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🔋college physics i – introduction review

$P = I^2R$

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

$P = I^2R$ is a fundamental equation in electrical engineering that describes the relationship between power (P), current (I), and resistance (R). It states that the power dissipated in a resistor is proportional to the square of the current flowing through it and the resistance of the circuit.

5 Must Know Facts For Your Next Test

  1. The $P = I^2R$ equation is derived from Ohm's law and Joule's first law, and it describes the power dissipated as heat in a resistor.
  2. In an electric circuit, the power dissipated in a resistor is equal to the product of the square of the current and the resistance of the resistor.
  3. The $P = I^2R$ equation is important in the analysis of electric circuits, as it allows for the calculation of power consumption and the design of efficient electrical systems.
  4. The back EMF in an electric motor or generator is proportional to the speed of the motor or generator, and it opposes the flow of current, reducing the overall power consumption of the device.
  5. The $P = I^2R$ equation is a fundamental principle in the design and analysis of electrical circuits, and it is essential for understanding the behavior of various electrical components and systems.

Review Questions

  • Explain how the $P = I^2R$ equation is derived from Ohm's law and Joule's first law.
    • The $P = I^2R$ equation is derived from the combination of Ohm's law and Joule's first law. Ohm's law states that the current through a conductor is directly proportional to the voltage applied across it and inversely proportional to the resistance of the circuit, expressed as $V = IR$. Joule's first law states that the amount of heat generated in a conductor is proportional to the product of the square of the current and the resistance of the conductor, expressed as $P = I^2R$. By substituting $V = IR$ into the Joule's first law equation, we arrive at the $P = I^2R$ equation, which describes the power dissipated as heat in a resistor.
  • Describe the relationship between back EMF and the $P = I^2R$ equation in the context of electric motors and generators.
    • In electric motors and generators, the back EMF is proportional to the speed of the motor or generator and opposes the flow of current. This back EMF reduces the overall power consumption of the device, as described by the $P = I^2R$ equation. Specifically, as the speed of the motor or generator increases, the back EMF also increases, which in turn reduces the current flowing through the device. Since power is proportional to the square of the current, the reduction in current due to the back EMF leads to a decrease in the overall power dissipated in the device, as described by the $P = I^2R$ equation.
  • Analyze how the $P = I^2R$ equation can be used to design efficient electrical systems and minimize power consumption.
    • The $P = I^2R$ equation can be used to design efficient electrical systems and minimize power consumption in several ways. First, by understanding that power is proportional to the square of the current, designers can optimize the circuit design to minimize the current required to deliver a given amount of power, thereby reducing power losses and improving efficiency. Second, the equation can be used to select appropriate resistor values to ensure that power dissipation is within acceptable limits, preventing overheating and energy waste. Finally, the $P = I^2R$ equation is crucial in the design of electric motors and generators, where the back EMF can be leveraged to reduce the overall power consumption of the device, as discussed in the previous question. By applying the principles of the $P = I^2R$ equation, electrical engineers can create more efficient and energy-conscious systems.