Electromagnetism
Charge in B field
Slide test-charge velocity — magnetic force F = qv×B appears in the readouts.
Charge in B field — interactive Electromagnetism simulation. Slide test-charge velocity — magnetic force F = qv×B appears in the readouts. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Magnetic force
Slide test-charge velocity — magnetic force F = qv×B appears in the readouts.
A test charge q moving with velocity v in uniform B feels magnetic force F_B = q(v×B), perpendicular to both v and B. The preset uses a uniform B-region (not a current wire) so only the Lorentz force applies. Yellow F_E and purple F_B arrows split electric and magnetic parts; combined F = F_E + F_B. Press Play to see motion along v.
- F = qv×B
- F ⊥ v and B
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
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?
Predictions to weigh
- 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.
Variable roles
What you set:
- Velocity x-component v_x (m/s)
- Velocity y-component v_y (m/s)
What you measure:
- Force magnitude F (µN)
How the investigation runs
- 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.
- Enable the force-magnitude readout on the test charge.
- Set the test charge's velocity components for each trial and record the force magnitude.
Governing equation
Lorentz Force (Magnetic) — F_B = q·v×B
A charge q moving with velocity v through a magnetic field B experiences a force F = qv×B, perpendicular to both its velocity and the field. Its magnitude is |F| = |q||v||B| when v is perpendicular to B.
What the printable worksheet asks students to work out
- 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.
- 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.
Where this shows up beyond the lab
- 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.
- 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.
- AP Physics 2 — Unit 12: Magnetism and Electromagnetism
- AP Physics C: Electricity and Magnetism — Unit 12: Magnetic Fields and Electromagnetism
- IB Physics — D.3 Motion in electromagnetic fields
- General High School Physics — Magnetism & electromagnetism
- NGSS High School Physics — Electric current and magnetic fields
- Welcome to Qv Cross B
- Select the test charge
- Press Play
- Force on a moving charge
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.