D.3 · IB Physics
Magnetic force on a current-carrying wire
Place a wire in a uniform magnetic field and see how the force depends on the current, the field strength, the length and the angle between wire and field. Reverse the current or the field to flip the force.
Predict first
Start from the default settings (θ = 90°, so the wire is perpendicular to the field). Reduce the angle θ to 30° and leave I, B and L unchanged. What happens to the force F?
F = BIL sinθ. Since sin 30° = 0.5 and sin 90° = 1, the force is multiplied by 0.5: it falls from 0.0625 N to 0.0313 N.
Check it in the model: Move the Angle θ between wire and field slider from 90° to 30° and compare the Force F readout.
current in wire magnetic field force (⊙ out of page, ⊗ into page)
B⊥ = B sinθ
direction: F = I L × B (left-hand rule: first finger field, second finger current, thumb force)
Try this
- Start at θ = 90° and lower the angle to 30°. Read F, then confirm it matches B I L sin 30°.
- Use the Parallel to field preset, then raise the current to its maximum. Why does the force stay at zero?
- Press Reverse current, then Reverse field. Which presses flip the direction ⊙ / ⊗, and what happens if you press both?
Common error: the angle in F = BIL sinθ is measured between the wire and the field lines, so a wire parallel to the field feels no force and the maximum force is at 90°.