CBSE Class 12 Physics Alternating Current Assignment

Question. If the rms current in a 50 Hz ac circuit is 5 A, the value of the current 1/300 seconds after its value becomes zero is
(a) 5√2 A
(b) 5(√3/2)A
(c) 5/6 A
(d) 5/√2 A

Answer: B

Question. An alternating current generator has an internal resistance R_g and an internal reactance X_g. It is used to supply power to a passive load consisting of a resistance R_g and a reactance X_L. For maximum power to be delivered from the generator to the load, the value of X_L is equal to
(a) zero
(b) X_g
(c) – X_g
(d) R_g

Answer: C

Question. When a voltage measuring device is connected to ac mains, the meter shows the steady input voltage of 220 V. This means
(a) input voltage cannot be ac voltage, but a dc voltage.
(b) maximum input voltage is 220 V.
(c) the meter reads not V but <V²> and is calibrated to read √< V² > .
(d) the pointer of the meter is stuck by some mechanical defect.

Answer: C

Question. To reduce the resonant frequency in an LCR series circuit with a generator
(a) the generator frequency should be reduced.
(b) another capacitor should be added in parallel to the first.
(c) the iron core of the inductor should be removed.
(d) dielectric in the capacitor should be removed.

Answer: B

Question. In a pure capacitive circuit, the current
(a) lags behind the applied emf by angle π/2
(b) leads the applied emf by an angle π
(c) leads the applied emf by angle π/2
(d) and applied emf are in same phase

Answer: C

Question. In an ac circuit, the emf (ε) and the current (i) at any instant are given by f = E₀ sin ωt,  i= I₀ sin (ωt – Φ)
Then average power transferred to the circuit in one complete cycle of ac is
(a) E₀ I₀
(b) (1/2)E₀ I₀
(c) (1/2)E₀ I₀ sin Φ
(d) (1/2)E₀ I₀ cos Φ

Answer: D

Question. In an ac circuit, the maximum value of voltage is 423 volts. Its effective voltage is
(a) 400 volt
(b) 300 volt
(c) 323 volt
(d) 340 volt

Answer: B

Question. The peak voltage of 220 V ac mains is
(a) 155.6 V
(b) 220.0 V
(c) 311 V
(d) 440 V

Answer: C

Question. An inductive circuit have zero resistance. When ac voltage is applied across this circuit, then the current lags behind the applied voltage by an angle
(a) 30°
(b) 45°
(c) 90°
(d) 0°

Answer: C

Question. If an LCR circuit contains L = 8 henry; C = 0.5 mF, R = 100 Ω in series. Then the resonant angular frequency will be:
(a) 600 rad/s
(b) 500 rad/s
(c) 600 Hz
(d) 500 Hz

Answer: B

Question. The average power dissipation in pure inductance is
(a) (1/2) LI²
(b) (1/4)LI²
(c) 2LI²
(d) zero

Answer: B

Question. In an ac circuit, voltage V and current i are given by
V = 100 sin 100 t volt
i = 100 sin (100t + π/3) mA
The power dissipated in the circuit is
(a) 104W
(b) 10 W
(c) 2.5 W
(d) 5 W.

Answer: C

Question. Which of the following combinations should be selected for better tuning of an LCR circuit used for communication?
(a) R = 20 Ω, L = 1.5 H, C = 35 μF
(b) R = 25 Ω, L = 2.5 H, C = 45 μF
(c) R = 15 Ω, L = 3.5 H, C = 30 μF
(d) R = 25 Ω, L = 1.5 H, C = 45 μF

Answer: C

Question. Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is (are) correct?
(a) For a given power level, there is a lower current.
(b) Lower current implies less power loss.
(c) It is easy to reduce the voltage at the receiving end using step-down transformers.
(d) All of the options

Answer: D

Question. The reactance of a capacitance at 50 Hz is 5 Ω. If the frequency is increased to 100 Hz, the new reactance is
(a) 5 Ω
(b) 10 Ω
(c) 2.5 Ω
(d) 125 Ω

Answer: C

Question. In a pure inductive circuit, the current
(a) lags behind the applied emf by an angle p
(b) lags behind the applied emf by an angle p / 2
(c) leads the applied emf by an angle p / 2
(d) and applied emf are in same phase

Answer: B

Question. When an ac voltage of 220 V is applied to the capacitor C
(a) the maximum voltage between plates is 220 V.
(b) the current is in phase with the applied voltage.
(c) the charge on the plates is in phase with the applied voltage.
(d) power delivered to the capacitor is zero.

Answer: C, D

Question. An inductor of reactance 1 Ω and a resistor of 2 Ω are connected in series to the terminals of a 6 V (rms) ac source. The power dissipated in the circuit is
(a) 8 W
(b) 12 W
(c) 14.4 W
(d) 18 W

Answer: C

Question. The potential differences across the resistance, capacitance and inductance are 80 V, 40 V and 100 V respectively in an L-C-R circuit, the power factor for this circuit is
(a) 0.4
(b) 0.5
(c) 0.8
(d) 1.0

Answer: C

Question. The output of a step-down transformer is measured to be 24 V when connected to a 12 watt light bulb. The value of the peak current is
(a) 1/√2 A
(b) √2 A
(c) 2 A
(d) 2√2 A

Answer: A

Fill in the Blanks

Question. The average power supplied to an inductor over one complete cycle is _______.
Answer: zero

Question. The inductive reactance is directly proportional to the inductance and to the _______ of the circuit.
Answer: frequency

Question. The capacitive reactance limits the _______ in a purely capacitive circuit in the same way as the resistance limits the current in a purely resistive circuit.
Answer: amplitude of the current

4. The phenomenon of resistance is common among systems that have a tendency to oscillate at a particular frequency. This frequency is called the system’s _______.
Answer: natural frequency

Question. The quantity 2 ω₀/2Δω is regarded as a measure of the _______.
Answer: sharpness of resonance

Question. The average power dissipated depends not only on the voltage and current but also on the _______ of the phase angle φ between them.
Answer: cosine

Question. For many purposes, it is necessary to change an alternating voltage from one to another of greater or smaller value. This is done with a device called _______ using the principle of mutual induction.
Answer: transformer

Question. In an ac circuit, containing pure resistance, the voltage and current are in _______ phase.
Answer: same

Question. In a pure inductive circuit current _______ the voltage by a phase angle of π/2
Answer: lags

Question. In a pure capacitive circuit, the current _______ the voltage by a phase angle of π/2
Answer: leads