Science (English Medium) Class 12 - CBSE Important Questions for Physics

A horizontal straight wire 10 m long extending from east to west is falling with a speed of 5.0 m s⁻¹, at right angles to the horizontal component of the earth’s magnetic field, 0.30 × 10⁻⁴ Wb m⁻².

1. What is the instantaneous value of the emf induced in the wire?
2. What is the direction of the emf?
3. Which end of the wire is at the higher electrical potential?

A circular coil of cross-sectional area 200 cm² and 20 turns is rotated about the vertical diameter with angular speed of 50 rad s⁻¹ in a uniform magnetic field of magnitude 3.0 × 10⁻²T. Calculate the maximum value of the current in the coil.

When Puja, a student of 10^th class, watched her mother washing clothes in the open, she observed coloured soap bubbles and was curious to know why the soap bubbles appear coloured. In the evening when her father, an engineer by profession, came home, she asked him this question. Her father explained to her the basic phenomenon of physics due to which the soap bubbles appear coloured.
(a) What according to you are the values displayed by Puja and her father?
(b) State the phenomenon of light involved in the formation of coloured soap bubbles.

A bar magnet is moved in the direction indicated by the arrow between two coils PQ and CD. Predict the directions of induced current in each coil.

How are eddy currents generated in a conductor which is subjected to a magnetic field?

Write two examples of their useful applications .

How can the disadvantages of eddy currents be minimized?

Answer the following question.
When an inductor is connected to a 200 V dc voltage, a current at 1A flows through it. When the same inductor is connected to a 200 V, 50 Hz ac source, only 0.5 A current flows. Explain, why? Also, calculate the self-inductance of the inductor. 

Define 'quality factor' of resonance in a series LCR circuit. What is its SI unit?

Write the function of a transformer.

A device 'X' is connected to an ac source V = V₀ sin ωt. The variation of voltage, current and power in one cycle is shown in the following graph:

(a) Identify the device 'X'.
(b) Which of the curves A, B and C represent the voltage , current and the power consumed in the circuit? Justify your answer.
(c) How does its impedance vary with frequency of the ac source? Show graphically.
(d) Obtain an expression for the current in the circuit and its phase relation with ac voltage.

State the principle of transformer working with the help of a diagram

Mention various energy losses in transformer device

The primary coil of an ideal step-up transformer has 100 turns and the transformation ratio is also 100. The input voltage and power are 220 V and 1100 W, respectively. Calculate the 

(a) number of turns in secondary
(b) current in the primary
(c) a voltage across secondary
(d) current in secondary
(e) power in secondary

Show that the current leads the voltage in phase by π/2 in an AC circuit containing an ideal capacitor ?

Mention the two characteristic properties of the material suitable for making core of a transformer.

State the underlying principle of a transformer. How is the large scale transmission of electric energy over long distances done with the use of transformers?

A light bulb is rated 100 W for 220 V ac supply of 50 Hz. Calculate

(i) The resistance of the bulb;

(ii) The rms current through the bulb.

A series LCR circuit is connected to an ac source. Using the phasor diagram, derive the expression for the impedance of the circuit. Plot a graph to show the variation of current with frequency of the source, explaining the nature of its variation.

State the principle of a step-up transformer. Explain, with the help of a labeled diagram, its working ?