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The potentiometer and null methods Cambridge International AS & A Level Physics revision

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In plain words

A voltmeter always steals a little current from the circuit it is measuring, which changes the very thing you wanted to know. A potentiometer gets round that by balancing the unknown voltage against a known one until no current flows at all. Nothing is taken, so nothing is disturbed.

4 things to know

  1. A potentiometer is a uniform resistance wire with a steady current through it from a driver cell. The p.d. across any length of the wire is proportional to that length.
  2. The cell being tested is connected through a galvanometer to a sliding contact on the wire. At the balance point the galvanometer reads zero, and the p.d. across that length of wire equals the cell's e.m.f.
  3. To compare two e.m.f.s: ε₁ ÷ ε₂ = L₁ ÷ L₂, where L₁ and L₂ are their balance lengths.
  4. This is a null method: you adjust until a meter reads zero. The galvanometer only has to be sensitive, not accurately marked, and no current is taken from the cell being tested.

Worked example

A standard cell of e.m.f. 1.02 V balances at 34.0 cm on a potentiometer wire. A second cell balances at 50.0 cm. Find the e.m.f. of the second cell.

  1. E.m.f. is proportional to balance length.
  2. ε = 1.02 × 50.0 ÷ 34.0.
  3. = 1.50 V.

Tips and tricks

  • The driver cell must have a bigger e.m.f. than the cell being tested, or there is no balance point anywhere on the wire.
  • The cells must be connected the same way round, positive to the same end of the wire.
5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

The potentiometer and null methods: 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 the galvanometer read at the balance point?
    • zero (the answer)
    • its maximum
    • the e.m.f. of the cell
    • the current from the driver cell

    No current flows through it.

  2. What is the p.d. across a length of uniform, current-carrying wire proportional to?
    • that length (the answer)
    • the square of that length
    • the diameter of the wire
    • one over that length

    Each centimetre of the wire has the same resistance.

  3. Cell X balances at 40 cm and cell Y balances at 60 cm. What is the ratio of their e.m.f.s, X to Y?
    • 2 : 3 (the answer)
    • 3 : 2
    • 4 : 9
    • 1 : 1

    E.m.f. is proportional to balance length.

  4. Why doesn't the galvanometer in a null method need an accurate scale?
    • It only has to show when the current is zero. (the answer)
    • It is never used.
    • It measures voltage, not current.
    • It is always switched off.

    It needs to be sensitive, not accurately marked.

  5. A cell is swapped for one with a larger e.m.f. What happens to the balance length?
    • It increases. (the answer)
    • It decreases.
    • It stays the same.
    • It becomes zero.

    A longer piece of wire is needed to match the larger e.m.f.

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