Alternating Current  Important Questions for CBSE Class 12 Physics Introduction to Alternating Current

1.Alternating Current (AC) It is the current which varies in both magnitude as well as direction alternatively and periodically.
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-1

2.Effective Value or rms Value of AC It is defined as the value of AC over a complete cycle which would generate same amount of heat in a given resistors that is generated by steady current in the same resistor and in the same time during a complete cycle.
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-2
The 70.7% of peak value of current gives effective or rms value of AC

3. Average or Mean Value of AC It is defined as the value of AC which would send same amount of charge through a circuit in half-cycle that is sent by steady current in the same time.
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-3

The 63.7% of peak value of AC gives average or mean value of AC.

NOTE In a complete cycle of AC, the mean value of AC will be zero.

4. Alternating emf or Voltage It is the emf which varies in both magnitude as well as direction alternatively and periodically. The instantaneous alternating emf is given by
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-4
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-5

Previous Years’  Examination Questions

1 Mark Questions

1.Why is the use of AC voltage  preferred over DC voltage? Give two reasons.       [All India 2014]

Ans. The use of AC voltage is preferred over DC voltage because of

(i) the loss of energy in transmitting the AC voltage over long distances with the help of step-up transformers is negligible as compared to DC voltage.

(ii) AC voltage can be stepped up and stepped down as per the requirement by using a transformer.

2.The current flowing through a pure  inductance 2mH is, l  =(15 cos 300t) A. What is the (i) rms and  average value of current for a complete cycle? [Foreign 2011]
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-2qa

3. Define the term wattless current.  [Delhi 2011]
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-3qa

4. Define the term rms value of the  current. How is it related to the peak value?             [All India 2010c]

Ans.  It is defined as the value of Alternating Current (AC) over a complete cycle which would generate same amount of heat in a given resistor that is generated by steady current in the same resistor and in the same time during a complete cycle .lt is also called virtual value or effective value of AC.
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-4qa

5. How much average power over a  complete cycle, does an AC source supply to a capacitor?    [Delhi 2009c]
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-5qa

important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-6q
Ans.  It is defined as the value of Alternating Current (AC) over a complete cycle which would generate same amount of heat in a given resistor that is generated by steady current in the same resistor and in the same time during a complete cycle .lt is also called virtual value or effective value of AC.
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-4qa

7. The instantaneous current and voltage of an AC circuit are given by I  =(10 sin 300t) A and V= (200 sin 300t) V. What is the power dissipation in the circuit?  [All India 2008]
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-7qa

8. The instantaneous current and voltage of an AC circuit are given by I  = (10 sin 314t) A and V= (50 sin 314t) V. What is the power dissipation in the circuit?  [All India 2008]
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-7qa
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-9q
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-9qa
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-9qa2

2 Marks Questions

10.A light bulb is rated 150 W for 220 V AC supply of 60 Hz. Calculate

  • the resistance of the bulb
  • the rms current through the bulb.  [All India 2012]

important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-10qa
11. Prove that an ideal capacitor in an AC circuit does not dissipate power.  [Delhi 2011]
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-11qa

3 Marks Questions

important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-12q
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-12qa
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-12qa2
13. (i) When an AC source is connected to an  ideal capacitor, then show that the average power supplied by the source over a complete cycle is zero.

(ii) A lamp is connected in series with a capacitor. Predict your observations when the system is connected first across a DC and then  AC source. What happens in each case if the capacitance of the capacitor is reduced?                       [Delhi 2013C]
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-13qa
important-questions-for-class-12-physics-cbse-introduction-to-alternating-current-13qa2
Important Questions for Class 12 PhysicsClass 12 PhysicsNCERT Solutions Home Page