# Physics class 12 chapter 7 Objective Questions

## Alternating Current Objective

1. Alternating voltage (V) is represented by the equation

(a) V(t) = Vm eωt

(b) V(t) = Vm sin ωt

(c) V(t) = Vm cot ωt

(d) V(t) = Vm tan ωt

Answer: (b) V(t) = Vm sin ωt

1. In the case of an inductor

(a) voltage lags the current by π/2

(b) voltage leads the current by π/2

(c) voltage leads the current by π/3

(d) voltage leads the current by π/4

Answer: (b) voltage leads the current by π/2

1. In a pure capacitive circuit if the frequency of ac source is doubled, then its capacitive reactance will be

(a) remains same

(b) doubled

(c) halved

(d) zero

1. In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?

(a) Only resistor

(b) Resistor and inductor

(c) Resistor and capacitor

(d) Only inductor

Answer: (c) Resistor and capacitor

1. In which of the following circuits the maximum power dissipation is observed?

(a) Pure capacitive circuit

(b) Pure inductive circuit

(c) Pure resistive circuit

(d) None of these

Answer: (c) Pure resistive circuit

1. In a series LCR circuit the voltage across an inductor, capacitor and resistor are 20 V, 20 V and 40 V respectively. The phase difference between the applied voltage and the current in the circuit is

(a) 30°

(b) 45°

(c) 60°

(d) 0°

1. At resonance frequency the impedance in series LCR circuit is

(a) maximum

(b) minimum

(c) zero

(d) infinity

1. At resonant frequency the current amplitude in series LCR circuit is

(a) maximum

(b) minimum

(c) zero

(d) infinity

1. Quality factor and power factor both have the dimensions of

(a) time

(b) frequency

(c) work

(d) angle

1. A transformer works on the principle of

(a) self induction

(b) electrical inertia

(c) mutual induction

(d) magnetic effect of the electrical current

Answer: (c) mutual induction

1. For an ideal-step-down transformer, the quantity which is constant for both the coils is

(a) current in the coils

(b) voltage across the coils

(c) resistance of coils

(d) power in the coils

Answer: (d) power in the coils

1. In an LCR series ac circuit, the voltage across each of the components L, C and R is 50 Volt. The voltage across LC combination is :

(A) 50 Volt

(B) 25 Volt

(C) 100 Volt

(D) 0 Volt

Answer: (D) 0 Volt

1. The phase difference between current and voltage in an AC circuit is θ. Then power factor will be:

(A) cosθ

(B) sinθ

(C) tanθ

(D) 1 θ

1. The working principle of choke coil is based on

(A) Conservation of angular momentum

(B) Self induction

(C) Mutual induction

(D) Conservation of momentum

Answer: (B) Self induction

1. In a step up transformer the number of coils in primary coils is N1 and N2 in secondary. Then :-

(A) N1 = N2

(B) N1 < N2

(C) N1 > N2

(D) None of these

Answer: (B) N1 < N2

1. The device that increases AC voltage is called –

(A) resistor

(B) step-down transformer

(C) step-up transformer

(D) transformer

Answer: (C) step-up transformer

1. The oscillation frequency of an LC circuit is f. If both the capacitance and inductance be doubled, the frequency will become –

(A) f / 4

(B) 2f

(C) 4f

(D) f / 2

Answer: (D) f / 2

1. In LCR-circuit, L = 8.0 H, C = 0.5 μF, and R = 100 Ω are connected in series. The resonant frequency will be

(C) 600 hertz

(D) 500 hertz

1. A choke coil is used to control current in a circuit of

(A) ac only

(B) dc only

(C) both ac and dc

(D) None of these

Answer: (A) ac only

1. Step-down transformer increases

(A) current

(B) voltage

(C) wattage

(D) None of these

1. The thermal effect of alternating current is practically

(A) Joule heating

(B) Peltier heating

(C) Thomson effect

(D) None of these

Answer: (A) Joule heating

1. The power factor of a choke coil is about

(A) 90°

(B) 0

(C) 1

(D) 180°

1. The power factor of a condenser is

(A) 90°

(B) 1

(C) 180°

(D) 0

1. For which of the following, capacitor works as infinite resistance ?

(A) DC

(B) AC

(C) both DC and AC

(D) None of these

1. L-C circuit without AC-source is called

(A) oscillatory circuit

(B) drift circuit

(C) hysteresis circuit

(D) None of these

Answer: (A) oscillatory circuit

1. The unit of Impedance is

(A) henry

(B) ohm

(C) tesla

(D) None of these

1. The equation of an AC is I = 60sin100πt. The root mean square value will be

(A) 60√2

(B) 60 / √2

(C) 100

(D) zero

Answer: (B) 60 / √2

1. The value of phase difference between current and emf in an AC circuit with resistance only, is

(A) zero

(B) π / 2

(C) π

(D) 2π

1. The unit of reactance is

(A) ohm

(C) ampere

(D) mho