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 \text{ m}

carries

5 \text{ A}

perpendicular to a

0.3 \text{ T}

magnetic field. Find the force on it.

Example 3

medium

A motor coil's two sides are

0.2

m apart, each feeling

0.3

N in opposite directions. Find the torque magnitude.

Example 4

medium

A motor coil has

50

turns, area

0.02 \text{ m}^2

, carries

4 \text{ A}

, and lies in a

0.6 \text{ T}

field. Find the maximum torque (

\tau = NBIA

Example 5

hard

A motor's input is

480 \text{ W}

. At a torque of

1.5 \text{ N} \cdot \text{m}

and angular speed

200 \text{ rad/s}

, find the efficiency.

Example 6

medium

A motor's torque is

0.4 \text{ N} \cdot \text{m}

and its shaft spins at

1200 \text{ rpm}

. Find the mechanical power output. (

1 \text{ rpm} = 2\pi/60 \text{ rad/s}

Example 7

medium

A motor with

4 \Omega

coil resistance is connected to

24 \text{ V}

. At the instant it starts (before it spins), what is the starting current?

Example 8

challenge

A DC motor on

230 \text{ V}

has armature resistance

2 \Omega

. At full load, it draws

10 \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 \text{ V}

and draws

2.5 \text{ A}

. It produces a torque of

1.2 \text{ N m}

2400 \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 \text{ W}

electrical and outputs

150 \text{ W}

mechanical. Find its efficiency.

Example 13

challenge

A motor's back-EMF is proportional to angular speed:

\varepsilon_{\text{back}} = k\omega

with

k = 0.05 \text{ V} \cdot \text{s}

. On

12 \text{ V}

with

R = 1 \Omega

, find the no-load speed (where the motor draws negligible current).

Example 14

medium

A motor running on

36 \text{ V}

draws

3 \text{ A}

. Its coil resistance is

2 \Omega

. Find the back-EMF.

Example 15

easy

A motor wire of length

0.2

m carries

3

A in a

0.5

T field (perpendicular). Find the force.

Example 16

medium

A motor runs on

12

V and develops

10

V of back-EMF. The coil resistance is

1\,\Omega

. Find the current.

Example 17

hard

A motor with

0.5 \Omega

coil resistance runs on

12 \text{ V}

, drawing

4 \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

0.3

m carries

5

A in a

0.4

T field (perpendicular). Find the force.

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Related Concepts

Magnetic Force Electric Current Magnetic Field