Guess Paper – 2012

Class – XII

Alternating Current And Electrical Machines

Question 1. A Sinusoidal Voltage V=200 sin 314t is Applied to a Resistor of 10 ohms Resistance. Calculate

(i) rms value of the voltage,
(ii) rms value of the current and
(iii) power dissipated as heat in watts.

Question 2. An ideal inductor is in turn put across 220V, 50 Hz and 220V, 100 Hz supplies. Will the current flowing through it in the two cases be the same or different?

Question 3. What is the phase relationship between the current flowing and the voltage applied in an ac circuit for a pure inductor?

Question 4.A virtual current of 4 A flows in a coil when it is connected in a circuit having alternating current of frequency 50 Hz. Power consumed in the circuit is 240 W. Calculate the inductance of the coil if the virtual p.d. across it is 100 V.

Question 5. An ac source E = Eo Sinwt is applied across an inductor of inductance L. Show mathematically that the current lags behind voltage by phase angle of p/2.

Question 6. A capacitor is connected an ac source. If the frequency of the source be increased, how does the current in the capacitor is affected?

Question 7. A coil has an inductance of 1 henry.

(a) At what frequency will it have a reactance of 3142 ohms?
(b) What should be the capacity of a capacitor, which has the same reactance at that frequency?

Question 8. A 1mf capacitor is connected to a 220 V, 50 Hz ac source. Calculate the rms value of the current through the circuit. Also find the peak value of voltage across the capacitor.

Question 9. A capacitor C and a bulb B are connected in series to the ac mains as shown. The bulb glows with some brightness. How will the glow of the bulb change when a dielectric slab is introduced between the plates of the capcitor? Give reason in support of your answer.

Question 10. An ac circuit consists of a 220 V 50 Hz supply connected across a 100 ohms resistance. What inductance should be connected in the circuit, in series with the resistance so that the current is reduced to half?