Science (English Medium) कक्षा १२ - CBSE Important Questions

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

A 100 Ω resistor is connected to a 220 V, 50 Hz ac supply.

1. What is the rms value of current in the circuit?
2. What is the net power consumed over a full cycle?

An iron cored coil is connected in series with an electric bulb with an AC source as shown in figure. When iron piece is taken out of the coil, the brightness of the bulb will ______.

If the reading of the voltmeter V₁ is 40 V, then the reading of voltmeter V₂ is ______.

An a.c. source generating a voltage ε = ε₀ sin ωt is connected to a capacitor of capacitance C. Find the expression for the current I flowing through it. Plot a graph of ε and I versus ωt to show that the current is ahead of the voltage by π/2.

An ac voltage V = V₀ sin ωt is applied across a pure inductor of inductance L. Find an expression for the current i, flowing in the circuit and show mathematically that the current flowing through it lags behind the applied voltage by a phase angle of `π/2`. Also draw graphs of V and i versus ωt for the circuit.

A series RL circuit with R = 10 Ω and L = `(100/pi)` mH is connected to an ac source of voltage V = 141 sin (100 πt), where V is in volts and t is in seconds. Calculate

1. the impedance of the circuit
2. phase angle, and
3. the voltage drop across the inductor.

A resistor of 50 Ω, a capacitor of `(25/pi)` µF and an inductor of `(4/pi)` H are connected in series across an ac source whose voltage (in volts) is given by V = 70 sin (100 πt). Calculate:

1. the net reactance of the circuit
2. the impedance of the circuit
3. the effective value of current in the circuit.

Which of the following statements about a series LCR circuit connected to an ac source is correct?

An alternating voltage of 220 V is applied across a device X. A current of 0.22 A flows in the circuit and it lags behind the applied voltage in phase by π/2 radian. When the same voltage is applied across another device Y, the current in the circuit remains the same and it is in phase with the applied voltage.

1. Name the devices X and Y and,
2. Calculate the current flowing in the circuit when the same voltage is applied across the series combination of X and Y.

An ideal inductor is connected across an AC source of voltage. The current in the circuit ______.

The primary coil having N_P turns of an ideal transformer is supplied with an alternating voltage V_P. Obtain an expression for the voltage V_S induced in its secondary coil having N_S turns.

Mention two main sources of power loss in real transformers.

What is the ratio of inductive and capacitive reactance in an ac circuit?

The large scale transmission of electrical energy over long distances is done with the use of transformers. The voltage output of the generator is stepped-up because of ______.

Draw the phasor diagram for a series LRC circuit connected to an AC source.

Plot a graph to show the variation of current with a frequency of the ac source, explaining the nature of its variation for two different resistances R₁ and R₂ (R₁ < R₂) at resonance.