Power: Students should be able to explain how the power transfer in any circuit device is related to the potential difference across it and the current through it, and to the energy changes over time:

The idea

Power is the rate of energy transfer. In a circuit device, power P in watts equals potential difference V across the device multiplied by current I through it (P = V I). Since V is energy transferred per unit charge and I is charge flow per second, their product gives energy per second. For a resistor, combining P = V I with V = I R gives P = I² R, showing power increases with the square of the current. Energy transferred over time is E = P t; a 60 W lamp on for 120 s transfers E = 60 × 120 = 7200 J. In a resistor, energy is transferred to the thermal store of the surroundings; in a motor, it is transferred mechanically to a kinetic store; in a lamp, it is transferred by radiation (light). These equations apply to any circuit device, whether energy is transferred usefully or by heating.

What earns the marks

• Power is the rate of energy transfer, measured in watts, where 1 W = 1 J/s.
• For any circuit device, power transfer is given by P = V I, where V is the potential difference across the device and I is the current through it.
• Because V is energy per unit charge and I is charge per second, the product V I gives energy transferred per second.
• For a resistor, P = I² R follows from P = V I and V = I R, showing that power depends on the square of the current.
• Energy transferred over a time interval is E = P t, so doubling the time doubles the energy transferred for a constant power.
• Energy may be transferred to thermal or kinetic stores, or transferred by pathways such as radiation (light), depending on the device.

Common mistakes and how to put them right

• Using P = V I for a device but forgetting that V must be the potential difference across that device, not the supply potential difference.

  Put it right by identifying the device and using the potential difference across it.

• Confusing power and energy.

  Put it right by stating that power is the rate of energy transfer in watts, while energy transferred is power multiplied by time in joules.

• Assuming all electrical energy is transferred usefully.

  Put it right by stating that some energy is often transferred by heating to the surroundings, so the useful energy output is less than the total energy input.

Exam tips

• Write the equation you are using, substitute values with units, and give the unit with your answer.
• When a question asks about energy changes over time, calculate E = P t and state the store or stores that gain energy, or the pathway used.