Required Practicals / Edexcel / Practical 13
13 A2 CP13

Force on a current-carrying conductor (CP13)

Investigate how the force on a current-carrying conductor in a magnetic field depends on current and length.

Apparatus

  • Current balance (stiff wire between magnadur magnets on an electronic balance)
  • Variable DC power supply and ammeter
  • Strong permanent magnets on a yoke
  • Ruler

Safety

  • Do not allow prolonged high currents; the wire heats up.
  • Ensure magnets are secured on the balance pan.

Method

  1. Place magnets on the balance and zero it with the circuit connected but no current.
  2. Set current I. Record the change in balance reading; convert to force $F = mg$.
  3. Vary I over at least six values. Plot F vs I at constant L.
  4. Repeat with wires of different lengths L inside the field. Plot F vs L at constant I.
  5. Calculate B from gradient: F vs I gives $B = \text{gradient}/L$.

Key Variables

Independent Current I (then length L)
Dependent Force F
Controlled B (same magnets); L when varying I; I when varying L

Analysis and Results

  • $F = BIL$. F vs I: gradient $= BL$. F vs L: gradient $= BI$.
  • Both gradients should give the same value of B.

Common Errors

  • Not zeroing the balance before passing current.
  • Measuring total wire length rather than the length L inside the magnetic field.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q1 3 marks

A wire of length 0.080 m in a $0.10$ T field carries a current of 4.5 A. Calculate the force and describe its direction relative to the field and current.

Q2 2 marks

The student plots F against I and obtains a straight line not passing through the origin. Suggest a reason.