How to Calculate the Power Delivered to Each Resistor

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Electrical power (or energy flow per second) measured in watts, sent through resistors is slowed by the resistor as it limits the amount of electricity that can flow through a circuit. (See References 1 and 2) The electrical power sent through a resistor is related to the voltage drop across the resistor as well as the current in the resistor (measured in amps). (See References 3) Resistors are either wired in series or parallel which changes how Ohm's law (voltage equals current times resistance), is applied. Resistors in series have equal currents while resistors in parallel have equal voltages. (See References 4)

Things You'll Need

  • Calculator

Resistors In Series

  • Add the values of the two resistors together to get the equivalent resistance in ohms. For example, say resistor one and resistor two are 5.0 and 10.0 ohms respectively, then the equivalent resistance is 15.0 ohms.

  • Divide the battery voltage in volts by the equivalent resistance to get the current flowing through both resistors in amps. Assuming a battery voltage of 12.0 volts, completing this step leads to 12.0 volts divided by 15.0 ohms equaling 0.80 amps.

  • Multiply each resistor's current by its resistance to obtain the voltage drop across each. Now you have for resistor one 0.80 amps times 5.0 ohms which equals 4.0 volts. Resistor two has a voltage of 0.80 amps times 10.0 ohms which equals 8.0 volts.

  • Multiply the current and voltage for each resistor together to arrive at power. Completing the exercise you have 0.80 amps times 4.0 volts or 3.2 watts for resistor one, and 0.80 amps times 8.0 volts or 6.4 watts for resistor two.

Resistors In Parallel

  • Divide the battery voltage by the resistance of the first resistor to obtain current flow in amps through resistor one. Using a battery voltage of 12.0 volts and a resistance of 5.0 ohms leads to a current of 2.4 amps.

  • Divide the battery voltage by the resistance of the second resistor to obtain current flow in amps through resistor two. Using a battery voltage of 12.0 volts and a resistance of 10.0 ohms leads to a current of 1.2 amps.

  • Multiply the current through a respective resistor by the battery voltage to get the power delivered to that resistor. The power through resistor one is 2.4 amps times 12.0 volts or 28.8 watts. The second resistor receives a power of 1.2 amps times 12 volts or 14.4 watts.

References

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