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

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 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 ?

Describe briefly and two energy losses, giving the reasons for their occurrence in actual transformers ?

Answer the following question.
In a series LCR circuit connected across an ac source of variable frequency, obtain the expression for its impedance and draw a plot showing its variation with frequency of the ac source. 

Answer the following question.
What is the phase difference between the voltages across the inductor and the capacitor at resonance in the LCR circuit? 

Answer the following question.
Draw the diagram of a device that is used to decrease high ac voltage into a low ac voltage and state its working principle. Write four sources of energy loss in this device.  

The selectivity of a series LCR a.c. circuit is large, when ______.

Why are microwaves considered suitable for radar systems used in aircraft navigation?

Name the phenomenon which shows the quantum nature of electromagnetic radiation.

Gamma rays and radio waves travel with the same velocity in free space. Distinguish between them in terms of their origin and the main application.