Explain how the power transfer in any circuit device is related to the…

The idea

The power transferred by a circuit device depends on both the potential difference across it and the current through it. Each coulomb of charge gains or loses energy equal to the potential difference in volts, and current measures how many coulombs pass each second. Multiplying the energy per coulomb (V) by the coulombs per second (I) gives the energy transferred each second, which is power: P = V × I. A device with 12 V across it and 2 A through it transfers 12 × 2 = 24 W. Increasing either the potential difference or the current increases the power. Combining with P = E ÷ t and V = E ÷ Q gives the same result, so the two equations are consistent.

What earns the marks

• Explains that potential difference is the energy transferred per unit charge passing through the device.
• Explains that current is the rate of flow of charge, so it measures charge passing per second.
• States that power equals potential difference multiplied by current, P = V × I.
• Explains that increasing the potential difference or the current increases the power transferred.
• Calculates the power transferred by a device from its potential difference and current.

Common mistakes and how to put them right

• Adds potential difference and current instead of multiplying them.

  Put it right: Power is the product V × I.

• Treats current as energy rather than rate of charge flow.

  Put it right: Current is charge per second.

• Assumes power depends only on potential difference.

  Put it right: Both V and I determine the power.

• Confuses potential difference with power.

  Put it right: Potential difference is energy per unit charge, not energy per second.

Exam tips

• Link each quantity to its definition: V is energy per coulomb, I is coulombs per second, so V × I is joules per second.
• Show the substitution P = V × I with units before giving the answer.
• Use a worked example such as 12 V and 2 A to check the reasoning about increasing either quantity.