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
- Place magnets on the balance and zero it with the circuit connected but no current.
- Set current I. Record the change in balance reading; convert to force $F = mg$.
- Vary I over at least 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 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.
Q13 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.
Q22 marks
The student plots F against I and obtains a straight line not passing through the origin. Suggest a reason.