Magnetic Flux Density and the Force on a Current-Carrying Wire
The strength of a magnetic field is defined by the force it produces: F = BIL when the wire is at right angles to the field, and BIL sin theta otherwise. One tesla is one newton per amp per metre.
What a learner can do afterwards
- Uses the left hand rule to give the direction of the force on a current-carrying wire
- Calculates the force for a wire set at an angle to the field and says when it becomes zero
- Defines the tesla from the defining equation rather than quoting it from memory
1 · Read
A wire carrying current in a magnetic field feels a force of B I L sin theta, where theta sits between the wire and the field. The force is largest at ninety degrees and zero when the wire runs parallel.
A 5 cm wire carrying 20 A across a 1.5 T field feels 1.5 N at right angles. This push is strong enough to move the wire, and motors are built from loops that exploit it.
The direction comes from the left hand rule: field, current, then thumb for force. Use conventional current, since electron flow points the other way and would flip the answer.
Read the tesla from the defining equation: one tesla is one newton per amp per metre. Always check the angle first, since sin zero is zero.
Size comes from B I L sin theta, and direction from the left hand rule.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.