Assertion Reason Physics Chapter 7 Alternating Current

Questions of Assertion Reason of Physics Chapter 7 Alternating Current CBSE Class 12 are very simple to understand as this chapter deals with Alternating Current. To solve assertion reason type questions of Physics, a deeper understanding of concept about AC Voltage Applied to a Resistor, Representation of AC Current and Voltage by Rotating Vectors — Phasors, AC Voltage Applied to an Inductor, AC Voltage Applied to a Capacitor, AC Voltage Applied to a Series LCR Circuit, Power in AC Circuit: The Power Factor, and Transformers is required. In order to help the student in solving Assertion Reason type of questions, detailed process is explained.

How to solve Assertion Reason Type Questions?
In Assertion-Reason type of question, two statements are given, first is Assertion and second is called Reason. Student must have to think critically about both the statements in Assertion Reason Questions, since it combines multiple choice questions and true/false type of questions which requires a higher level of understanding.

How many types are there of Assertion Reason Type Questions?
Assertion-Reason type of questions can be asked either with four MCQ options or with five MCQ options. First four options are same in both the cases only one more options is being provided i.e. last one. One extra option increases the difficulty level of the questions.

In assertion reason type of questions, all 4 or 5 options are same for each question, which are as:

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.
(e) Both Assertion and Reason are false.

Then a question haunts in every student’s mind, which option is correct and when?
This can be understood clearly with the following table:

Assertion (A)Reason (R)MCQ Options (Fixed for all questions)
If TrueTrue(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
If TrueTrue(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
If TrueFalse(c) Assertion (A) is true but reason (R) is false.
If FalseTrue(d) Assertion (A) is false but reason (R) is true.
If FalseFalse(e) Both Assertion and Reason are false.

Now, lets practice some Assertion Reason Questions of Physics: Chapter 7 Alternating Current

Read instructions carefully before answering the questions.

For question given below, two statements are given- one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:

(a) Both A and R are true and R is correct explanation of the assertion.
(b) Both A and R are true but R is not the correct explanation of the assertion.
(c) A is true but R is false.
(d) A is false but R is true.

Question.1.
Assertion: In series LCR resonance circuit, the impedance is equal to the ohmic resistance.
Reason: At resonance, the inductive reactance exceeds the capacitive reactance.

Ans.1. (c)
A is true but R is false.

Question.2.
Assertion: A capacitor is connected to a direct current source. Its reactance is infinite.
Reason: Reactance of a capacitor is given by \chi_{c}=\frac{1}{\omega C} .

Ans.2. (a)
Both A and R are true and R is correct explanation of the assertion.

Question.3.
Assertion: Average value of ac over a complete cycle is always zero.
Reason: Average value of ac is always defined over half cycle.

Ans.3. (b)
Both A and R are true but R is not the correct explanation of the assertion.

Question.4.
Assertion: The alternating current lags behind the emf by a phase angle of, π/2 when AC flows through an inductor.
Reason: The inductive reactance increases as the frequency of AC source increases.

Ans.4. (b)
Both A and R are true but R is not the correct explanation of the assertion.

Question.5.
Assertion: The inductive reactance limits amplitude of the current in a purely inductive circuit.
Reason: The inductive reactance is independent of the frequency of the current.

Ans.5. (c)
A is true but R is false.

Question.6.
Assertion: The power is produced when a transformer steps up the voltage.
Reason: In an ideal transformer VI = constant.

Ans.6. (a)
Transformer cannot produce power, but it transfer from primary to secondary.

Question.7.
Assertion: A capacitor blocks direct current in the steady state.
Reason: The capacitive reactance of the capacitor is inversely proportional to frequency f of the source of emf.

Ans.7. (b)
Both A and R are true but R is not the correct explanation of the assertion.

Question.8.
Assertion: The voltage and current in a series AC circuit are given by V=V_{o} \sin \omega t    and I=I_{o}\cos \omega t . The power dissipated in the circuit is zero.
Reason: Power in AC circuit is given by P=V_{o}I_{o}\cos \left(\frac{\phi}{2}\right) .

Ans.8. (a)
Both A and R are true and R is correct explanation of the assertion.

Question.9.
Assertion: In a purely inductive or capacitive circuit, the current is referred to as wattless current.
Reason: No power is dissipated in a purely inductive or capacitive circuit even though a current is flowing in the circuit.

Ans.9. (a)
Both A and R are true and R is correct explanation of the assertion.

Question.10.
Assertion: The power in an ac circuit is minimum if the circuit has only a resistor.
Reason: Power of a circuit is independent of the phase angle.

Ans.10. (d)
Assertion is incorrect, reason is correct.

Question.11.
Assertion: When the frequency of the AC source in an LCR circuit equals the resonant frequency, the reactance of the circuit is zero, and so there is no current through the inductor or the capacitor.
Reason: The net current in the inductor and capacitor is zero.

Ans.11. (d)
The currents in capacitor and in inductor are opposite and so net current is zero.

Question.12.
Assertion: A laminated core is used in transformers to increase eddy currents.
Reason: The efficiency of a transformer increases with increase in eddy currents.

Ans.12. (d)
Large eddy currents are produced in non-laminated iron core of the transformer by the induced emf, as the resistance of bulk iron core is very small. By using thin iron sheets as core the resistance is increased. Laminating the core substantially reduces the eddy currents. Eddy current heats up the core of the transformer. More the eddy currents greater is the loss of energy and the efficiency goes down.

Question.13.
Assertion: Choke coil is preferred over a resistor to control the current in an AC circuit.
Reason: Power factor of an ideal inductor is zero.

Ans.13.(a)
Both A and R are true and R is correct explanation of the assertion.

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