CLASS 12 – PHYSICS -- Chapter 5: ALTERNATING CURRENT CIRCUIT

CLASS 12 – PHYSICS
Chapter 5: ALTERNATING CURRENT CIRCUIT
Short answer question:

Q1. A sinusoidal current has rms value 10A. What is the peak value of current?
Ans. Irms = 10A
I0 =?
We know that I0 = 1.414Irms
I = 1.414 * 10
I0 = 14.14A

Q2. Name the device that (a) permit the flow of direct current but oppose the flow of alternating current (b) permit the flow of alternating current but not the direct current.
Ans.
(a) The device which permits the flow of direct current but oppose the alternating current is the inductor.
(b) The device which permits the flow of alternating current but oppose the direct current is the capacitor.

Q3. How many times will an incandescent lamp reach maximum brilliance when connected to a 50 Hz source?
Ans. The alternating current in one cycle becomes maximum twice (one in the positive half cycle and other is the negative half cycle). So in one cycle the brilliance of the lamp will becomes maximum two times. Therefore in 50 Hz, the maximum brilliance will be 100 times.

Q4. A circuit contains an iron-cored inductor, a switch and a dc source arranged in series. The switch is closed and after an interval reopened. Explain why a spark jumps across the switch contacts?
Ans. In the given circuit when the switch is closed, the current increases from zero to maximum which changes the magnetic flux and hence induced emf produces but when the switch is reopened, the current decreases from maximum value to zero. This again changes the magnetic flux in given circuit (coil), and causing the back emf which produces spark jumps in the switch.

Q5. How does doubling frequency affect the reactance of?
(a) An inductor (b) a capacitor
Ans.
a) The inductive reactance is given by
XL = 2πfL
If the frequency is doubled, the inductive reactance XL will be becomes double.
b) The capacitive reactance is given by
Xc = 1/2πfc
When the frequency is double, the capacitive reactance will becomes half.

Q6. In a RL circuit, will the current lag or lead the voltage?
Ans. In RL circuit, the current lags behind the voltage by 90° [π/2 radian].

Q8. Explain the conditions under which electromagnetic waves are produced from a source.
Ans. When the alternating current is applied to the antenna the oscillating electric field produces, which accelerate the electrons. The accelerated electrons radiate energy carried by electric field. The changing electric field produces magnetic field. Thus each field produces the other. This changing electric and magnetic fields are both confined to a plane perpendicular to the direction of propagation which is the electromagnetic wave.

Q9. How the reception of a particular radio station is select on your set?
Ans. The particular radio station can be selected on the radio set by tuning its tuning circuit to adjust the frequency equal to the frequency of the incoming radio waves. This is done by using a variable capacitor in the tuning circuit. At this condition, resonance takes place; the current of tuning circuit. At this condition, resonance takes place; the current of the incoming radio signal becomes maximum. The frequency resonance is called resonant frequency is given by f = 1 / 2π√LC

Q10. What is meant by AM and FM?
Ans. AM stand for amplitude modulation, in this type of modulation the amplitude of the carrier wave is increased on or decreased as amplitude of the superposing modulating signal.
FM stand for frequency modulation, in this type of modulation the frequency of the carrier wave is increased or decreased as the modulating signal amplitude increases or decreases but amplitude of the carrier waves remains constant.

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