Force on a current-carrying conductor
Key ideas
A wire carrying a current in a magnetic field feels a force — this is the motor effect, the principle behind every electric motor.
The force relationship
- — force on the conductor (N)
- — magnetic field strength (T, tesla)
- — current (A)
- — length of conductor in the field (m)
This gives the maximum force, when the current is at right angles to the field.
Direction of the force
- The force is at right angles to both the current and the field.
- Use the right-hand slap rule (or Fleming's left-hand rule for conventional current): fingers point along the field, current along the thumb/fingers as taught, and the force pops out perpendicular.
- Reversing either the current or the field reverses the force; reversing both leaves it unchanged.
:::tip is only the length of wire inside the field, not the whole wire. If a cm wire passes through a magnet's cm gap, use m, not m. Using the full length is a classic over-estimate. :::
A wire carries A through a T field. The length of wire in the field is cm, at right angles to the field. Find the force.
Step 1 — Convert the length
Step 2 — Apply
Practice question
A m length of wire carries A at right angles to a T field. Find the force on it.
Worked solution: N.
Test yourself
Practice by grade
One question each at Achieved, Merit and Excellence. Have a go, then compare with the model answer.
A wire of length m carries a current of A at right angles to a magnetic field of T.
Calculate the force on the wire.
A wire experiences a force of N when carrying A through a T field at right angles.
Calculate the length of wire that is in the field.
Explain fully how the force on a current-carrying wire is used to make a simple electric motor spin continuously, referring to the direction of the force and the role of the split-ring commutator.