Physics / Unit 10: Magnetism and Modern Physics
10.3 Magnetic Force and Motors
Notes
A magnetic field exerts a force on a current-carrying wire (and on moving charges).
- Force on a straight wire perpendicular to the field: $F = BIL$ ($B$ in teslas, $I$ in amperes, $L$ in meters).
- The force is perpendicular to both the current and the field. If the wire is parallel to the field, the force is zero.
- An electric motor uses this force on a current loop to make it spin, changing electrical energy into mechanical energy.
- Example: a 0.2 m wire carrying 4 A at right angles to a 0.5 T field: $F = 0.5 \times 4 \times 0.2 = 0.4 \text{ N}$.
Practice
A 0.5 m wire carries 3 A at right angles to a 0.2 T magnetic field. What force acts on it?
$F = BIL = 0.2 \times 3 \times 0.5 = 0.3 \text{ N}$.
An electric motor converts:
Current in a magnetic field produces forces that make the motor turn.
A current-carrying wire lies parallel to a magnetic field. What is the magnetic force on it?
A wire parallel to the field feels no magnetic force.
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