Alternating Current
- 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
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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
- The core of a transformer is laminated because
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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
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Efficiency of the transformer
η = Poutput × 100 = 100 × 100 = 90.9% Pinput 220 × 0.5 Correct Option: B
Efficiency of the transformer
η = Poutput × 100 = 100 × 100 = 90.9% Pinput 220 × 0.5
- 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
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Since Vs = Ns VP Np
Where Ns = No. of turns across primary coil = 50
Np = No. of turns across secondary coil = 1500and Vp = dφ = d (φ0 + 4t) = 4 dt dt ⇒ Vs = 1500 × 4 = 120 V 50 Correct Option: A
Since Vs = Ns VP Np
Where Ns = No. of turns across primary coil = 50
Np = No. of turns across secondary coil = 1500and Vp = dφ = d (φ0 + 4t) = 4 dt dt ⇒ Vs = 1500 × 4 = 120 V 50
- 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
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V2 = 0.8 I1 V1 I2 ⇒ V2I2 = 0.8 V1I1
V1 = 220 V, I2 = 2.0 A, V2 = 440 VI1 = 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 VI1 = V2I2 × 10 = 440 × 2 × 10 = 5 A V1 8 220 × 8