Quick answer: A magnetic field exerts a force on moving charges and on current-carrying wires. This Physics 11-12 lab on magnetic fields gives the aim, the equipment, the safety points, a step-by-step method, sample results and 6 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How strong is the field between two magnet poles? You will not need a magnetic probe: the force on a wire carrying a known current pushes back on the magnet, and a top-pan balance turns that push into a mass reading you can analyse.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB D.3
Aim
To measure the flux density between two magnet poles by weighing the reaction force of a current-carrying wire on the magnet.
Equipment
- top-pan balance reading to 0.01 g
- pair of ceramic magnets on an iron yoke, with a 4.0 cm length of gap
- stiff copper wire held in a clamp stand, not touching the magnet
- variable low-voltage dc supply (0–3 A), ammeter, switch and leads
- metre rule
Safety
Keep the current below 3 A and switch off between readings because the wire warms up. Keep phones, bank cards and pacemakers away from the magnets. Do not touch the wire when it is hot.
Method
- Place the magnet assembly on the balance and fix the wire in the clamp so that it passes horizontally through the gap without touching.
- Switch the circuit off and press tare so the balance reads 0.00 g.
- Set the current to 0.5 A, switch on, and record the new balance reading at once.
- Switch off. Repeat for 1.0, 1.5, 2.0 and 2.5 A, taring each time.
- Reverse the current once and note the sign of the reading.
- Measure the length of wire inside the field with the rule.
Results
The wire length in the field is L = 0.040 m. Apparent mass change Δm:
| Current I / A | Δm / g | Δm / kg | F = Δm g / N |
|---|---|---|---|
| 0.5 | 0.17 | 1.7 × 10⁻⁴ | 1.7 × 10⁻³ |
| 1.0 | 0.36 | 3.6 × 10⁻⁴ | 3.5 × 10⁻³ |
| 1.5 | 0.52 | 5.2 × 10⁻⁴ | 5.1 × 10⁻³ |
| 2.0 | 0.70 | 7.0 × 10⁻⁴ | 6.9 × 10⁻³ |
| 2.5 | 0.86 | 8.6 × 10⁻⁴ | 8.4 × 10⁻³ |
Analysis questions
Question 1 Plotting
Using your table, draw a force–current graph with F on the y-axis and a straight best-fit line through the origin.Question 2 Gradient
Find the gradient of your line in N/A.Question 3 Flux density
Use the gradient to calculate B. State the formula you use.Question 4 Direction
Explain why the balance reading changed sign when the current was reversed.Question 5 Uncertainty
The balance reads to 0.01 g. Calculate the percentage uncertainty in the smallest Δm and comment.Question 6 Improvement
Suggest two improvements that would make B more accurate.
