Magnetic force on a charge
A moving charge in a magnetic field feels a force F = q v B sin theta, at right angles to both. It changes direction, not speed.
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Analogy
A hand always pushing sideways on a moving ball.
Example
F = q v B= 1.6e-19 x 1e6 x 0.01= 1.6e-15 N
In practice
An electron at 1e6 m/s across a 0.01 T field feels 1.6e-15 N.
Practice
Flashcards
- Does a magnetic force change a charge's speed?No. It is at right angles to the motion and only changes direction.
Interview questions
- Explain why a charged particle in a uniform magnetic field moves in a circle and derive the radius.
- Force is at right angles to velocity
- Magnetic force provides the centripetal force
- qvB = mv^2/r gives r = mv/qB
Questions people ask
Why does a magnetic force do no work?
The force always acts at right angles to the velocity, so it never pushes forward or back along the motion. Work equals force along the displacement, which is zero here. A magnetic field changes the direction of a charge but not its kinetic energy.
What is the right-hand rule for force?
For a positive charge, point your fingers along the velocity, curl them toward the field direction, and your thumb shows the force. For a negative charge such as an electron, the force points the opposite way. Check carefully, because many exam marks hinge on this sign.