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.
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 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 kilowatts
Q: 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
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 1.8-kilowatt hair dryer for 0.2 hours, how much electrical energy does she use?
- Electrical Energy = 1.8 kilowatts × 0.2 hours = 0.36 kilowatt-hours
Electrical energy use is typically expressed in kilowatt-hours, as in this example. How much energy is this? One kilowatt-hour equals 3.6 million joules of energy.
Q: Suppose Miranda were to use a 1.2-kilowatt 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 kilowatt-hours
- 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.
- Define electric power. What is the SI unit for power?
- A microwave is the only electric device in a 120-volt 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.