Physics
Magnetic Field, Ampère Force and Lorentz Force
Magnetic Field
A magnetic field arises around moving charges (current). The unit of magnetic induction is the tesla (T).
Ampère and Lorentz Forces
| Force | Formula | What it acts on |
|---|---|---|
| Ampère | F = B·I·l·sinα | a current-carrying conductor |
| Lorentz | F = q·v·B·sinα | a moving charge |
- The direction of the force is found using the left-hand rule. For the Lorentz force, the rule applies to a positive charge; for a negative charge the force is reversed.
- The Lorentz force does no work, because it is perpendicular to the velocity. The magnitude of the particle's velocity and its kinetic energy do not change.
Motion of a Particle
If a charged particle enters a uniform magnetic field perpendicular to it, it moves along a circle:
R = mv/(qB)
If the speed increases 3 times, the radius also increases 3 times.
Solenoid
The field inside a solenoid: B = μ₀·n·I, where n is the number of turns per unit length.
Example: if B = 0,2 T, I = 5 A, l = 0,3 m, α = 90°, then F = 0,2·5·0,3·1 = 0,3 N.