Key Fact
Transformer
V_s/V_p = N_s/N_p. For ideal transformer (no losses): V_p I_p = V_s I_s. Step-up: N_s > N_p; step-down: N_s < N_p.
-- NCERT Class 12 Physics, Ch. 7, p. 195Transformer
8 MCQs1 revision card9-step worked example
Source: NCERT Electromagnetic Induction and Alternating CurrentsPYQ coverage: NEET 2021, 2023, 2024Official key: NTA-verifiedLast updated: 25 Sep 2026
Transformer, explained for NEET
The transformer question is almost always one line of arithmetic, and the mark is lost on direction. A step-up transformer raises voltage — so it must have more turns on the secondary than the primary, and it delivers less current. Students who memorise "step-up means bigger" without asking bigger what invert the ratio and land on a distractor that is sitting there waiting.
Hold the single relation: V_s/V_p = N_s/N_p. Voltage tracks turns, in that order. NCERT Class 12 Physics Chapter 7 (page 196) states it directly for the ideal case, alongside the power condition V_p I_p = V_s I_s.
That power condition is the whole of the current behaviour, and it is worth deriving rather than memorising as a second formula. An ideal transformer has no losses, so power in equals power out. If the secondary voltage is ten times the primary, the secondary current must be one-tenth, or the output would carry ten times the input power. Hence I_p/I_s = N_s/N_p — the current ratio is the inverse of the voltage ratio. A step-up transformer steps current down. Nothing is gained.
Two further facts the exam uses as one-liners. A transformer works on mutual induction and therefore requires a changing flux — connect the primary to a DC supply and the steady current produces no flux change, so the secondary EMF is zero. And the frequency is unchanged: the secondary EMF is driven by the same alternating primary flux, so 50 Hz in gives 50 Hz out, whatever the turns ratio does to voltage.
Watch-out for the exam hall: when the question gives you a turns ratio as a bare number like "1 : 20", check which coil the first number describes before substituting. Write N_p and N_s as labelled quantities on your rough sheet rather than dropping the ratio straight into the formula. That one labelling step is what separates the students who get this mark from the students who compute a correct number for the wrong coil.
Can you answer these Transformer MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
For an ideal transformer, the ratio of secondary to primary voltage is equal to
Show answer and why every option is right or wrong
Answer: B. The defining ideal-transformer relation is V_s/V_p = N_s/N_p — voltage ratio equals turns ratio, taken in the same order. Stated in NCERT Class 12 Physics Chapter 7, page 196.
Why A is wrong: A is wrong because it inverts the ratio; N_p/N_s gives V_p/V_s, not V_s/V_p, and substituting it turns every step-up answer into a step-down one.
Why C is wrong: C is wrong because the current ratio is the inverse of the voltage ratio: I_s/I_p = N_p/N_s. Setting V_s/V_p equal to I_s/I_p would mean power increases across an ideal transformer.
Why D is wrong: D is wrong because no squaring appears in the transformer relation; the turns ratio enters voltage linearly.
MCQ 2Easy RecallPractice
A transformer with its primary connected to a steady DC source produces a secondary EMF that is
Show answer and why every option is right or wrong
Answer: C. A transformer works by mutual induction, which requires a changing flux. A steady DC primary current gives a constant flux, so no EMF is induced in the secondary. NCERT Class 12 Physics Chapter 7, page 196.
Why A is wrong: A is wrong because the turns-ratio relation presupposes an alternating flux; with DC there is no flux change for the ratio to act on.
Why B is wrong: B is wrong because it assumes some EMF survives; the secondary EMF depends on dΦ/dt, which is zero for steady DC.
Why D is wrong: D is wrong because constant power delivery is irrelevant — induction responds to the rate of change of flux, not to the magnitude of power supplied.
MCQ 3Easy RecallPractice
An ideal step-up transformer, compared with its primary side, delivers a secondary that has
Show answer and why every option is right or wrong
Answer: B. Step-up raises voltage; the ideal power condition V_p I_p = V_s I_s then forces the secondary current down by the same factor. NCERT Class 12 Physics Chapter 7, page 196.
Why A is wrong: A is wrong because raising both voltage and current would raise the output power above the input power, which an ideal (lossless) transformer cannot do.
Why C is wrong: C is wrong because it describes a step-down transformer — this inverted-ratio reading is the standard distractor for this pattern.
Why D is wrong: D is wrong because holding the current fixed while raising the voltage again violates V_p I_p = V_s I_s.
MCQ 4Direct ApplicationPractice
An ideal transformer has 2.0 × 10² turns on the primary and 8.0 × 10² turns on the secondary. The primary is connected to a 1.1 × 10² V AC supply. The secondary voltage is
Show answer and why every option is right or wrong
Answer: C. V_s = V_p × (N_s/N_p) = 1.1 × 10² × (8.0 × 10²)/(2.0 × 10²) = 1.1 × 10² × 4.0 = 4.4 × 10² V. NCERT Class 12 Physics Chapter 7, page 196.
Why A is wrong: A is wrong because it applies the inverted ratio N_p/N_s = 0.25, treating a step-up transformer as step-down.
Why B is wrong: B is wrong because it leaves the primary voltage unchanged, ignoring the turns ratio entirely.
Why D is wrong: D is wrong because it multiplies by the turns ratio twice (4.0 × 4.0 = 16), as if the ratio entered the voltage relation squared.
MCQ 5Direct ApplicationPractice
An ideal transformer steps 2.20 × 10² V down to 1.10 × 10¹ V. The primary draws a current of 5.0 × 10⁻¹ A. The secondary current is
Show answer and why every option is right or wrong
Answer: C. For an ideal transformer V_p I_p = V_s I_s, so I_s = (2.20 × 10² × 5.0 × 10⁻¹)/(1.10 × 10¹) = 1.10 × 10²/(1.10 × 10¹) = 1.0 × 10¹ A. NCERT Class 12 Physics Chapter 7, page 196.
Why A is wrong: A is wrong because it scales the current the same way as the voltage (÷20) instead of inversely; a step-down transformer steps current up.
Why B is wrong: B is wrong because it assumes the current is unchanged, ignoring the power condition V_p I_p = V_s I_s.
Why D is wrong: D is wrong because it uses a ratio of 40 rather than the correct factor of 20, doubling the voltage ratio before applying it.
MCQ 6Direct ApplicationPractice
An ideal transformer is operated from a 50 Hz AC mains supply with a turns ratio N_s : N_p of 1 : 4. The frequency of the secondary EMF is
Show answer and why every option is right or wrong
Answer: B. The secondary EMF is induced by the same alternating flux that the primary current produces, so it alternates at the source frequency. The turns ratio scales voltage, never frequency. NCERT Class 12 Physics Chapter 7, page 196.
Why A is wrong: A is wrong because it applies the turns ratio to the frequency, dividing 50 Hz by 4 — the ratio governs voltage only.
Why C is wrong: C is wrong because it applies the inverted turns ratio to the frequency; neither direction is correct, as frequency is unchanged.
Why D is wrong: D is wrong because stepping down reduces the secondary voltage, not the secondary frequency, and a step-down transformer still delivers an alternating EMF.
MCQ 7CalculationPractice
An ideal transformer supplies a 1.10 × 10² V, 2.20 × 10² W lamp from a 2.20 × 10³ V AC line. The number of turns on the primary is 4.00 × 10³. The number of turns on the secondary, and the current drawn from the line, are respectively
Show answer and why every option is right or wrong
Answer: A. N_s = N_p × (V_s/V_p) = 4.00 × 10³ × (1.10 × 10²)/(2.20 × 10³) = 2.00 × 10² turns. The lamp draws 2.20 × 10² W, and for an ideal transformer the primary supplies the same power, so I_p = P/V_p = (2.20 × 10²)/(2.20 × 10³) = 1.0 × 10⁻¹ A. NCERT Class 12 Physics Chapter 7, page 196.
Why B is wrong: B is wrong in the second step: 2.0 A is the secondary current (P/V_s), not the line current. The primary is the high-voltage side, so it carries the smaller current.
Why C is wrong: C is wrong in the first step: it multiplies the primary turns by the inverted voltage ratio (×20 instead of ÷20), giving a secondary with more turns than the primary for a step-down transformer.
Why D is wrong: D is wrong in the second step: 1.0 A = P/(2.20 × 10² V) comes from misreading the line voltage as 2.20 × 10² V instead of 2.20 × 10³ V.
MCQ 8Concept TrapPractice
A transformer is described as having a turns ratio of 1 : 25, and the label does not state which coil each number refers to. It is connected to a fixed AC supply. Which statement is certainly true?
Show answer and why every option is right or wrong
Answer: C. The magnitude of the voltage ratio is fixed by the turns ratio at 25, but the direction — step-up or step-down — depends on which coil holds which number, which the label does not say. This is the labelling ambiguity to resolve before substituting. NCERT Class 12 Physics Chapter 7, page 196.
Why A is wrong: A is wrong because it assumes the secondary is the 25 side; with the assignment reversed the secondary voltage would be one twenty-fifth of the supply.
Why B is wrong: B is wrong for the same reason, and also reads the current ratio in the same sense as the voltage ratio rather than inversely.
Why D is wrong: D is wrong because an ideal transformer conserves power, V_p I_p = V_s I_s, whatever the turns ratio; no transformer generates power.
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Transformer: quick recall before you leave
How do you solve a Transformer question? A worked example
- 1
Given
• Primary (line) voltage V_p = 2.20 × 10³ V• Secondary (heater) voltage V_s = 1.10 × 10² V• Secondary power P_s = 5.50 × 10² W• Primary turns N_p = 6.00 × 10³ turns (exact — a count)• Transformer is ideal
- 2
Required
• Secondary turns N_s• Primary current I_p
- 3
Concept
Voltage ratio equals turns ratio. Because the transformer is ideal, no power is lost in the core or windings, so the power drawn from the line equals the power delivered to the heater. These two statements are enough; no reactance or phase reasoning is needed.
- 4
Formula
• V_s/V_p = N_s/N_p• V_p I_p = V_s I_s = P (ideal)
- 5
Substitution
• N_s = N_p × (V_s/V_p) = 6.00 × 10³ × (1.10 × 10²)/(2.20 × 10³)• I_p = P/V_p = (5.50 × 10²)/(2.20 × 10³)
- 6
Calculation
• (1.10 × 10²)/(2.20 × 10³) = 5.00 × 10⁻²• N_s = 6.00 × 10³ × 5.00 × 10⁻² = 3.00 × 10² turns• I_p = (5.50 × 10²)/(2.20 × 10³) = 2.50 × 10⁻¹ A
The turns count N_p = 6.00 × 10³ is an exact integer — a count of windings, not a measurement — so it does not limit the significant figures. The precision of the answer is set by the measured voltages and power, each given to three significant figures. - 7
Final answer
N_s = 3.00 × 10² turns; I_p = 2.50 × 10⁻¹ A.
Sanity check: the secondary has fewer turns and lower voltage (step-down), and correspondingly carries the larger current — I_s = P/V_s = 5.00 A, twenty times I_p, matching the twenty-fold voltage step-down. - 8
Common trap
Computing I_p as P/V_s instead of P/V_p. That gives 5.00 A, which is the secondary current, and it appears as a distractor because it is a correct number for the wrong coil. The guard is to write the coil label beside every symbol before substituting: the high-voltage coil always carries the smaller current in an ideal transformer.
- 9
Similar NEET-style question
An ideal transformer steps 2.40 × 10² V down to 1.20 × 10¹ V to run a 3.60 × 10¹ W device. If the secondary has 1.50 × 10² turns, find the primary turns and the primary current. *(Answer: N_p = 3.00 × 10³ turns; I_p = 1.50 × 10⁻¹ A.)*
What to remember before solving Transformer questions
Which Transformer formulas do you need for NEET?
1 formula — click to collapse
Transformer ratios
Voltage ratio = turns ratio. Ideal transformer: V_p I_p = V_s I_s.
| Symbol | Quantity | SI Unit |
|---|---|---|
| V_p, V_s | primary, secondary V | V |
| N_p, N_s | turns | - |
| I_p, I_s | currents | A |
Valid when
- Ideal transformer (no losses)
- Common iron core
More in Electromagnetic Induction and Alternating Currents: 4 exam traps and mistakes · 10 formulas · 5 question patterns from its other lessons.
Transformer questions from past NEET papers
3 questions from NEET 2021, 2023, 2024. Answers verified against NTA official keys. — click to collapse
NEET 2024Revised key
11 : 2
22 : 1
31 : 1
41 : 4
NTA Answer: Option 2(revised_final)
NEET 2023
12.7 A
23.7 A
30.37 A
40.27 A
NTA Answer: Option 4(final)
NEET 2021
14 A
20.2 A
30.4 A
42 A
NTA Answer: Option 2(final)
All 20 past-paper questions from Electromagnetic Induction and Alternating Currents →
How does NEET ask about Transformer?
1 recurring pattern from past papers — click to collapse
Transformer: V_s/V_p = N_s/N_p. Step-up vs step-down. Ideal: V_p I_p = V_s I_s.
Direct ApplicationEasy
Common distractors
inverts ratio direction
Confuses step-up vs step-down assignment
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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