http://www.ck12.org Chapter 5. Energy
5.73 Electric Power and Electrical Energy Use
- 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.
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 calledelectric 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 120-volt 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 kilowattsQ:If a different hair dryer is plugged into a 120-volt 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