Practice Electric Motor in Physics

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

Quick 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.

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?

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?

Example 3

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 4

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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