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Foundation6 past questions

AC Generators and Transformers

AC generators convert mechanical energy to electrical energy using electromagnetic induction, while transformers transfer electrical energy between circuits, often changing voltage levels.

Why this shows up in the exam

NEET tests your understanding of the working principles, efficiency, and power relations in these devices.

How NEET tests this

Numerical · 7 QsDirect recall

Learn the idea

Electromagnetic induction makes a changing magnetic flux produce an emf; in a generator mechanical motion creates that flux change, while in a transformer the same changing flux links two coils so that voltages are related by their turn ratios.

🧠 Memory hook: Big N gives big V; small N gives small V – the coil‑size decides the voltage, the opposite decides the current.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Faraday’s law: emf = -N dΦ/dt
  • Lenz’s law: induced emf opposes the cause of flux change
  • Transformer relation: Vp/Vs = Np/Ns and Ip/Is = Ns/Np
  • Efficiency η = (output power / input power)×100%
  • Power loss in a line = I²R
  • Step‑down transformer: V secondary < V primary, I secondary > I primary

How to approach it

  1. 1Read the question and note what is given – voltage, power, turns or resistance per unit length
  2. 2Write the appropriate relation: for transformers use Vp/Vs = Np/Ns or Ip/Is = Ns/Np; for line loss use I = Vdrop / Rtotal then Ploss = I²Rtotal
  3. 3Insert the numbers, keep units consistent, solve for the required quantity
  4. 4Check the answer against physical sense (e.g., current in primary of a step‑down is smaller than secondary)

Worked example — watch it click

Two cities are 150 km apart. Electric power is sent from one city to another city through copper wires. The fall of potential per km is 8 volt and the average resistance per km is 0.5 Ω. The power loss in the wire is:

  • A)19.2 W
  • ✅19.2 kW
  • C)19.2 J
  • D)12.2 kW

The concept behind this problem

The example forces you to convert a per‑kilometre voltage drop and resistance into total values, find the line current, and then apply the I²R formula for loss – a classic test of line‑loss reasoning.

Step by step

  1. 1Distance=150km.
  2. 2Voltage drop per km=8V, so total voltage drop=150×8=1200V.
  3. 3Resistance per km=0.5Ω, so total resistance=150×0.5=75Ω.
  4. 4Current I=V/R=1200/75=16A.
  5. 5Power loss=I²R=(16)²×75=256×75=19200W=19.2kW.

Watch out

Using the per‑kilometre resistance (0.5 Ω) as the whole‑line resistance instead of multiplying by the 150 km distance.

Common slip-ups that cost marks

  • •Mixing up primary and secondary quantities in the transformer ratio
  • •Treating the resistance per km as the total resistance of the whole line
  • •Ignoring the given efficiency when the problem asks for input or output values

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 6NEET 2021

A step down transformer connected to an AC mains supply of 220 V is made to operate at 11 V, 44 W lamp. Ignoring power losses in the transformer, what is the current in the primary circuit ?

Push further

More challenging

4 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 4

A step-down transformer has a turns ratio of 10:1 (primary to secondary). It is connected to a 200 V AC mains supply. If the secondary coil delivers 500 W of power to a resistive load, what is the current in the primary coil, assuming 90% efficiency?

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