Quick answer: A moving charge or a current produces a magnetic field, and a magnetic field pushes sideways on other moving charges. This AP Physics 2 lab on magnetic force gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How big is the magnetic force on a wire carrying current, and can a kitchen balance measure it? By using the reaction of the magnets, you can turn a tiny force into a mass reading and determine the field strength between two poles.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 12
Aim
Investigate how the magnetic force on a current-carrying wire depends on the current, and use the results to find the field of a pair of magnets.
Equipment
- pair of bar or disc magnets on a U-shaped iron yoke, with a gap of about 4 cm
- top-pan balance reading to 0.01 g
- rigid wire, clamped on a stand so that it passes through the gap without touching the magnets
- variable d.c. supply (0–6 V) with a rheostat and an ammeter (0–6 A)
- ruler
Safety
Currents of several amperes warm the wire. Switch on only while reading the balance and never hold the wire. Keep magnets away from phones and bank cards.
Method
- Place the magnet-and-yoke assembly on the balance and tare the reading to zero.
- Clamp the wire so that a length L of 4.0 cm lies inside the field, with no part touching the magnets. Measure L with the ruler.
- Choose the current direction so that the force on the wire is upward (the magnets are pushed down by the reaction).
- Set I = 1.0 A, switch on, read the balance change Δm, then switch off.
- Repeat for I = 2.0, 3.0, 4.0 and 5.0 A.
Results
Sample data, with L = 4.0 cm:
| Current I (A) | Change in balance reading Δm (g) |
|---|---|
| 1.0 | 0.50 |
| 2.0 | 0.98 |
| 3.0 | 1.46 |
| 4.0 | 1.95 |
| 5.0 | 2.44 |
Analysis questions
Question 1 Force from the readings
Convert Δm = 1.46 g (I = 3.0 A) into the force on the wire. Use g = 9.81 N/kg and state why the balance reading changes.
Question 2 Graph
Plot Δm (y-axis) against I (x-axis). Describe the graph and explain the shape using F = BIL.
Question 3 Slope of the line
Work out the slope of your best-fit line from its two end points, and state the unit of the slope.
Question 4 Magnetic field
Use the gradient, g and L to calculate the field B between the poles.
Question 5 Uncertainty
The length L is 4.0 cm with an uncertainty of ±1 mm, and Δm has an uncertainty of ±0.01 g. Estimate the percentage uncertainty in L and in Δm for I = 1.0 A. State which limits the experiment more.
Question 6 Changing the angle
The wire is turned in the horizontal plane until it makes 30° with the field, keeping the same length in the field. Predict the new reading for I = 4.0 A and explain.
Question 7 Improving the method
Suggest two improvements to make the value of B more reliable.
