Electric Motor

Fields
definition

Also known as: motor, DC motor, AC motor

Grade 9-12

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A device that converts electrical energy into mechanical energy (rotational motion) by exploiting the force exerted on a current-carrying conductor placed in a magnetic field. Electric motors power household appliances, electric vehicles, industrial robots, hard-drive spindles, and medical devices.

Definition

A device that converts electrical energy into mechanical energy (rotational motion) by exploiting the force exerted on a current-carrying conductor placed in a magnetic field.

💡 Intuition

Run current through a loop between magnets and it spins — the magnetic force on each side of the loop pushes in opposite directions, creating rotation.

🎯 Core Idea

Motors are the reverse of generators — electrical current becomes mechanical rotation through magnetic force.

Example

The motor in an electric fan: current flows through a coil between permanent magnets, creating a torque that spins the blades.

Notation

\tau is the torque in N·m, N is the number of turns, I is the current in amperes, A is the coil area in m², B is the magnetic field in tesla, and \theta is the angle between the magnetic moment and the field.

🌟 Why It Matters

Electric motors power household appliances, electric vehicles, industrial robots, hard-drive spindles, and medical devices. They consume roughly half of all electricity generated worldwide, making motor efficiency a major factor in global energy use.

💭 Hint When Stuck

When analysing an electric motor, identify the current direction, the magnetic field direction, and use the left-hand rule (or \vec{F} = I\vec{L} \times \vec{B}) to find the force on each side of the coil. The forces on opposite sides act in opposite directions, creating a couple (torque) that spins the coil. A commutator reverses the current every half-turn to keep the rotation going.

Formal View

The torque on a current loop of N turns, area A, carrying current I in a field \vec{B} is \tau = NIAB\sin\theta, where \theta is the angle between the magnetic moment \vec{m} = NIA\hat{n} and \vec{B}. Maximum torque occurs when the coil plane is parallel to the field.

🚧 Common Stuck Point

A motor and a generator are the same device used in reverse — many electric cars use their motors as generators during braking.

⚠️ Common Mistakes

  • Thinking the coil keeps spinning on its own once it reaches one half-turn — without a commutator to reverse the current direction, the coil would oscillate back and forth instead of rotating continuously.
  • Confusing the motor effect with electromagnetic induction — a motor converts electrical energy to mechanical energy using F = BIL; induction converts mechanical to electrical using a changing flux.
  • Forgetting that a running motor also acts as a generator (back-EMF) — the spinning coil induces a voltage that opposes the supply voltage, which is why motors draw more current at startup than at full speed.

Frequently Asked Questions

What is Electric Motor in Physics?

A device that converts electrical energy into mechanical energy (rotational motion) by exploiting the force exerted on a current-carrying conductor placed in a magnetic field.

When do you use Electric Motor?

When analysing an electric motor, identify the current direction, the magnetic field direction, and use the left-hand rule (or \vec{F} = I\vec{L} \times \vec{B}) to find the force on each side of the coil. The forces on opposite sides act in opposite directions, creating a couple (torque) that spins the coil. A commutator reverses the current every half-turn to keep the rotation going.

What do students usually get wrong about Electric Motor?

A motor and a generator are the same device used in reverse — many electric cars use their motors as generators during braking.

How Electric Motor Connects to Other Ideas

To understand electric motor, you should first be comfortable with magnetic force, electric current and magnetic field.

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