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Potential difference and power Cambridge International AS & A Level Physics revision

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In plain words

Potential difference tells you how much energy each coulomb of charge hands over as it passes through a component. Multiply that by how many coulombs pass each second, and you have the power.

Three things to know

  1. Potential difference V = W ÷ Q: the energy transferred per unit charge. One volt is one joule per coulomb.
  2. Power P = VI. Putting in V = IR gives two more forms: P = I²R and P = V² ÷ R.
  3. Energy transferred W = VIt.

Worked example

A 12 V supply drives a current of 3.0 A through a heater for 5.0 minutes. Find the power and the energy transferred.

  1. P = VI = 12 × 3.0 = 36 W.
  2. Time = 5.0 × 60 = 300 s.
  3. W = Pt = 36 × 300 = 10 800 J, about 1.1 × 10⁴ J.

Tips and tricks

  • Pick the power formula that uses the two things you know: V and I, I and R, or V and R.
  • Time must be in seconds for the energy to come out in joules.
5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Potential difference and power: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. What does potential difference measure?
    • the charge passing each second
    • the energy transferred per unit charge (the answer)
    • the resistance per metre
    • the number of electrons

    One volt is one joule per coulomb.

  2. A 6.0 V battery delivers a current of 2.0 A. What power does it supply?
    • 3.0 W
    • 8.0 W
    • 12 W (the answer)
    • 72 W

    P = VI.

  3. A 10 Ω resistor has a p.d. of 20 V across it. What power does it dissipate?
    • 2.0 W
    • 20 W
    • 40 W (the answer)
    • 200 W

    P = V² ÷ R.

  4. Which of these is the same as one volt?
    • one joule per coulomb (the answer)
    • one coulomb per second
    • one joule per second
    • one ampere per ohm

    V = W ÷ Q.

  5. A lamp transfers 600 J when 50 C of charge passes through it. What is the p.d. across it?
    • 0.083 V
    • 12 V (the answer)
    • 550 V
    • 30 000 V

    600 ÷ 50.

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