Electric Motor Examples in Physics

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Electric Motor.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept Recap

A device that converts electrical energy into mechanical energy (rotation) using the force on a current-carrying conductor in a magnetic field.

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.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

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

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.

Worked Examples

Example 1

easy
A DC motor draws 4 \text{ A} from a 12 \text{ V} supply. If the motor is 80\% efficient, what is its useful mechanical power output?

Solution

  1. 1
    Electrical power input: P_{\text{in}} = VI = 12 \times 4 = 48 \text{ W}.
  2. 2
    Useful mechanical power: P_{\text{out}} = \eta \times P_{\text{in}} = 0.80 \times 48 = 38.4 \text{ W}.
  3. 3
    The remaining 9.6 \text{ W} is lost as heat in the coils and friction.

Answer

P_{\text{out}} = 38.4 \text{ W}
An electric motor converts electrical energy to mechanical energy. The efficiency tells us what fraction of the input power becomes useful work; the rest is wasted as heat and friction.

Example 2

medium
A motor coil has 200 turns, area 0.01 \text{ m}^2, carries 3 \text{ A}, and is in a 0.5 \text{ T} field. What is the maximum torque on the coil?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
An electric drill motor operates at 240 \text{ V} and draws 2.5 \text{ A}. It produces a torque of 1.2 \text{ N m} at 2400 \text{ rpm}. What is the efficiency?

Example 2

hard
A DC motor has a back-EMF of 10 \text{ V} when running at full speed. The supply voltage is 12 \text{ V} and the coil resistance is 0.5 \text{ } \Omega. What is the running current, the starting current (back-EMF = 0), and the mechanical power output?
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Background Knowledge

These ideas may be useful before you work through the harder examples.

magnetic forceelectric currentmagnetic field