# MCQ Questions for Class 12 Physics Chapter 7 Alternating Current with Answers

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## Alternating Current Class 12 MCQs Questions with Answers

Question 1.
Ohm’s law expressed as E = IR.
(a) may never be applied to a.c. circuits
(b) always applied to a.c. circuits as to d.c. circuits when Z is substituted for R.
(c) tells us that Eeff = 0.707 E0 for a.c.
(d) None of these.

Answer: (b) always applied to a.c. circuits as to d.c. circuits when Z is substituted for R.

Question 2.
The capacitive reactance in an a.c. circuit is:
(a) effective wattage
(b) effective voltage
(c) effective resistance due to capacity
(d) None of these.

Answer: (c) effective resistance due to capacity

Question 3.
Of the following about the capacitive reactance which is correct:
(a) capacitive reactance is inversely proportional to the frequency of a.c.
(b) capacitive reactance is measured in Farad.
(c) The reactance of the capacitor is directly proportional to its ability to store charge.
(d) None of these.

Answer: (a) capacitive reactance is inversely proportional to the frequency of a.c.

Question 4.
With increase in frequency of applied a.c. supply, the impedance of LCR series circuit:
(a) increases
(b) decreases
(c) first decreases, becomes minimum and then increases.
(d) remains constant.

Answer: (c) first decreases, becomes minimum and then increases.

Question 5.
The current in an LCR series circuit when frequency of applied a.c. supply is increased:
(a) increases
(b) decreases
(c) first increases becomes maximum and then decreases.
(d) first decreases becomes minimum and then increases.

Answer: (c) first increases becomes maximum and then decreases.

Question 6.
When 100 V d.c. applied across a solenoid a current of 1.0 A flows in it. When 100 V a.c. supply is applied across the same coil, the current drops to 0.5 A. If the frequency of a.c. source is 50 Hz, the impedance and inductance of the solenoid are:
(a) 200 Ω and 0.55 H
(b) 200 Ω and 1.0 H
(c) 100 Ω and 0.93 H
(d) 100 Ω and 0.80 H

Answer: (a) 200 Ω and 0.55 H

Question 7.
In an a.c. circuit, the current
(b) always lags behind the voltage
(c) is always in phase with the voltage
(d) may lead or lag behind or be in the same phase with the voltage.

Answer: (d) may lead or lag behind or be in the same phase with the voltage.

Question 8.
For LCR series circuit, the current will be maximum when:
(a) XL = 0
(b) XC = O
(c) XL = XC
(d) R = XL + XC.

Question 9.
An alternating voltage is connected in series with a resistance R and an inductance L. If the potential drop across the resistance is 200 V and across the inductance is 150 V, then the applied voltage is:
(a) 350 V
(b) 500 V
(c) 250 V
(d) 300 V

Question 10.
In an a.c. circuit, the instantaneous values of current and e.m.f. are given by: I = I0 sin(ωt +$$\frac {π}{2}$$) and E = E0 sin ωt. The power P consumed in the circuit is:
(a) $$\frac {E_0I_0}{√2}$$
(b) EI √2
(c) Zero
(d) $$\frac {E_0I_0}{2}$$

Question 11.
Which of the following does not decrease the efficiency of a transformer:
(a) laminating the core
(b) use of iron core
(c) core made of material having narrow hysteresis loop
(d) All of the above.

Answer: (d) All of the above.

Question 12.
A choke coil should have:
(a) low resistance and high inductance
(b) low resistance and low inductance
(c) high resistance and high inductance
(d) high resistance and low inductance.

Answer: (a) low resistance and high inductance

Question 13.
Hot wire ammeters are used for measuring:
(a) a.c. only
(b) d.c. only
(c) neither a.c. nor d.c.
(d) both a.c. and d.c.

Answer: (d) both a.c. and d.c.

Question 14.
Transformers are used to
(a) step up d.c. voltage
(b) step up or step down a.c. voltage
(c) converts a.c. into d.c.
(d) converts d.c. in to a.c.

Answer: (b) step up or step down a.c. voltage

Question 15.
Which of the following phenomenon is helpful in the working f a choke coil?
(a) self induction
(b) mutual induction
(c) Eddy currents
(d) none of these.

Question 16.
For long distance transmission, the a.c. is stepped up because transmission at high voltage is:
(a) economical
(b) faster
(c) not dangerous
(d) not damped.

Question 17.
An electric bulb rated 220 V is connected to 220 V, 5 Hz A.C. source. Then the bulb:
(a) glows continuously
(b) does not glow
(c) glows infermittantly
(d) fuses.

Fill in the Blanks

Question 1.
In an a.c. circuit, the power is dissipated only due to ……………….. Thus in an a.c. circuit containing inductance coil, the power is dissipated Only due to the ……………….. of the coil and not due to its ………………..

Answer: Ohmic resistance, ohmic resistance, inductance.

Question 2.
The frequency of a.c. supplied to our houses in India is ………………..

Question 3.
The power factor of a purely resistive a.c. circuit is ……………….. while E and I are in ……………….. in it.

Question 4.
In a series LCR circuit, the phase difference between the current flowing in the inductor and capacitor is ………………..

Question 5.
In the parallel resonant circuit, the current and the voltage are in ……………….. phase at resonance while in series resonant circuit, the current and voltage are in ……………….. phase.

Question 6.
In an a.c. circuit, the voltage and current are represented by
V = 200 sin (100 π t)V
I = 25 sin (100 π t + $$\frac {π}{3}$$)A
The impedence of the circuit is ……………….. and average power consumed is ……………….. watt.

Question 7.
An alternating current circuit consists of an ohmic resistance only. If the frequency of the source is increased first and then decreased, then the current flowing in the circuit will ………………..

Question 8.
A ……………….. is a device that controls the alternating current without much loss of electrical energy in the form of heat.

Question 9.
If the frequency of alternating current is doubled then the inductive reactance of the circuit is ……………….. while capacitive reactance is ………………..

Question 10.
The core of a transformer is made of a magnetic material of ………………..

Question 11.
………………. measures the sharpness or the selectivity of a resonant circuit.

Question 12.
The current due to the flow of which no power is consumed in an a.c. circuit is called ………………..

Question 13
……………….. is the reciprocal of the impedance of ah a.c. circuit.

Question 14.
A ……………….. behaves as a conductor at very high frequency of a car it offers no resistance to the flow of a.c.

Question 15.
The dissipation of energy is much reduced if the transmission of electric energy is done at ……………….. and ………………..

Answer: high voltage and low current.

Question 16
……………….. is produced due to the repeated magnetisation and demagnetisation of the iron core in a transformer.

Question 17.
The power factor in a circuit connected to a.c. power supply is unity when the circuit contains ………………..