An electric motor converts electrical energy into rotational mechanical energy. It matters because motors run fans, pumps, toys, power tools, electric cars, and many machines used in industry. A simple DC motor shows the core idea clearly: a current-carrying coil feels forces when it sits in a magnetic field.
Those forces create a torque that makes the coil spin.
Key Facts
- Magnetic force on a straight current-carrying wire: F = BIL sin(theta)
- Motor torque on a rectangular coil: tau = N B I A sin(theta)
- A DC motor uses a split-ring commutator to reverse the coil current every half-turn.
- The two sides of the coil feel opposite magnetic forces, forming a turning pair that produces torque.
- Increasing current, magnetic field strength, coil area, or number of turns increases motor torque.
- Electrical input power is approximately P = VI, while useful mechanical output power is P = tau omega.
Vocabulary
- Armature
- The rotating coil or set of coils in a motor that carries current and experiences magnetic torque.
- Magnetic field
- A region around a magnet or current where magnetic forces can act on moving charges or currents.
- Commutator
- A split metal ring that reverses the current in the coil at the right time so the motor keeps spinning in the same direction.
- Torque
- A turning effect of a force that causes or changes rotational motion.
- Brushes
- Conducting contacts that press against the commutator and carry current from the power source into the rotating coil.
Common Mistakes to Avoid
- Forgetting the angle in F = BIL sin(theta) is wrong because the magnetic force depends on how the wire is oriented relative to the magnetic field.
- Thinking the commutator makes the coil spin by itself is wrong because the commutator only reverses current, while the magnetic forces on the coil provide the torque.
- Drawing both sides of the coil with forces in the same direction is wrong because opposite current directions on opposite sides create opposite forces that form a turning pair.
- Assuming more voltage always means a safe stronger motor is wrong because higher voltage can cause larger current, heating, and damage if the motor is not designed for it.
Practice Questions
- 1 A straight wire segment in a motor carries 3.0 A through a 0.080 m length in a 0.50 T magnetic field. The wire is perpendicular to the field. What magnetic force acts on the wire?
- 2 A motor coil has 50 turns, area 0.020 m^2, current 2.0 A, and magnetic field 0.30 T. If the coil is positioned so sin(theta) = 1, what is the maximum torque?
- 3 A simple DC motor keeps rotating in one direction even though the coil turns through positions where the torque becomes zero. Explain how the commutator and the coil's inertia help maintain continuous rotation.