Practice Electric Motor in Physics

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

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Quick Recap

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.

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.

Showing a random 20 of 50 problems.

Example 1

easy
Which rule predicts the direction of force on a motor's current-carrying wire?

Example 2

easy
A wire of length 0.4 m0.4 \text{ m} carries 5 A5 \text{ A} perpendicular to a 0.3 T0.3 \text{ T} magnetic field. Find the force on it.

Example 3

medium
A motor coil's two sides are 0.20.2 m apart, each feeling 0.30.3 N in opposite directions. Find the torque magnitude.

Example 4

medium
A motor coil has 5050 turns, area 0.02 m20.02 \text{ m}^2, carries 4 A4 \text{ A}, and lies in a 0.6 T0.6 \text{ T} field. Find the maximum torque (τ=NBIA\tau = NBIA).

Example 5

hard
A motor's input is 480 W480 \text{ W}. At a torque of 1.5 N⋅m1.5 \text{ N} \cdot \text{m} and angular speed 200 rad/s200 \text{ rad/s}, find the efficiency.

Example 6

medium
A motor's torque is 0.4 N⋅m0.4 \text{ N} \cdot \text{m} and its shaft spins at 1200 rpm1200 \text{ rpm}. Find the mechanical power output. (1 rpm=2π/60 rad/s1 \text{ rpm} = 2\pi/60 \text{ rad/s}.)

Example 7

medium
A motor with 4Ω4 \Omega coil resistance is connected to 24 V24 \text{ V}. At the instant it starts (before it spins), what is the starting current?

Example 8

challenge
A DC motor on 230 V230 \text{ V} has armature resistance 2Ω2 \Omega. At full load, it draws 10 A10 \text{ A}. Find the back-EMF, total power input, ohmic loss, mechanical output, and efficiency.

Example 9

medium
An electric drill motor operates at 240 V240 \text{ V} and draws 2.5 A2.5 \text{ A}. It produces a torque of 1.2 N m1.2 \text{ N m} at 2400 rpm2400 \text{ rpm}. What is the efficiency?

Example 10

easy
Why does a motor draw a large current at the instant it starts?

Example 11

easy
A motor converts which kind of energy into which?

Example 12

medium
A motor uses 200 W200 \text{ W} electrical and outputs 150 W150 \text{ W} mechanical. Find its efficiency.

Example 13

challenge
A motor's back-EMF is proportional to angular speed: εback=kω\varepsilon_{\text{back}} = k\omega with k=0.05 V⋅sk = 0.05 \text{ V} \cdot \text{s}. On 12 V12 \text{ V} with R=1ΩR = 1 \Omega, find the no-load speed (where the motor draws negligible current).

Example 14

medium
A motor running on 36 V36 \text{ V} draws 3 A3 \text{ A}. Its coil resistance is 2Ω2 \Omega. Find the back-EMF.

Example 15

easy
A motor wire of length 0.20.2 m carries 33 A in a 0.50.5 T field (perpendicular). Find the force.

Example 16

medium
A motor runs on 1212 V and develops 1010 V of back-EMF. The coil resistance is 1 Ω1\,\Omega. Find the current.

Example 17

hard
A motor with 0.5Ω0.5 \Omega coil resistance runs on 12 V12 \text{ V}, drawing 4 A4 \text{ A}. Find the resistive power lost in the coil and the mechanical power output.

Example 18

medium
Without a commutator, what does a current loop in a magnetic field do instead of rotating continuously?

Example 19

easy
A spinning motor also acts as a generator, producing what opposing voltage?

Example 20

medium
A motor wire of length 0.30.3 m carries 55 A in a 0.40.4 T field (perpendicular). Find the force.
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