Free Transformers MCQs with Answers

1. A transformer works on the principle of

• A. self induction only
• B. mutual induction between two coils linked by a common changing flux
• C. electrostatic attraction
• D. the motor effect

Explanation: Alternating current in the primary produces a continuously changing flux, which the laminated iron core carries to the secondary, where it induces an alternating emf. Since the process depends entirely on a changing flux, a transformer cannot work on direct current. Self induction is the related effect a single coil has on itself.

2. A step down transformer has

• A. more turns on the secondary than the primary
• B. fewer turns on the secondary than the primary, giving a lower output voltage
• C. the same number of turns on both
• D. no core

Explanation: Voltage divides in the ratio of the turns, so fewer secondary turns give a lower voltage, and for an ideal transformer the current rises in the same proportion so that power is conserved. This is what a phone charger does, dropping 220 V to a few volts. A step up transformer reverses the turns ratio.

3. A transformer with 1000 primary turns and 100 secondary turns is supplied at 220 V. The secondary voltage is

• A. 2200 V
• B. 22 V
• C. 220 V
• D. 2.2 V

Explanation: The turns ratio is 10 to 1 down, so the voltage falls by the same factor, from 220 V to 22 V. If the transformer is ideal the secondary current is ten times the primary current, keeping the power the same on both sides. Multiplying instead of dividing gives the 2200 V distractor.

4. Electrical power is transmitted over long distances at very high voltage because

• A. high voltage travels faster
• B. the current is then small, so the I squared R heating loss in the cables is small
• C. high voltage cables are cheaper
• D. consumers need high voltage

Explanation: For a given power, raising the voltage lowers the current in proportion, and since the loss goes as the square of the current, a tenfold rise in voltage cuts the wasted heat a hundredfold. Transformers then step the voltage back down for safe use at the consumer end. This is the main practical reason the grid runs on alternating current.

5. The core of a transformer is laminated in order to

• A. make it lighter
• B. increase the flux through it
• C. reduce energy lost as heat through eddy currents in the core
• D. insulate the two coils from each other

Explanation: Thin sheets separated by insulating varnish break up the circulating paths available to eddy currents, so much less energy is dissipated as heat in the core. Soft iron is used because it is easily magnetised and demagnetised, keeping hysteresis loss low as well. The coils are insulated separately by the enamel on the wire.

6. An ideal transformer is one in which

• A. the output power equals the input power
• B. the output voltage equals the input voltage
• C. the output current equals the input current
• D. no flux passes through the core

Explanation: Ideal means lossless, so all the power delivered to the primary appears at the secondary, and voltage and current change in inverse proportion. Real transformers approach 98 per cent efficiency but lose a little to eddy currents, hysteresis, resistance in the windings and flux leakage. A transformer never changes the frequency.

7. A transformer cannot be used with a direct current supply because

• A. direct current is too dangerous
• B. steady direct current gives no changing flux, so no emf is induced in the secondary
• C. the core would melt
• D. direct current cannot flow in a coil

Explanation: Induction requires a changing flux, and a steady current produces a steady flux, so the secondary voltage is zero except at the instants of switching on and off. Connecting a transformer to a DC supply also risks burning out the primary, since only its low resistance and not its inductive reactance limits the current. This is a fundamental reason the grid uses alternating current.

8. In an ideal transformer, if the primary voltage is doubled and the turns ratio remains the same, what happens to the secondary current?

• A. Doubles
• B. Halves
• C. Remains the same
• D. Becomes four times

Explanation: With the turns ratio fixed, doubling the primary voltage doubles the secondary voltage, and for a given load resistance the secondary current rises in proportion by Ohm's law. The power drawn from the primary rises fourfold to match. The halving answer would apply to comparing primary with secondary current at a fixed input, which is a different question.