Resistance is one of the most basic concepts that physics students learn about electricity. If you picture electricity as a group of electrons flowing through a wire to create current, then resistance is a measure of a material’s inherent barriers to electron flow. Each material has a different resistance to electrical flow; some things, like copper wire, allow electrons to move freely while others, like rubber, have huge barriers that barely allow electrons to move.
Calculating Resistance from Current and Voltage
Write down all of the information that the problem gives you. Most simple physics problems that ask you to calculate resistance give you values for current and the voltage in the problem.
Convert all of the units in the problem to volts and amperes. Tricky physics teachers may give you the voltage in kilovolts (kV) or the current in milliamperes (mA). This method of calculating resistance will not work unless you convert all of the factors to their proper units.
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Divide the voltage by the current to get your resistance. This formula, known as Ohm’s Law, is a fundamental law of electronics and states that the voltage is equal to the resistance multiplied by the current. For example, a 120 volt circuit that generates 10 amperes of current would have a resistance of 12 ohms.
Calculating Resistance from Power and Current
Write down the information that the problem gives you; in this case, the problem will probably give you the circuit’s power and current. Many physics teachers use problems requiring you to calculate resistance from power and current as a slightly more difficult way to test your knowledge of electrical circuits.
Convert all of your factors to the correct units. In this case, your power should be in watts (not in kilowatts or kilowatt-hours) and your current should be in amperes. If the problem gives you the power in units of kilowatt-hours, you will have to do a significantly more complicated conversion.
Square the current. For a circuit with a current of 10 amperes, you should get 100 amperes squared.
Divide the power by the square of the current to get the final resistance. For a 120 watt circuit with a squared current of 100, you should get a resistance of 1.2 ohms.