Science (English Medium) Class 12 - CBSE Important Questions

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.

Draw a labeled diagram of a full wave rectifier circuit. State its working principle. Show the input-output waveforms ?

Show that in an a.c. circuit containing a pure inductor, the voltage is ahead of current by π/2 in phase ?

A coil Q is connected to low voltage bulb B and placed near another coil P as shown in the figure. Give reasons to explain the following observations:

(a) The bulb ‘B’ lights

(b) Bulb gets dimmer if the coil Q is moved towards left.

A series LCR circuit is connected to a source having voltage v = v_m sin ωt. Derive the expression for the instantaneous current I and its phase relationship to the applied voltage.

Obtain the condition for resonance to occur. Define ‘power factor’. State the conditions under which it is (i) maximum and (ii) minimum.

 Derive an expression for the average power consumed in a series LCR circuit connected to a.c. source in which the phase difference between the voltage and the current in the circuit is Φ.

Describe briefly, with the help of labelled diagram, working of a step-up transformer.

A step-up transformer converts a low voltage into high voltage. Does it not violate the principle of conservation of energy? Explain.

An ac circuit as shown in the figure has an inductor of inductance L and a resistor or resistance R  connected in series. Using the phasor diagram, explain why the voltage in the circuit will lead the  current in phase.

The potential difference across the resistor is 160V and that across the inductor is 120V. Find the  effective value of the applied voltage. If the effective current in the circuit be 1.0 A, calculate the total impedance of the circuit.

What will be the potential difference in the circuit when direct current is passed through the circuit? 

Show that the time period (T) of oscillations of a freely suspended magnetic dipole of magnetic moment (m) in a uniform magnetic field (B) is given by `"T" = 2pi sqrt("I"/"mB")`, where I is a moment of inertia of the magnetic dipole.

The potential difference across the resistor is 160 V and that across the inductor is 120 V. Find the effective value of the applied voltage. If the effective current in the circuit is 1⋅0 A, calculate the total impedance of the circuit.

Use the expression for Lorentz force acting on the charge carriers of a conductor to obtain the expression for the induced emf across the conductor of length l moving with velocity v through a magnetic field B acting perpendicular to its length.

Using the phasor diagram, derive the expression for the current flowing in an ideal inductor connected to an a.c. source of voltage, v= v_o sin ωt. Hence plot graphs showing the variation of (i) applied voltage and (ii) the current as a function of ωt.

Derive an expression for the average power dissipated in a series LCR circuit.

Choose the correct answer from given options
The phase difference between the current and the voltage in series LCR circuit at resonance is