Transformers

Practice Questions:

Question 1: moderate

In step-down transformer of turn ratio 6 : 1, an alkali accumulator of 12 V; 3A is connected to the primary. Then the voltage developed in its secondary is :

1. 2V

2. 72V

3. 2/3 V

4. zero

View Answer

The voltage developed in the secondary is zero.

A transformer operates on the principle of mutual induction, which requires a changing magnetic flux produced by Alternating Current (AC). Since an alkali accumulator provides Direct Current (DC), the magnetic flux remains constant, and no electromotive force (EMF) is induced in the secondary coil.

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Question 2: easy

In a step up transformer, the voltage in the primary is \(220\text{ V}\) and the current is \(5\text{ A}\). The secondary voltage is found to be \(22000\text{ V}\). The current in the secondary coil (neglect losses) is:

1. \(5\text{ A}\)

2. \(50\text{ A}\)

3. \(500\text{ A}\)

4. \(0.05\text{ A}\)

View Answer

In an ideal transformer, input power equals output power: \(V_p I_p = V_s I_s\). Therefore, \(I_s = \frac{V_p I_p}{V_s} = \frac{220 \times 5}{22000} = 0.05\text{ A}\).

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Question 3: easy

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

1. 4 A

2. 0.2 A

3. 0.4 A

4. 2 A

View Answer

Assuming zero power losses, the input power in the primary circuit equals the output power in the secondary circuit: \(P_{\text{in}} = P_{\text{out}} = 44\text{ W}\). Since \(P_{\text{in}} = V_p I_p\), we get \(I_p = \frac{P_{\text{in}}}{V_p} = \frac{44}{220} = 0.2\text{ A}\).

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Question 4: easy

In an ideal transformer, the turns ratio is \(\frac{N_p}{N_s} = \frac{1}{3}\). The ratio of \(I_s : I_p\) is equal to (symbols carry their usual meaning):

1. 1 : 3

2. 1 : 1

3. 9 : 1

4. 2 : 1

View Answer

For an ideal transformer, input power equals output power: \(V_p I_p = V_s I_s ⇒ \frac{I_s}{I_p} = \frac{V_p}{V_s} = \frac{N_p}{N_s} = \frac{1}{3}\).

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Question 5: easy

The ratio of secondary to primary turns in a transformer is 4 : 1. If the power input is P, then output power neglecting all losses must be equal to

1. \(\frac{P}{4}\)

2. 4P

3. P

4. \(\frac{2P}{3}\)

View Answer

In an ideal transformer, there is no energy loss. Therefore, the output power is equal to the input power, which is P.

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Question 6: easy

Assertion (A): A transformer cannot work on D.C. supply.

Reason (R): D.C. changes neither in magnitude nor in direction.

1. Both A & R are true and the (R) is the correct explanation of the (A)

2. Both A & R are true but the (R) is not the correct explanation of the (A)

3. (A) is true but (R) is false

4. Both (A) and (R) are false

View Answer

Transformers rely on mutual induction, which requires a changing magnetic flux. DC current, being constant (R), produces a steady flux, thus no induced EMF. Both A and R are true, and R correctly explains A.

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Question 7: easy

Assertion (A): If we use a battery across the primary of a step up transformer, no voltage is obtained across secondary.

Reason (R): Battery gives a steady current.

1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)

2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)

3. (A) is true but (R) is false

4. Both (A) and (R) are false

View Answer

Assertion (A) is true. Transformers require a changing magnetic flux to induce voltage. Reason (R) is true; a battery provides DC current. This steady current causes constant flux, hence no induced emf in the secondary. (R) correctly explains (A).

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