Force on a Moving ChargeSt Peter's Science Portal · F = BqV
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Electromagnetism

Charge in a Field · F = BqV

Drag the charged particle through the magnetic field. A force F = Bqv acts on it, perpendicular to both its velocity and the field. The green arrow from the particle's centre shows the force — its length grows with the field strength, the charge, and how fast you drag.

Controls

Sign sets direction: positive = into screen (⊗), negative = out (⊙).
Multiples of e. Negative flips the force direction.
Trail: on
Field
⊗ in
Charge sign
+

Practice question

Press “New question” to begin.
×10^
Enter your answer in scientific notation — e.g. for 0.00202 type 2.02 ×10^ -3.
+ grab the charge, then flick to launch it SPEED 0.00 m/s FORCE 0.00 N
Velocity v Force F = Bqv ⊗ into screen   ⊙ out of screen
Flick the charge and let go — the perpendicular force curves it into a circle. Click the charge again to grab it and re-launch. Press space to pause.
The physics behind it

A charge q moving with velocity v through a magnetic field B feels a force F = Bqv (when v is perpendicular to B). Its direction is given by F = qv × B: always perpendicular to the velocity. Because the force is always sideways to the motion, a free charged particle travels in a circle — the basis of mass spectrometers, cyclotrons and the aurora. Here the field points into or out of the screen and you drag the charge in the screen plane, so the force lies in the plane, at right angles to your drag. Reverse the field or flip the charge sign and the force flips too.