Table of Contents

**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.

**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.

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.

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

**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]**

**3. Define the term wattless current. [Delhi 2011]**

**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.

**5. How much average power over a complete cycle, does an AC source supply to a capacitor? [Delhi 2009c]**

**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.

**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]**

**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]**

**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]**

**11. Prove that an ideal capacitor in an AC circuit does not dissipate power. [Delhi 2011]**

**3 Marks Questions**

**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]**