Required Practicals / OCR A / Practical 7
7 A2 PAG 7

Force on a current-carrying conductor (PAG 7)

Investigate how the force on a wire in a magnetic field varies with current and length.

Apparatus

  • Stiff wire on a current balance above an electronic balance
  • Magnadur magnets on a steel yoke
  • Variable DC power supply and ammeter
  • Ruler

Safety

  • Avoid prolonged high currents to prevent overheating.
  • Secure the magnet assembly.

Method

  1. Zero the balance with magnets in place and circuit connected (no current).
  2. Pass current I; record change in balance reading and convert to force F.
  3. Vary I (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 gradients.

Key Variables

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

Analysis and Results

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

Common Errors

  • Not zeroing balance before measurement.
  • Measuring full wire length rather than length within the field.

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

Q1 3 marks

A 0.10 m wire carrying 2.0 A sits in a 0.075 T field at 90 degrees. Calculate the force.

Q2 2 marks

The wire is rotated until it is parallel to the field. State and explain the force on it.