PhysicsICSE Class 10 · Magnetic Effect of Current and Electromagnetic Induction

Transformer: Principle, Step-Up and Step-Down Types, Turns Ratio and Uses (ICSE Class 10)

Q: What is a transformer?

A transformer is a device that increases or decreases an alternating voltage. It has a primary and a secondary coil on a laminated soft iron core and works on electromagnetic induction; for an ideal one, Vs/Vp = Ns/Np = Ip/Is.

Q: What is the difference between a step-up and a step-down transformer?

A step-up transformer has more turns on the secondary, so it raises the voltage and lowers the current. A step-down transformer has fewer secondary turns, so it lowers the voltage and raises the current.

Q: Why doesn't a transformer work on DC?

A steady direct current produces a steady magnetic field in the core. Induction needs a changing field, so no emf is induced in the secondary.

Updated 16 October 2026 · Fact-checked

A transformer changes the voltage of an alternating supply. It has a primary and a secondary coil wound on a laminated soft iron core, and works by electromagnetic induction. For an ideal transformer Vs/Vp = Ns/Np = Ip/Is. A step-up transformer has more secondary turns; a step-down transformer has fewer.

The coil with more turns gets the larger voltage; Vs/Vp = Ns/Np, and the current changes in the opposite ratio (Ip/Is).

Illustration drawn for QuestionBuddy.

Understand Transformer: Step-Up and Step-Down

What a transformer does. It raises (steps up) or lowers (steps down) an alternating voltage, with very little loss of power. It has no moving parts.

Construction.

Primary coil: connected to the AC supply (input).
Secondary coil: connected to the load (output).
Core: the two coils, insulated from each other, are wound on a laminated soft iron core. Soft iron is easily magnetised and demagnetised, so it follows the changing current and carries almost all the magnetic field of the primary through the secondary. Making it of thin insulated sheets (laminations) reduces the energy wasted as heat in the core.

Principle: electromagnetic induction. An alternating current in the primary sets up a changing magnetic field in the core. This changing field passes through the secondary and induces an alternating emf in it, of the same frequency.

A transformer does not work on steady DC: a steady current gives a steady field, which induces nothing in the secondary.

The transformer equations (ideal transformer).

$\frac{V _{s}}{V _{p}} = \frac{N _{s}}{N _{p}} = \frac{I _{p}}{I _{s}}$

$\frac{N _{s}}{N _{p}}$ is the turns ratio. The voltage changes in the same ratio as the number of turns.
In an ideal transformer, no energy is lost: input power = output power, $V_{p} I_{p} = V_{s} I_{s}$ . So if the voltage goes up, the current goes down in the same ratio, and vice versa.

Step-up and step-down transformers.

Feature	Step-up	Step-down
Turns	$N_{s} > N_{p}$	$N_{s} < N_{p}$
Turns ratio $\frac{N _{s}}{N _{p}}$	More than 1	Less than 1
Output voltage	Higher than input	Lower than input
Output current	Lower than input	Higher than input
Thicker wire	Primary (larger current)	Secondary (larger current)
Uses	At power stations, to raise the voltage for transmission	At substations; in phone chargers, doorbells, low-voltage lamps and radios

Which coil is thicker, and why? The coil that carries the larger current is made of thicker wire. A thick wire has a lower resistance, so less heat ( $I^{2} R t$ ) is produced in it. In a step-down transformer, the secondary has fewer turns but carries more current, so the secondary is thicker. (More turns does not mean thicker wire.)

A real transformer. Some energy is lost as heat in the coils and the core, so the output power is slightly less than the input power. Good transformers waste only a small fraction of the input.

Transformers and power transmission. At a power station the generator output is stepped up to a very high voltage, so the current in the long transmission lines is small and the heating loss $I^{2} R$ in them is small. Near towns, step-down transformers bring the voltage down in stages to 220 V for homes. This is possible only because the supply is AC, which is one of the main advantages of AC over DC.

Transformer: Step-Up and Step-Down: formulas and key facts

Transformer equation (ideal): $\frac{V _{s}}{V _{p}} = \frac{N _{s}}{N _{p}} = \frac{I _{p}}{I _{s}}$; Turns ratio = Nₛ/Nₚ.
Power (ideal): $V_{p} I_{p} = V_{s} I_{s}$; Input power = output power.
Types: $S t e p - u p : N_{s} > N_{p}, V_{s} > V_{p}, I_{s} < I_{p} ∣ S t e p - d o w n : N_{s} < N_{p}, V_{s} < V_{p}, I_{s} > I_{p}$; The coil with the larger current is thicker.
Line loss: $P_{l oss} = I^{2} R$; Why transmission is at high voltage.

How to solve Transformer: Step-Up and Step-Down questions

Write the full equation chain, fill in the known values, and solve for the unknown; then check the type.

1Write $\frac{V _{s}}{V _{p}} = \frac{N _{s}}{N _{p}} = \frac{I _{p}}{I _{s}}$ and list the given values with the correct subscripts.
2Use the pair of ratios that contains the unknown and one complete known pair.
3Decide the type: if $V_{s} > V_{p}$ (or $N_{s} > N_{p}$ ) it is step-up; otherwise step-down.
4For currents, remember they change the opposite way to voltage; check with $V_{p} I_{p} = V_{s} I_{s}$ .
5For 'which coil is thicker', name the coil with the larger current and give the reason (lower resistance, less heating).

Transformer: Step-Up and Step-Down in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Inside a phone charger

Suppose you open an old phone charger (safely unplugged) and find a small transformer marked 'input 230 V AC, output 5 V'. One coil is of fine wire with many turns; the other has a few turns of thicker wire.

Which coil is the primary, and why is the other one thicker?

Try it first, then show the solutionSolution

It is a step-down transformer, so the primary has more turns: the fine wire with many turns is the primary.
The secondary gives a lower voltage and so carries a larger current; thicker wire has lower resistance and heats up less.

Answer: Fine, many-turn coil: primary. Few-turn thick coil: secondary (larger current).

Memory hook: Big current, thick wire.

The pole transformer

Imagine the large transformer mounted on a pole near your colony, fed by overhead lines at several thousand volts.

What type is it, and why is it needed?

Try it first, then show the solutionSolution

Homes need 220 V, but the local lines carry a much higher voltage to keep the current and losses low.
So the pole transformer lowers the voltage: it is a step-down transformer, with fewer turns on its secondary.

Answer: Step-down: it brings the distribution voltage down to the 220 V household supply.

Memory hook: Up to travel, down to deliver.

Tricks to remember Transformer: Step-Up and Step-Down

Short memory aids. Say each one aloud once, then use it on a question the same day.

S over P, three times
Vs/Vp = Ns/Np = Ip/Is: voltage and turns go 'S on top'; current flips to 'P on top'.
Use it when: Writing the transformer equation correctly.
Volts up, amps down
Whatever the voltage does, the current does the opposite.
Use it when: Checking current answers.
Thick follows current
Not turns, not voltage: the coil with more current is thicker.
Use it when: Thicker-coil questions.
No change, no transformer
Steady DC means a steady field, so nothing is induced.
Use it when: Why AC is needed.

Transformer: Step-Up and Step-Down: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: The core material · 1 mark · MCQBoard level

The core of a transformer is made of:
(a) steel
(b) copper
(c) laminated soft iron
(d) aluminium

Try it first, then show the solutionSolution

Soft iron is easily magnetised and demagnetised as the current alternates; laminations reduce heat loss in the core.

Answer: (c) laminated soft iron

Example 2: Comparing the two coils · 3 marks · Short answerBoard level

For a step-down transformer, compare the primary and secondary coils in terms of (a) number of turns, (b) thickness of wire, (c) current. Give a reason for (b).

Try it first, then show the solutionSolution

(a) The primary has more turns than the secondary ( $N_{s} < N_{p}$ ). (1)
(c) The secondary carries the larger current, since $\frac{I _{p}}{I _{s}} = \frac{N _{s}}{N _{p}} < 1$ . (1)
(b) So the secondary is made of thicker wire: lower resistance, less heating. (1)

Answer: (a) Primary more turns (b) Secondary thicker (c) Secondary larger current

Example 3: A doorbell transformer · 3 marks · Short answerHarder board level

A doorbell needs 12 V and is run from the 240 V mains through a transformer with 1200 turns on its primary. Find (a) the number of secondary turns, (b) the primary current when the bell draws 2 A (ideal transformer), (c) the power supplied.

Try it first, then show the solutionSolution

(a) $N_{s} = N_{p} \times \frac{V _{s}}{V _{p}} = 1200 \times \frac{12}{240} = 60$ turns. (1)
(b) $I_{p} = I_{s} \times \frac{V _{s}}{V _{p}} = 2 \times \frac{12}{240} = 0.1$ A. (1)
(c) $P = V_{s} I_{s} = 12 \times 2 = 24$ W (check: $240 \times 0.1 = 24$ W). (1)

Answer: (a) 60 turns (b) 0.1 A (c) 24 W

Example 4: A step-up for a neon sign · 3 marks · Short answerHarder board level

A transformer for a neon sign has 150 turns on the primary and 3000 turns on the secondary. The primary is connected to 230 V AC. (a) Find the turns ratio and the output voltage. (b) Name the type. (c) Which coil uses the thicker wire?

Try it first, then show the solutionSolution

(a) $\frac{N _{s}}{N _{p}} = \frac{3000}{150} = 20$ ; $V_{s} = 230 \times 20 = 4600$ V. (1)
(b) Step-up (turns ratio more than 1). (1)
(c) The primary, because it carries the larger current (20 times the secondary current). (1)

Answer: (a) 20, 4600 V (b) Step-up (c) Primary

Example 5: Transformer on a battery · 3 marks · Short answerHarder board level

A student connects the primary of a transformer to a 12 V battery. (a) What is the secondary output once the current has become steady? (b) Explain. (c) Why might the primary overheat?

Try it first, then show the solutionSolution

(a) Zero (apart from a brief pulse at the moment of switching on). (1)
(b) A steady current gives a steady magnetic field; with no change in the field, no emf is induced in the secondary. (1)
(c) On AC the changing field limits the primary current; on DC only the small resistance of the wire limits it, so a large current flows and heats the coil. (1)

Answer: (a) No output (b) No changing field, no induction (c) Large current limited only by resistance

Example 6: Phone charger design · 3 marks · Short answerHOTS / competency

A charger's transformer steps 230 V down to 5 V and supplies 2 A to the phone. Its secondary has 40 turns. Assuming it is ideal, find (a) the turns ratio $\frac{N _{p}}{N _{s}}$ , (b) the primary turns, (c) the primary current.

Try it first, then show the solutionSolution

(a) $\frac{N _{p}}{N _{s}} = \frac{V _{p}}{V _{s}} = \frac{230}{5} = 46$ . (1)
(b) $N_{p} = 46 \times 40 = 1840$ turns. (1)
(c) $I_{p} = \frac{V _{s} I _{s}}{V _{p}} = \frac{5 \times 2}{230} \approx 0.043$ A: a tiny current, so the primary can be fine wire. (1)

Answer: (a) 46 (b) 1840 turns (c) ≈ 0.043 A

Example 7: Case study: sending power across the state · 4 marks · Case-basedHOTS / competency

A generator delivers 1.1 MW at 22 kV. A transformer steps this up to 220 kV for a line of total resistance 10 Ω.
(a) Find the turns ratio $\frac{N _{s}}{N _{p}}$ of this transformer and name its type.
(b) Find the line current if the power were sent at 22 kV, and at 220 kV.
(c) Find the power lost in the line in each case.
(d) Why can this not be done with DC?

Try it first, then show the solutionSolution

(a) $\frac{220 000}{22 000} = 10$ ; step-up. (1)
(b) At 22 kV: $I = \frac{1.1 \times 1 0 ^{6}}{22 000} = 50$ A; at 220 kV: $I = 5$ A. (1)
(c) $5 0^{2} \times 10 = 25000$ W (25 kW); $5^{2} \times 10 = 250$ W: 100 times less. (1)
(d) A transformer needs a changing magnetic field, which only AC provides; DC cannot be stepped up this way. (1)

Answer: (a) 10, step-up (b) 50 A and 5 A (c) 25 kW and 250 W (d) Transformers work only on AC

Where students lose marks in Transformer: Step-Up and Step-Down

The slips examiners see most often, and what to write instead.

Mistake: Wrong type from a diagram
The type depends on which coil has more turns, not on which is drawn on the left or looks bigger.
Wrong working
Calling a transformer with fewer secondary turns 'step-up'.
Right working
Count turns: Nₛ < Nₚ means step-down.
Do this: Identify the primary (input) side first, then compare turns.
Usually costs: Reported by CISCE examiners for 2024 Q8(ii).
Mistake: More turns taken to mean thicker wire
Thickness depends on current: the coil with the larger current needs lower resistance.
Wrong working
'The primary of a step-down transformer is thicker because it has more turns.'
Right working
'The secondary is thicker because it carries more current; thick wire has lower resistance, so less heat is produced.'
Do this: Link thickness to current and resistance, never to turns or voltage.
Usually costs: Reported by CISCE examiners for 2024 Q8(ii).
Mistake: Vague answer for the turns ratio
The turns ratio Nₛ/Nₚ of a step-up transformer is more than 1; 'large' or 'high' is not precise.
Wrong working
'The turns ratio is big.'
Right working
'The turns ratio is more than 1.'
Do this: Compare with 1 in every type question.
Usually costs: Reported by CISCE examiners for 2025 Q2(i)(e).
Mistake: Current ratio written the wrong way up
Current changes opposite to voltage, so the current ratio is Iₚ/Iₛ, not Iₛ/Iₚ.
Wrong working
Is = Ip × Ns/Np for a step-up transformer (current increases).
Right working
Is = Ip × Np/Ns (current decreases when voltage rises).
Do this: Check with power: VₚIₚ = VₛIₛ.
Usually costs: Usually costs the answer mark.

Exam tips

In a labelled diagram, show the primary and secondary on a closed laminated soft iron core, with AC input and output marked.
When asked about thickness, use the words 'larger current', 'lower resistance' and 'less heating'.
State the type with the turns ratio compared with 1.
Mention 'ideal transformer' when you use $V_{p} I_{p} = V_{s} I_{s}$ .

Practice questions on Transformer: Step-Up and Step-Down

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which statement is true for a step-down transformer?
(a) $N_{s} > N_{p}$
(b) $V_{s} > V_{p}$
(c) $I_{s} > I_{p}$
(d) the turns ratio $\frac{N _{s}}{N _{p}}$ is more than 1
Hint
Voltage down, current up.
Answer
(c) $I_{s} > I_{p}$
Q2 · 1 mark · Very short answerBoard level
State the principle on which a transformer works.
Hint
Changing field.
Answer
Electromagnetic induction: an alternating current in the primary produces a changing magnetic field in the core, which induces an emf in the secondary.
Q3 · 2 marks · Short answerBoard level
Name the type of transformer used (a) at a generating station before transmission, (b) in a mobile phone charger.
Hint
Raise for travel, lower for use.
Answer
(a) Step-up. (b) Step-down.
Q4 · 2 marks · Short answerHarder board level
A transformer has 600 primary turns and 30 secondary turns. The primary is connected to 240 V AC. Find the output voltage and name the type.
Hint
$V_{s} = V_{p} \frac{N _{s}}{N _{p}}$ .
Answer
$V_{s} = 240 \times \frac{30}{600} = 12$ V; step-down.
Q5 · 2 marks · Short answerHarder board level
A 6 V supply is needed from 240 V mains. The secondary has 50 turns. How many turns must the primary have?
Hint
$N_{p} = N_{s} \frac{V _{p}}{V _{s}}$ .
Answer
$N_{p} = 50 \times \frac{240}{6} = 2000$ turns.
Q6 · 2 marks · Short answerHarder board level
An ideal transformer takes 0.5 A from a 220 V supply and gives an output of 11 V. Find the output current.
Hint
$V_{p} I_{p} = V_{s} I_{s}$ .
Answer
$I_{s} = \frac{220 \times 0.5}{11} = 10$ A.
Q7 · 2 marks · Short answerHarder board level
In a step-up transformer, why is the secondary made of thinner wire than the primary?
Hint
Which coil carries less current?
Answer
The secondary of a step-up transformer carries a smaller current than the primary ( $I_{s} < I_{p}$ ). A smaller current produces less heat, so thinner wire (higher resistance) is enough; the primary, with the larger current, needs thick, low-resistance wire.
Q8 · 2 marks · Short answerHarder board level
Why is the core of a transformer (a) made of soft iron, (b) laminated?
Hint
Magnetising and heating.
Answer
(a) Soft iron is easily magnetised and demagnetised, so it follows the alternating current and links the field of the primary with the secondary. (b) Thin insulated sheets reduce the energy wasted as heat in the core.
Q9 · 2 marks · Short answerHOTS / competency
A real transformer takes 2 A from a 230 V supply and delivers 19 A at 22 V. (a) Find the input and output powers. (b) Why is the output less than the input?
Hint
P = VI on each side.
Answer
(a) Input $= 230 \times 2 = 460$ W; output $= 22 \times 19 = 418$ W. (b) Some energy (42 W) is lost as heat in the coils and the core, so a real transformer is not ideal (about 91% of the input reaches the output).
Q10 · 2 marks · Short answerHOTS / competency
A transformer with a turns ratio $\frac{N _{s}}{N _{p}} = 4$ is connected to a 12 V car battery. A student expects 48 V at the output. What is the actual output, and why?
Hint
What does a battery supply?
Answer
Zero (except a momentary pulse when switched on or off). A battery gives steady DC, which makes a steady field in the core; with no change in the field, no emf is induced in the secondary.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Principle of a transformer?Show answer
Electromagnetic induction.
2.Turns ratio of a step-up transformer?Show answer
More than 1.
3.Thicker coil in a step-down transformer?Show answer
The secondary.
4.Does a transformer work on DC?Show answer
No.
5.Core material?Show answer
Laminated soft iron.

Transformer: Step-Up and Step-Down: frequently asked questions

What is a transformer?

A transformer is a device that increases or decreases an alternating voltage. It has a primary and a secondary coil on a laminated soft iron core and works on electromagnetic induction; for an ideal one, Vs/Vp = Ns/Np = Ip/Is.

What is the difference between a step-up and a step-down transformer?

A step-up transformer has more turns on the secondary, so it raises the voltage and lowers the current. A step-down transformer has fewer secondary turns, so it lowers the voltage and raises the current.

Why doesn't a transformer work on DC?

A steady direct current produces a steady magnetic field in the core. Induction needs a changing field, so no emf is induced in the secondary.