Science (English Medium) कक्षा १२ - CBSE Important Questions for Physics

In a series LCR circuit, V_L = V_C ≠ V_R. What is the value of power factor?

You are given three circuit elements X, Y and Z. When the element X is connected across an a.c. source of a given voltage, the current and the voltage are in the same phase. When the element Y is connected in series with X across the source, voltage is ahead of the current in phase by π/4 . But the current is ahead of the voltage in phase by π/4when Z is connected in series with X across the source. Identify the circuit elements X, Y and Z. When all the three elements are connected in series across the same source, determine the impedance of the circuit.Draw a plot of the current versus the frequency of applied source and mention the significance of this plot.

Why does current in a steady state not flow in a capacitor connected across a battery? However momentary current does flow during charging or discharging of the capacitor. Explain. 

State the principle of the step-down transformer and its working.

Express the turn ratio in terms of voltages.

Find the ratio of primary and secondary currents in terms of turn ratio in an ideal transformer.

How much current is drawn by the primary of a transformer connected to 220 V supply when it delivers power to a 110 V − 550 W refrigerator?

A light bulb and a solenoid are connected in series across an ac source of voltage. Explain, how the glow of the light bulb will be affected when an iron rod is inserted in the solenoid.

A source of ac voltage v = v₀ sin ωt, is connected across a pure inductor of inductance L. Derive the expressions for the instantaneous current in the circuit. Show that average power dissipated in the circuit is zero.

Describe, with the help of  a suitable diagram, the working principle of a step-up transformer. Obtain the relation between input and output voltages in terms of the number of turns of primary and secondary windings and the currents in the input and output circuits.

Given the input current 15 A and the input voltage of 100 V for a step-up transformer having 90% efficiency, find the output power and the voltage in the secondary if the output current is 3 A.  

A 2 µF capacitor, 100 Ω resistor and 8 H inductor are connected in series with an AC source.
(i) What should be the frequency of the source such that current drawn in the circuit is maximum? What is this frequency called?
(ii) If the peak value of e.m.f. of the source is 200 V, find the maximum current.
(iii) Draw a graph showing variation of amplitude of circuit current with changing frequency of applied voltage in a series LRC circuit for two different values of resistance R₁ and R₂ (R₁ > R₂).
(iv) Define the term 'Sharpness of Resonance'. Under what condition, does a circuit become more selective?

In a series LCR circuit, obtain the condition under which the impedance of the circuit is minimum ?

In a series LCR circuit, obtain the condition under which watt-less current flows in the circuit ?

 A light bulb is rated at 100 W for a 220 V a.c. supply. Calculate the resistance of the bulb.

A light bulb is rated at 120 W for a 220 V a.c. supply. Calculate the resistance of the bulb.

A light bulb is rated at 125 W for a 250 V a.c. supply. Calculate the resistance of the bulb.

Calculate the quality factor of a series LCR circuit with L = 2.0 H, C = 2μF and R = 10 Ω. Mention the significance of quality factor in LCR circuit.

Calculate the quality factor of a series LCR circuit with L = 4.0 H, C = 1, μF and R = 20 Ω.

Mention the significance of quality factor in LCR circuit.

State the principle of working of a transformer. Can a transformer be used to step up or step down a d.c. voltage? Justify your answer.