A charge moving across a magnetic field feels a sideways force, F = qvB, that curves it into a circle. Drag the sliders below to change the charge, speed, field strength and mass, and watch the force and the circle's radius respond in real time.
Some forces only steer. A magnetic force pushes a moving charge strictly to the side of wherever it is heading, so it can never add energy to the motion or drain any away — it can only bend the path. This sim makes that visible: fire a charged particle across a uniform field pointing into the page (drawn as ×) and watch it wheel around in a closed circle instead of flying straight.
Set the Charge, the Speed v, the Magnetic field strength B, and the Mass (in electron masses), then pick a particle. The force readout follows F = q·v·B: raise the charge, the speed, or the field and the sideways push grows. Because that push always aims perpendicular to the velocity, it does no work — the speed you set stays fixed while only the direction keeps rotating. Drop v to zero and the force vanishes entirely; a still charge feels nothing.
The radius readout r = m·v/(q·B) is where the shape lives. A heavier or faster particle sweeps a wider circle; a bigger charge or stronger field pulls a tighter one. That is the magnetic force serving as the centripetal force, since q·v·B = m·v²/r rearranges straight into that radius — the same physics that curves ions in a mass spectrometer. Put numbers to it with the magnetic field calculator, or bend more particles across our full set of physics simulators.
It is F = qvB when the velocity is perpendicular to the field, where q is the charge, v the speed and B the field strength. The force is always perpendicular to the motion.
Because the force is always perpendicular to the velocity, it does no work — it can only change the direction of motion, not the speed. So the particle circles at a constant speed.
The radius is r = mv/(qB). A heavier or faster particle traces a wider circle; a larger charge or a stronger field makes a tighter one.
Nothing — the magnetic force F = qvB is zero when v = 0. A charge has to be moving (and not straight along the field) to feel any magnetic force.