The Lorentz Force: A Moving Charge in a Magnetic Field

1 · Predict

A charged particle moves through a uniform magnetic field region. Does the magnetic force on it depend on the particle's speed, or only on the field strength?

• The force depends only on the field strength, not speed.
• The magnetic force doesn't depend on either quantity in a simple way.
• The force depends on both the field strength and the particle's speed — faster particles feel a stronger magnetic force.

2 · Set Up

1. Open the qv-cross-b preset and press Reset. The field region has a fixed Bz = 0.12 T; the test charge is q = 3×10⁻¹⁰ C.
2. Enable the force-magnitude readout on the test charge.
3. Set the test charge's velocity components for each trial and record the force magnitude.

3 · Collect Data

Plot force magnitude F (y-axis) against speed |v| = √(v_x² + v_y²) (x-axis) for your three trials.

4 · Analyze

1. For one trial, compute |F| = |q|·|B|·|v| using q = 3×10⁻¹⁰ C, B = 0.12 T, |v| = √(v_x² + v_y²). Compare to the table.
2. Explain why the magnetic force magnitude depends on the particle's total speed |v|, not on the direction of its velocity — even though the force's direction does depend on which way it's moving.

5 · Extend

1. The magnetic force is always perpendicular to the velocity (F = qv×B), so it can change a particle's direction but never its speed. Explain why a magnetic field alone can never speed up or slow down a charged particle, only steer it.
2. A charged particle moving perpendicular to a uniform magnetic field travels in a circle, because the magnetic force always points toward the circle's center. Explain, using F = qvB, why a faster particle in the same field traces a bigger circle.

The Physics Behind This Experiment

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