Transformer Examples: 43 Problems with Answers

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

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 changes AC voltage using electromagnetic induction between two coils on a shared iron core.

A transformer trades voltage for current (or vice versa) — like a gear system trades speed for torque.

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: Transformer starts by naming the source, the object affected, and how the field or potential changes through space.

Common stuck point: Students often know a formula related to transformer but skip the recognition step: Am I using a field or potential to explain how one object influences another across space? That leads to a correct-looking substitution attached to the wrong physical model.

Sense of Study hint: Ask: Am I using a field or potential to explain how one object influences another across space?

Worked Examples

Example 1

easy

A step-up transformer has

100

turns on the primary coil and

500

turns on the secondary coil. If the input voltage is

240 \text{ V}

, what is the output voltage?

Answer

V_{s} = 1200 V

First step

Transformer equation:

\frac{V_s}{V_p} = \frac{N_s}{N_p}

Full solution

2
$V_{s} = V_{p} \times \frac{N_{s}}{N_{p}} = 240 \times \frac{500}{100} = 240 \times 5 = 1200 V$
3
The turns ratio is $5:1$ , so the voltage is stepped up by a factor of $5$ .

A transformer changes AC voltage using electromagnetic induction between two coils. A step-up transformer has more secondary turns and increases voltage, while a step-down transformer decreases it.

Example 2

medium

An ideal transformer steps down

11{,}000 \text{ V}

220 \text{ V}

. If the secondary current is

50 \text{ A}

, what is the primary current?

Example 3

medium

An ideal transformer steps

230 \text{ V}

down to

46 \text{ V}

. The secondary load draws

5.0 \text{ A}

. Find the primary current.

Example 4

medium

A step-down transformer takes

11{,}000 \text{ V}

230 \text{ V}

. If the primary current is

0.50 \text{ A}

(ideal), find the secondary current.

Example 5

hard

A transformer takes

240 \text{ V}

AC and supplies a

12\text{ V}

36\text{ W}

lamp. Find the primary and secondary currents, assuming ideal operation.

Example 6

medium

A transformer is rated

400 \text{ VA}

with

V_p = 200 \text{ V}

V_s = 50 \text{ V}

. Find the rated full-load currents on both sides.

Practice Problems

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

Example 1

medium

A phone charger transformer converts

240 \text{ V}

AC to

5 \text{ V}

AC. If the primary coil has

960

turns, how many turns does the secondary coil have?

Example 2

hard

A transformer with

90\%

efficiency steps up

240 \text{ V}

12{,}000 \text{ V}

. If the secondary delivers

2 \text{ A}

, what is the primary current and the power wasted as heat?

Example 3

easy

Write the transformer voltage-turns relation.

Example 4

easy

Can a transformer work with direct current (DC)?

Example 5

easy

A transformer has

N_p=100

N_s=200

V_p=10

V. Find

V_s

Example 6

easy

A transformer with more primary turns than secondary turns is which type?

Example 7

easy

In an ideal transformer, what quantity is conserved between primary and secondary?

Example 8

easy

A transformer steps voltage up. What happens to the current?

Example 9

easy

A transformer has

N_p=200

N_s=50

V_p=240

V. Find

V_s

Example 10

easy

What links the two coils of a transformer to share flux efficiently?

Example 11

medium

A transformer:

N_p=500

N_s=100

V_p=120

V. Find

V_s

and identify the type.

Example 12

medium

An ideal transformer:

V_p=120

I_p=2

V_s=240

V. Find

I_s

Example 13

medium

A transformer steps

240

V down to

12

V. If the secondary draws

5

A, find the primary current (ideal).

Example 14

medium

A transformer should output

V_s=6

V from

V_p=120

V with

N_s=20

turns. Find

N_p

Example 15

medium

An ideal transformer delivers

60

W to its load. What power is drawn from the source?

Example 16

medium

A step-up transformer has a turns ratio

N_s/N_p = 5

. If

V_p=20

V and the secondary current is

0.4

A, find the primary current (ideal).

Example 17

medium

Why does a transformer hum and warm up even with no load connected?

Example 18

challenge

Power

P=10

kW is sent over a line of resistance

2\,\Omega

. Compare the line power loss at

100

V versus

1000

V transmission. (

I=P/V

, loss

=I^2R

Example 19

challenge

A transformer is

90\%

efficient. The primary draws

200

W. Find the output power and, if

V_s=20

V, the secondary current.

Example 20

challenge

A transformer has

N_p=600

N_s=150

. The secondary feeds a

5\,\Omega

resistor. If

V_p=240

V (ideal), find the secondary current and the power delivered.

Example 21

medium

A transformer:

N_p=300

N_s=900

V_p=20

V. Find

V_s

and the type.

Example 22

medium

An ideal transformer:

V_p=240

I_p=1

V_s=120

V. Find

I_s

Example 23

easy

A transformer has

N_p = 600

turns and

N_s = 150

turns. If

V_p = 120 \text{ V}

, find

V_s

Example 24

easy

A transformer has

N_p = 50

and

N_s = 200

. What is the turns ratio

N_s/N_p

, and is it step-up or step-down?

Example 25

medium

A transformer increases voltage from

120 \text{ V}

480 \text{ V}

. If the primary coil has

300

turns, how many turns are on the secondary?

Example 26

medium

A transformer has

N_p = 400

and

N_s = 100

. The primary current is

0.5 \text{ A}

. Find the secondary current in an ideal transformer.

Example 27

hard

A transformer is

95\%

efficient. The primary draws

2.0 \text{ A}

240 \text{ V}

. Find the secondary power output.

Example 28

medium

Why does a transformer NOT work on DC?

Example 29

easy

A transformer has a turns ratio (secondary : primary) of

1:8

and

V_s = 24 \text{ V}

. Find

V_p

Example 30

medium

An ideal transformer supplies

40 \text{ W}

20 \text{ V}

on the secondary. If the primary voltage is

240 \text{ V}

, find the primary current.

Example 31

medium

A transformer has

V_p = 100 \text{ V}

V_s = 25 \text{ V}

, and primary current

0.2 \text{ A}

. If efficiency is

80\%

, find the secondary current.

Example 32

easy

A transformer has

V_p = 240 \text{ V}

N_p = 1200

N_s = 60

. Find

V_s

Example 33

hard

98\%

efficient transformer feeds

400 \text{ kW}

to a factory from a

33 \text{ kV}

feed at

230 \text{ V}

. Find the primary current.

Example 34

medium

A bell circuit uses a transformer that steps

240 \text{ V}

down to

8 \text{ V}

to drive a

4 \text{ W}

bell. Find the primary current (assume ideal).

Example 35

challenge

A transmission line of resistance

5.0 \text{ }\Omega

delivers

100 \text{ kW}

. Compare wire-loss percentage at

1000 \text{ V}

10{,}000 \text{ V}

delivery.

Example 36

medium

A transformer steps

24 \text{ V}

AC up to

120 \text{ V}

for a neon display. What is the turns ratio

N_s : N_p

Example 37

hard

A transformer measures

V_p = 240 \text{ V}

I_p = 0.50 \text{ A}

V_s = 12 \text{ V}

I_s = 9.0 \text{ A}

. Find its efficiency.

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Related Concepts

Faraday's Law Generator

Background Knowledge

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

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