Required Practicals / AQA / Practical 10
10 A2 3.7.5.2

Force on a current-carrying conductor

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

Apparatus

  • Stiff copper wire or conducting rod on a support
  • Two magnadur magnets mounted on a steel yoke (uniform magnetic field between poles)
  • Electronic balance (milligram resolution)
  • Variable DC power supply and ammeter
  • Ruler and connecting leads

Safety

  • Do not allow high currents to flow for extended periods; the wire heats up and could burn or melt insulation.
  • Ensure the magnet yoke is stable on the balance pan.

Method

  1. Place the magnets on the balance pan so the wire sits horizontally between the poles in the uniform field. Zero the balance with the circuit connected but no current flowing.
  2. Pass a current I through the wire and record the change in balance reading (in grams); convert to force: $F = m \times g$.
  3. Vary I in steps from 0.5 A to 4 A. Record F for each current. Plot F vs I (at constant L).
  4. Repeat with wires of different lengths L between the poles. Plot F vs L at constant I.
  5. Determine B from the gradient of each graph: F vs I gives gradient $= BL$; F vs L gives gradient $= BI$.

Key Variables

Independent Current I (then length L)
Dependent Force F
Controlled Magnetic flux density B (same magnets, same separation); L when varying I; I when varying L

Analysis and Results

  • $F = BIL$: force is proportional to both I and L.
  • F vs I (constant L): gradient $= BL$, so $B = \text{gradient}/L$.
  • F vs L (constant I): gradient $= BI$, so $B = \text{gradient}/I$.
  • Compare B values obtained from both graphs as a consistency check.

Common Errors

  • Not zeroing the balance before passing current.
  • Measuring the total wire length rather than only the length L within the magnetic field.
  • Reversing the current direction unexpectedly, causing the force to act downwards instead of upwards.
  • Moving the magnets between measurements, changing B.

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 0.12 m carries a current of 3.5 A at right angles to a uniform magnetic field of flux density 0.085 T. Calculate the force on the wire.

Q2 3 marks

A student plots F against I and obtains a straight line through the origin with gradient 0.018 N A$^{-1}$. The length of wire in the field is 0.15 m. Determine B.

Q3 2 marks

Explain how the student should orient the wire to ensure $F = BIL$ applies exactly.