Potential Difference and Power

Overview

Potential difference and power describe energy transfers in circuits. This lesson links voltage to energy per unit charge, then uses power equations to compare how quickly electrical energy is transferred by different components.

What You Need to Know

• Use potential difference to track the energy transferred by charges passing through a component.
• Use V = W / Q, where V is potential difference, W is work done or energy transferred, and Q is charge.
• One volt means one joule of energy transferred per coulomb of charge.
• Electrical power is the rate of energy transfer.
• Use P = VI when current and potential difference are known.
• Combine P = VI with V = IR to use P = I^2R or P = V^2 / R when resistance is included.

How to Work Through It

1. Begin with energy transfer in a simple circuit: the source gives energy to each coulomb of charge, and components transfer that energy to the surroundings.
2. Practise using V = W / Q in both directions, checking that joules, coulombs, and volts match the question.
3. Use P = VI for direct power questions, then choose P = I^2R or P = V^2 / R when resistance data makes those forms quicker.
4. Compare answers by thinking about whether a larger p.d., current, or resistance should increase the rate of energy transfer.

Check Your Understanding

• What does a potential difference of 6.0 V tell you about each coulomb of charge?
• How much energy is transferred when 20 C passes through a component with a p.d. of 12 V?
• Which power equation is most efficient when a question gives current and resistance?
• Why is potential difference not the same quantity as current?

Common Mistakes

• Saying voltage is the amount of charge rather than energy transferred per unit charge.
• Using Q = It when the question is really asking about energy and potential difference.
• Forgetting that P = I^2R squares the current.
• Choosing P = V^2 / R and then accidentally multiplying by resistance instead of dividing.

Next Steps

• Practise selecting the most useful energy or power equation from the quantities given.
• Use the voltage, current, and power ideas in the next lesson on resistance and resistivity.