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F.Sc ICS Notes: Physics XII: Chapter 16 Alternating Current Exercise Short Questions:

FSc ICS Notes: Physics XII: Chapter 16 Alternating Current 

                             Exercise Short Questions:

Question 16.1 A sinusoidal current has rms value of 10A. What is the maximum or peak value?
Answer 16.1           Irms = 10 A
                               Io    = ?
                            We know that
                               Irms = Io / √2
                               Io    = Irms √2
                               Io    = 1.414 * 10
                               Io    = 14.14 A

Question 16.2 Name the device that will 
  1. permit flow of direct current but oppose the flow of alternating current
  2. permit flow of alternating current but not the direct current.
Answer 16.2 (1) An inductor is a device which permits the flow of D.C and opposes the flow of A.C.
(2) A capacitor is a device which permits the flow of A.C but oppose the flow of D.C.

Question 16.3 How many times per second will an incandescent lamp reach maximum brilliance when connected to a 50 Hz source?
Answer 16.3 In one A.C cycle the current becomes maximum two times. Hence a lamp connected to 50Hz source will reach to its maximum brilliance (2 * 50) = 100 times in one second.

Question 16.4 A circuit contains an iron-cored inductor, a switch and a D.C. source arranged in series. The switch is closed and after an interval reopened. Explain why a spark jumps across the switch contacts?
Answer 16.4 Let us consider a series circuit an iron core inductor a switch and a D.C source. When the circuit is made on, then current grows from zero to maximum value, this changing current produces change of magnetic flux and induced emf is produced. When the switch is reopened, then again current value change from its maximum value to zero. Again induced emf is developed across the chock coil in the form of back emf. This back emf produced spark at the ends of switch contacts
Question 16.5 How does doubling the frequency affect the reactance of (a) an inductor (b) a capacitor?
Answer 16.5 (a) The reactance of inductor is  XL = 2π fL
                                                           as XL∝ f
on doubling the frequency f the inductive reactance is doubled.
(b) In case of capacitor XC = 1 / 2πfC
                  or                           XC∝ 1 / f
Therefore on doubling the frequency the capacitive reactance will become one half.

Question 16.6 In a R-L circuit, will the current lag or lead the voltage? Illustrate your answer by a vector diagram.
Answer 16.6  From vector diagram, it can be seen, that applied V leads the current by an angle θ
             Where  tan θ = ωL / R
              or             θ  = tan(-1)(ωL / R)

Question 16.7 A choke coil placed in series with an electric lamp in an A.C. circuit causes the lamp to become dim. Why is it so? A variable capacitor added in series in this circuit may be adjusted until the lamp glows with normal brilliance. Explain, how this is possible?
Answer 16.7 When a circuit contains only choke connected to series with the bulb then due to inductance the inductive reactance will be very high, This will allow very small current through the circuit. Therefore the bulb will shine dimmer. On connecting a variable capacitor in series, it will become R-L-C series resonance circuit. By adjusting the value of C resonance frequency can be made equal to the frequency of A.C source. At this stage resonance takes place and the impedence of circuit become minimum. This will allow a very large current through the circuit and the bulb will shine to its normal brilliance.

Question 16.8 Explain the conditions under which electromagnetic waves are produced from a source?
Answer 16.8 When antenna is connected to A.C source it accelerates electrons up and down along the wire. This will change the polarity of the ends of the antenna. If an electric charge at a moment is +q then half cycle it will become -q. This will produce a changing electric field which in turn produce a changing magnetic field. This will result in propagation of electromagnetic waves.

Question 16.9 How the reception of a particular radio station is selected on your radio set?
Answer 16.9 A particular radio station can be tuned by adjusting the value of C such that the frequency of LC circuit is same as that of transmitted signal. The circuit will resonate under driving action of antenna. Consequently, the L.C. circuit will build up large response to the incoming signal, which is to be tuned.

Question 16.10 What is meant by A.M. and F.M. ?
Answer 16.10 A.M means amplitude modulation. In this mode of transmission the amplitude of carrier wave is made to change by variation of amplitude of modulating signal. Its frequency range is 540KHz to 1600KHz.
F.M means frequency modulation. In this mode of transmission frequency of carrier wave is made to change with variation of amplitude of modulating signal. Its frequency range of transmission is 88MHz to 108MHz.

Written By: Asad Hussain.