energy per second. High wattages are often expressed in kilowatts, where 1 kilowatt equals 1000 watts. The power of an electric device, such as a hair dryer, can be calculated if you know the voltage of the circuit and how much current the device receives. The following equation is used:


 Power (watts) = Current (amps) × Voltage (volts)

Assume that Miranda’s hair dryer is the only electric device in a 120volt circuit that carries 15 amps of current. Then the power of her hair dryer in kilowatts is:


 Power = 15 amps × 120 volts = 1800 watts, or 1.8 kilowatts

Q: If a different hair dryer is plugged into a 120volt circuit that carries 10 amps of current. What is the power of the other hair dryer?
A: Substitute these values in the power equation:


 Power = 10 amps × 120 volts = 1200 watts, or 1.2 kilowatts

Electrical Energy Use
Did you ever wonder how much electrical energy it takes to use an appliance such as a hair dryer? Electrical energy use depends on the power of the appliance and how long it is used. It can be calculated with this equation:


 Electrical Energy = Power × Time

If Miranda uses her 1.8kilowatt hair dryer for 0.2 hours, how much electrical energy does she use?


 Electrical Energy = 1.8 kilowatts × 0.2 hours = 0.36 kilowatthours

Electrical energy use is typically expressed in kilowatthours, as in this example. How much energy is this? One kilowatthour equals 3.6 million joules of energy.
Q: Suppose Miranda were to use a 1.2kilowatt hair dryer for 0.2 hours. How much electrical energy would she use then?
A: She would use:


 Electrical Energy = 1.2 kilowatts × 0.2 hour = 0.24 kilowatthours

Summary
 Electric power is the rate at which a device changes electric current to another form of energy. The SI unit of power is the watt. Electric power can be calculated as current times voltage.
 Electrical energy use equals the power of the appliance multiplied by the amount of time the appliance is used.
Review
 Define electric power. What is the SI unit for power?
 A microwave is the only electric device in a 120volt circuit. If the circuit has 7.5 amps of current, what is the electric power of the microwave?
 Calculate the electrical energy use of the microwave in question 2 if it used for 0.05 hours.
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Image Attributions
 Explain how to calculate the electric power of an appliance.
 Identify the SI unit for power, including electric power.
 Show how to calculate the electrical energy used by an appliance.
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