Intermediate · 12 min
Magnetic fields and forces on wires
Describe magnetic field direction and calculate the force on a perpendicular current-carrying wire.
Represent a magnetic field
A magnetic field describes magnetic influence around magnets and electric currents. Outside a bar magnet, field lines point from its north pole towards its south pole and form part of closed loops. A compass's north-seeking end points along the local field. Magnetic flux density B is measured in teslas (T). A current-carrying wire in an external magnetic field can experience a force perpendicular to both current and field.
Worked example
A straight wire carrying conventional current through a magnetic field can be pushed sideways. This interaction is used in electric motors.
Use the perpendicular-wire model
For a straight wire perpendicular to a uniform field, force magnitude is F = B × I × L. Here L is the length actually inside the field. A wire parallel to the field has zero magnetic force in this model. Reversing either current or field reverses the force direction while keeping its magnitude unchanged. Reversing both leaves the force direction unchanged.
Worked example
A 0.1 m wire segment carries 2 A perpendicular to a 0.3 T field. Force magnitude = 0.3 × 2 × 0.1 = 0.06 N
Explore a motor model on paper
- Calculate the force on a 0.2 m wire carrying 3 A perpendicular to a 0.4 T field.
- Predict what happens to the force magnitude if the current doubles, and to its direction if only the current reverses.