Electric motors turn electrical energy into mechanical motion using the force between magnetic fields and electric current. They matter because they power fans, tools, appliances, electric vehicles, robots, and many other devices. A simple DC motor shows the core idea clearly: a current carrying coil sits between magnets and experiences forces that make it rotate.
The direction of the force depends on the direction of current and the direction of the magnetic field.
Inside the motor, the stator magnets create a magnetic field across the rotating coil, called the armature. When current flows through opposite sides of the coil, each side feels a magnetic force in opposite directions, creating a turning effect called torque. A commutator reverses the current every half turn so the torque keeps pushing the coil in the same rotational direction.
Stronger magnetic fields, larger currents, more coil turns, and larger coil area all increase the motor torque.
Key Facts
- Magnetic force on a current carrying wire: F = BIL sin(theta)
- Torque on a current loop: tau = NIBA sin(theta)
- Motor effect: a current carrying conductor in a magnetic field experiences a force.
- The commutator reverses current every half rotation in a simple DC motor.
- Maximum force occurs when the wire is perpendicular to the magnetic field, so sin(theta) = 1.
- Increasing current I, magnetic field B, number of turns N, or coil area A increases torque.
Vocabulary
- Stator
- The stationary part of a motor that provides the magnetic field, often using permanent magnets or electromagnets.
- Armature
- The rotating coil or set of coils in a motor where current interacts with the magnetic field.
- Commutator
- A split ring switch that reverses current in the coil every half turn to keep torque in the same direction.
- Torque
- A turning effect produced by a force acting at a distance from an axis of rotation.
- Magnetic field
- A region around a magnet or current where magnetic forces can act on moving charges or magnetic materials.
Common Mistakes to Avoid
- Confusing current direction with electron motion, which is wrong because conventional current points opposite to electron flow in metal wires.
- Forgetting the angle term in F = BIL sin(theta), which is wrong because the magnetic force depends on the wire's angle to the field.
- Thinking one side of the coil makes all the rotation, which is wrong because opposite sides feel opposite forces that form a torque pair.
- Leaving out the commutator, which is wrong because without current reversal the torque would reverse direction after half a turn and the motor would stall or rock back and forth.
Practice Questions
- 1 A straight wire segment in a motor carries 3.0 A through a 0.12 m length inside a 0.50 T magnetic field. If the wire is perpendicular to the field, what magnetic force acts on the wire?
- 2 A rectangular coil has 40 turns, area 0.020 m^2, current 2.5 A, and is in a 0.30 T magnetic field. What is the maximum torque on the coil?
- 3 Explain why a simple DC motor needs a commutator to keep spinning in one direction instead of stopping after part of a turn.