Chapter 7 Alternating Current Chapter assertation & reasoning Questions class 12th physics

Assertion-Reasoning Based MCQ

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1. Both assertion and reason are true and reason is the correct explanation of assertion.
2. Both assertion and reason are true but reason is not the correct explanation of assertion.  
3. Assertion is true but reason is false.
4. Assertion is false but reason is true. 

1. Assertion AC is more dangerous in use than DC

Reason It is because the peak value of AC is greater than indicated value

2. Assertion Average value of AC over a complete cycle is always zero

Reason average value of AC is always defined over half cycle

3. Assertion The alternating current lags behind the EMF by a phase angle of when AC flows through and inductor

Reason The inductive reactance increases as the frequency of AC source decreases

4. Assertion Capacitor serves as a block for DC and offers an easy path to AC

Reason Capacitive reactance is inversely proportional to frequency

5. Assertion In series LCR resonant circuit the impedance is equal to the ohmic resistance

Reason At resonance the inductive reactance exceeds the capacitive reactance

6. Assertion An alternating current shows magnetic effect

Reason Alternating current varies with time

7. Assertion In series LCR circuit resonance can take place

Reason Resonance takes place in inductance and capacitive reactance are equal and opposite

8. Assertion Power factor correction is must in heavy machinery

Reason A low power factor implies larger power loss in transmission

9. Assertion Choke coil is preferred over a registered to adjust current in an AC circuit

Reason Power factor for inductance is zero

10. Assertion When AC circuit containing resistor only its power is minimum

Reason Power of a circuit is independent of phase angle

11. Assertion A transformer cannot work on DC supply

Reason DC change is neither in magnitude nor in direction

12. Assertion A laminated core is used in transformer to increase eddy currents

Reason The efficiency of a transformer increases with increase in eddy currents

13. Assertion Soft iron is used as a core of transformer

Reason Area of hysteresis loop for soft iron is small

14. Assertion An AC generator is based on the phenomenon of electromagnetic induction

Reason In single coil we consider self induction only 

Assertion-Reasoning Based MCQ Answers

1. (1)

AC is more dangerous in use than DC. It is because the peak value of AC is greater than the indicated value.

2. (2)

The mean or average value of alternating current or EMF during half cycle is given by

I_m = 0.636 I_o
E_m = 0.6363 E_o

During the next half cycle, the mean value of AC will be equal in magnitude but opposite in direction. For this reason the average value of AC over a complete cycle is always zero. So the average value is always defined over a half cycle of AC.

3. (3)

When AC flows through an inductor current lags behind the EMF, by phase of π/2 inductive reactance

XL = ωL = 2πfL

So, when frequency increases correspondingly inductive reactance also increases.

4. (1)

The capacitive reactance of capacitor is given by

XC = 1/ ωC = 1/2πfC

So this is infinite for DC and has a very small value for AC. Hence, a capacitor blocks DC.

5. (3)

In series resonance circuit inductive reactance is equal to capacitive reactance.

ωL = 1/ωC

6. (2)

Like direct current, an AC also produces magnetic field. But the magnitude and direction of the field goes on changing continuously with time.

7. (1)

At resonant frequency,

X_L = X_C, Z = R (minimum)

8. (2)

A heavy machinery requires a large power.

The average power is given by,

P_av = E_rmsI_rmscosΦ

The required power can be supplied to the heavy machinery either by supplying larger current or by improving power factor. The first method is costly. Hence, the second one is used.

9. (1)

We can use a capacitor of suitable capacitance as a choke coil, because average power consumed per cycle in an ideal capacitor is zero. Therefore, like a choke coil a condenser can reduce AC without power dissipation.

10. (4)

The power of an AC circuit is given by,

P = EIcosΦ

Where cosΦ is a power factor and is Φ phase angle. In case of circuit containing resistance only, phase angle is zero and power factor is equal to 1. Therefore power is maximum in case of circuit containing resistor only.

11. (1)

Transformer works on AC only AC changes in magnitude as well as in direction and induced EMF.

12. (4)

Large eddy currents are produced in non laminated iron core of the transformer by induced EMF, as the resistance of bulk iron core is very small. By using thin iron sheets are score the resistance is increased. Laminating the core substantially reduces the eddy currents. Eddy currents heat up the core of the transformer. More the eddy current greater the loss of energy and efficiency goes down.

13. (1)

Hysteresis loss in the core of transformer is directly proportional to the hysteresis loop area of the core material. Since soft iron has narrow hysteresis loop area, that is why soft iron core is used in transformer.

14. (2)

According to electromagnetic induction, whenever the magnetic flux changes and EMF will be induced in the coil.