## Alternating Current Objective

**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

**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

**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

Answer: (c) halved

**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

**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

**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°

Answer: (d) 0°

**At resonance frequency the impedance in series LCR circuit is**

(a) maximum

(b) minimum

(c) zero

(d) infinity

Answer: (b) minimum

**At resonant frequency the current amplitude in series LCR circuit is**

(a) maximum

(b) minimum

(c) zero

(d) infinity

Answer: (a) maximum

**Quality factor and power factor both have the dimensions of**

(a) time

(b) frequency

(c) work

(d) angle

Answer: (d) angle

**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

**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

**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

**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

Answer: (A) cosθ

**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

**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

**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

**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

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

(A) 600 radian/second

(B) 500 radian/second

(C) 600 hertz

(D) 500 hertz

Answer: (B) 500 radian/second

**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

**Step-down transformer increases**

(A) current

(B) voltage

(C) wattage

(D) None of these

Answer: (A) current

**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

**The power factor of a choke coil is about**

(A) 90°

(B) 0

(C) 1

(D) 180°

Answer: (B) 0

**The power factor of a condenser is**

(A) 90°

(B) 1

(C) 180°

(D) 0

Answer: (D) 0

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

(A) DC

(B) AC

(C) both DC and AC

(D) None of these

Answer: (A) DC

**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

**The unit of Impedance is**

(A) henry

(B) ohm

(C) tesla

(D) None of these

Answer: (B) ohm

**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

**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

Answer: (A) zero

**The unit of reactance is**

(A) ohm

(B) farad

(C) ampere

(D) mho

Answer: (A) ohm

**The unit of capacitive reactance is**

(A) farad

(B) ohm

(C) maxwell

(D) ampere

Answer: (B) ohm