Alternating Current(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

ALTERNATING CURRENT MCQs

Total Questions : 30 | Page 1 of 3 pages

Question 1. A bulb and a capacitor are in series with an ac source. On increasing frequency how will glow of the bulb change

The glow decreases
The glow increases
The glow remain the same
The bulb quenches

Discuss Question

Answer: Option B. -> The glow increases
:
B
The glow increases because, when frequency vis increased, the capacitive reactance

X_{c} = \frac{1}{2 π v C}

decreases and hence the current through the bulb increases which will lead to increase in glow

Question 2. If power factor is

\frac{1}{2}

in a series RL circuit R = 100

Ω

. AC mains is used then L is

√3π Henry
π Henry
π√3 Henry
None of these

Discuss Question

Answer: Option A. -> √3π Henry
:
A

c o s ϕ = \frac{1}{2} \Rightarrow ϕ = 60^{\circ}, t a n 60^{\circ} = \frac{ω L}{R} \Rightarrow L = \frac{\sqrt{3}}{π}

Question 3. The reading of ammeter in the circuit shown will be

2A
2.4 A
Zero
1.7 A

Discuss Question

Answer: Option A. -> 2A
:
A
Given

X_{L} = X_{C} = 5 Ω

this is the condition of resonance. Since

V_{L} = V_{C}

net voltage across L and C combination will be zero.
Voltage drop across the resistance = 110v. Thus current flowing though Ammeter reading =

\frac{110 v}{55} = 2 A

Question 4. In the adjoining figure the impedance of the circuit will be

120 ohm
50 ohm
60 ohm
45 ohm

Discuss Question

Answer: Option D. -> 45 ohm
:
D

i_{L} = \frac{90}{30} = 3 A,

i_{c} = \frac{90}{90} = 1 A

Net current through circuit

i = i_{c} - i_{L} = 2 A

∴ Z = \frac{V}{i} = \frac{90}{2} = 45 Ω

Question 5. A resistor R, an inductor L and a capacitor C are connected in series to an oscillator of frequency n. If the resonant frequency is , then the current lags behind voltage, when

n = 0
n
n= nr
n> nr

Discuss Question

Answer: Option D. -> n> nr
:
D
The current will lag behind the voltage when reactance of inductance is more than the reactance of condenser. Thus,

ω L > \frac{1}{w C}

ω > \frac{1}{\sqrt{L C}}

n > \frac{1}{2 π \sqrt{L C}}

n > n_{r}

where

n_{r}

= resonant frequency

Question 6. Following figure shows an ac generator connected to a "block box" through a pair of terminals. The box contains possible R, L, C or their combination, whose elements and arrangements are not known to us. Measurements outside the box reveals that e = 75 sin (sin

ω

t) volt, i = 1.5 sin

(ω t + 45^{\circ})

amp then, the wrong statement is

Following Figure Shows An Ac Generator Connected To A

There must be a capacitor in the box
There must be no inductor in the box
There must be a resistance in the box
The power factor is 0.707

Discuss Question

Answer: Option B. -> There must be no inductor in the box
:
B
Since voltage is lagging behind the current, so there must be no inductor in the box.

Question 7. The variation of the instantaneous current (I) and the instantaneous emf (E) in a circuit is as shown in fig. Which of the following statements is correct