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Use the steps below to calculate energy flow in an electrical circuit. Choose a resistor. Connect the resistor to a current source. Run a current through the circuit for a set period of time. Measure the strength of the current. Use the formula P = I2 x R. Multiply the power by the amount of time elapsed.
The steps below are written as a practical example, but you can use the method to understand written physics problems as well. First, we'll calculate the power P using the formula P = I2 x R, where I is the current in amperes (amps) and R is the resistance in ohms. These units give us the power in watts, so from there, we' can use the formula in the previous step to calculate the energy in joules. Resistors are rated in ohms, with the rating either labeled directly or indicated with a series of colored bands. You can also test a resistor's resistance by connecting it to an ohmmeter or multimeter. For this example, we'll assume the resistor is rated at 10 ohms. Either connect wires to the resistor with Fahnestock or alligator clips, or plug the resistor into a testing board. For this example, we'll use a period of 10 seconds. Do this with an ammeter or a multimeter. Most household current is in milliamperes, or thousandths of an ampere, so we'll assume the current is 100 milliamperes, or 0.1 ampere. To find the power, multiply the square of the current by the resistance. This yields the power output in watts. Squaring 0.1 gives 0.01, multiplied by 10, gives a power output of 0.1 watt, or 100 milliwatts. This gives the energy output in joules. 0.1 watt x 10 seconds equals 1 joule of electrical energy. As joules are small units, and because appliances commonly use watts, milliwatts, and kilowatts to indicate how much power they use, utilities commonly measure their energy output in kilowatt-hours. One watt equals 1 joule per second, or 1 joule equals 1 watt-second; a kilowatt equals 1 kilojoules per second and a kilojoule equals 1 kilowatt-second. As there are 3,600 seconds in an hour, 1 kilowatt-hour equals 3,600 kilowatt-seconds, 3,600 kilojoules, or 3,600,000 joules.