You probably use at least 25 different electrical devices before eating lunch every day. Each converts electrical power into other forms of energy, such as heat, light, and motion. On page 519, we used the equation for electrical power (P = IV) as a practical definition of the volt: One volt means one watt per ampere. A current of one ampere flowing across a potential difference of one volt dissipates one watt of power.
How much current do appliances use?
Electrical devices are designed to draw the correct amount of current to transfer the power they need to operate when connected to the proper voltage. The power equation can be used to calculate the current drawn by a given appliance from its power requirement and the operating voltage. For most household appliances the operating voltage is 120 V. The current can therefore be found by dividing the power by the voltage.
What kind of circuits are in houses?
Household outlets are typically wired in parallel circuits. Each outlet provides the full voltage from the power line regardless of what other appliances are connected because each outlet is on its own branch of the circuit. This means that some outlets can have appliances plugged in and running while others do not.
Lights and light switches
Lights are typically also wired in parallel. That way you can turn off lights in one room while leaving the lights in another room unaffected. A light switch, however, is placed in series with the lights on that branch (i.e., within a room) because you want the switch to turn off all the lights connected to it. The light switch changes the circuit for that room from open to closed.
Energy consumption measured in kWh
Electric power companies use the kilowatt-hour (kWh) to measure and collect payment for the total electrical energy you use. One kWh is 1,000 W (1 kW) used for 1 hr, or 3,600 s. A kilowatt-hour is a measure of energy, not power. Why? Remember that P = E/t, so E = Pt. A 100 W bulb, used for 10 hr continuously, will use 100 W × 10 hr = 1.0 kWh of energy.
What happens if you run a hair dryer, toaster oven, and microwave from the same parallel circuit? Each appliance draws about 10 A of current. The 12-gauge wire used to connect kitchen outlets can safely carry only 20 A before overheating. To protect against fire the circuit breaker would trip and disconnect the entire parallel circuit. When a circuit breaker trips you must turn off and disconnect some of the appliances overloading the circuit before resetting the circuit breaker.
Some electrical devices contain fuses to protect against too much current. A fuse is similar to a circuit breaker, except that the electrical wire inside the fuse permanently breaks when too much current is drawn. A fuse has to be replaced after it has blown.
Test your knowledge
How much current is drawn by a 150 W television when it is plugged into a household outlet?
The answer is a, 1.25 A. As described on this page, the current is given by I = P/V = (150 W)/(120 V) = 1.25 A.