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
- Zero the balance with magnets in place and circuit connected (no current).
- Pass current I; record change in balance reading and convert to force F.
- Vary I (six values); plot F vs I at constant L.
- Repeat with wires of different lengths L inside the field; plot F vs L at constant I.
- 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.
Q13 marks
A 0.10 m wire carrying 2.0 A sits in a 0.075 T field at 90 degrees. Calculate the force.
Q22 marks
The wire is rotated until it is parallel to the field. State and explain the force on it.