Alternating Current Easy Questions and Answers | Page - 3

Alternating Current

The primary and secondary coil of a transformer have 50 and 1500 turns respectively. If the magnetic flux φ linked with the primary coil is given by φ = φ₀ + 4t, where φ is in webers, t is time in seconds and φ0 is a constant, the output voltage across the secondary coil is

1. 120 volts
1. 220 volts
1. 30 volts
1. 90 volts

View Hint View Answer Discuss in Forum

Since V_s = N_s
V_P N_p

Where N_s = No. of turns across primary coil = 50
N_p = No. of turns across secondary coil = 1500
and V_p = dφ = d (φ₀ + 4t) = 4
dt dt

⇒ V_s = 1500 × 4 = 120 V
50

Correct Option: A

Since V_s = N_s
V_P N_p

Where N_s = No. of turns across primary coil = 50
N_p = No. of turns across secondary coil = 1500
and V_p = dφ = d (φ₀ + 4t) = 4
dt dt

⇒ V_s = 1500 × 4 = 120 V
50

The core of a transformer is laminated because

1. the weight of the transformer may be reduced
1. rusting of the core may be prevented
1. ratio of voltage in primary and secondary may be increased
1. energy losses due to eddy currents may be minimised

View Hint View Answer Discuss in Forum

When there is change of flux in the core of a transformer due to change in current around it, eddy current is produced. The direction of this current is opposite to the current which produces it, so it will reduce the main current. We laminate the core so that flux is reduced resulting in the reduced production of eddy current.

Correct Option: D

When there is change of flux in the core of a transformer due to change in current around it, eddy current is produced. The direction of this current is opposite to the current which produces it, so it will reduce the main current. We laminate the core so that flux is reduced resulting in the reduced production of eddy current.

A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 amp, the efficiency of the transformer is approximately

1. 50%
1. 90%
1. 10%
1. 30%.

View Hint View Answer Discuss in Forum

Efficiency of the transformer
η = P_output × 100 = 100 × 100 = 90.9%
P_input 220 × 0.5

Correct Option: B

Efficiency of the transformer
η = P_output × 100 = 100 × 100 = 90.9%
P_input 220 × 0.5

A transistor-oscillator using a resonant circuit with an inductor L (of negligible resistance) and a capacitor C in series produce oscillations of frequency f. If L is doubled and C is changed to 4C, the frequency will be

1. 8f
1. f/2√2
1. f/2
1. f/4

View Hint View Answer Discuss in Forum

We know that frequency of electrical oscillation in L.C. circuit is
ƒ = (1/2π)√(1/LC)
Now, L = 2L & C = 4C
ƒ' = (1/2π)√(1/2L.4C) = (1/2π) √(1/LC) × (1/2√2)
⇒ ƒ' = 1 × ƒ
2√2

Correct Option: B

We know that frequency of electrical oscillation in L.C. circuit is
ƒ = (1/2π)√(1/LC)
Now, L = 2L & C = 4C
ƒ' = (1/2π)√(1/2L.4C) = (1/2π) √(1/LC) × (1/2√2)
⇒ ƒ' = 1 × ƒ
2√2

A 220 volts input is supplied to a transformer. The output circuit draws a current of 2.0 ampere at 440 volts. If the efficiency of the transformer is 80%, the current drawn by the primary windings of the transformer is

1. 3.6 ampere
1. 2.8 ampere
1. 2.5 ampere
1. 5.0 ampere

View Hint View Answer Discuss in Forum

V₂ = 0.8 I₁
V₁ I₂

⇒ V₂I₂ = 0.8
V₁I₁

V₁ = 220 V, I₂ = 2.0 A, V₂ = 440 V
I₁ = V₂I₂ × 10 = 440 × 2 × 10 = 5 A
V₁ 8 220 × 8

Correct Option: D

V₂ = 0.8 I₁
V₁ I₂

⇒ V₂I₂ = 0.8
V₁I₁

V₁ = 220 V, I₂ = 2.0 A, V₂ = 440 V
I₁ = V₂I₂ × 10 = 440 × 2 × 10 = 5 A
V₁ 8 220 × 8