Force on a Current
Force on a Current
- A wire has its own magnetic field when it carries current. If the wire is placed in another magnetic field, the two fields interact.
- The wire can then experience a force. This is the motor effect.
Find the force direction
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- Use Fleming's left-hand rule. Hold your left thumb, first finger and second finger at right angles.
- First finger: magnetic field. Second finger: conventional current. Thumb: force or motion.
- The force is perpendicular to both the current and the magnetic field.
Magnetic flux density
Definition
2 marksWhat is meant by magnetic flux density?
Model answer: The force per unit current per unit length on a straight wire placed at right angles to the magnetic field.
The mark scheme requires force per unit current, force per unit length and the right-angle condition. Two points earn one mark; all three earn two marks (9702/41/M/J/25 Q7(a)).
Symbols
- = force on the wire (N)
- = magnetic flux density (T)
- = current (A)
- = length of wire inside the field (m)
- = angle between current and magnetic field (—)
- The force is maximum when the wire is perpendicular to the field.
- The force is zero when the wire is parallel to the field.
- One tesla is one newton per ampere per metre.
Use the force equation
Worked example
Measure a field with a current-carrying wire
A 5.0 cm wire carries 0.50 A at right angles to a field. The measured force is 2.0 × 10⁻⁴ N. Find the magnetic flux density.
- Convert the length to metres.
- The wire is perpendicular to the field, so the sine factor is one.
- Rearrange and substitute.
A 0.20 m wire carries 3.0 A perpendicular to a 0.40 T field. Find the force.
Show worked answer
A current balance measures the force through a change in a balance reading. A Hall probe gives a direct field reading; it is explained in the last lesson.