The method and the results are on the magnetic force lab. Do the analysis yourself first, then check the answers below.
Quick answer: A moving charge or a current produces a magnetic field, and a magnetic field pushes sideways on other moving charges. These answers go with the AP Physics 2 lab on magnetic force: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
The sample readings give a straight line through the origin with gradient about 0.49 g/A. The answers below turn that gradient into a field strength and discuss how reliable it is.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 12
Question 1 Force from the readings
F = Δm g = (1.46 × 10⁻³ kg)(9.81) = 1.43 × 10⁻² N. The wire pushes up on the magnets (Newton’s third law), so the balance reads a different mass.
Question 2 Graph
The graph is a straight line through the origin, so Δm is proportional to I. This matches F = BIL, where B and L are constant.
Question 3 Slope of the line
Slope = (2.44 − 0.50) / (5.0 − 1.0) = 1.94 / 4.0 = 0.485 g/A, which is 4.85 × 10⁻⁴ kg/A.
Question 4 Magnetic field
F = Δm g = BIL gives B = (gradient × g) / L = (4.85 × 10⁻⁴)(9.81) / 0.040 = 0.12 T.
Question 5 Uncertainty
For L: 1/40 = 2.5 %. For Δm at 1.0 A: 0.01 / 0.50 = 2 %. At 1.0 A the two are similar. At larger currents the balance uncertainty shrinks as a percentage, so the length L becomes the main limit.
Question 6 Changing the angle
F = BIL sin θ, so the force is multiplied by sin 30° = 0.5. At 4.0 A the reading was 1.95 g, so it becomes about 0.98 g.
Question 7 Improving the method
(1) Use a larger range of currents and take readings on the way up and down to average out heating. (2) Measure the length of wire in the field more carefully with a longer pole gap, or use a Hall probe to compare the field value. B ≈ 0.12 T is the result to quote.
