## December 13, 2012

### Answers to Review Questions: Ch # 16 Electronics

XII BSEK ( Karachi Board)

Q.16.1 Under what circumstances does a charge radiate electromagnetic wave?
Ans.The Static charge will produce an electric field around it while the moving charge will produce a magnetic field around it so to produce both filed that is electromagnetic field the charges should be accelerated.

Q.16.2 In an electromagnetic wave, what is the relationship, if any b/w the variations in the magnetic and electric fields?
Ans. Yes there is a big relationship between the electric and magnetic field because they propagates at right angles in electromagnetic waves and to the direction of propagation.

Q.16.3 A radio transmitter has a vertical antenna. does it matter whether the receiving antenna is vertical or horizontal?
Ans.The receiving antenna should be vertical because in this position it will receive more radiation as it does in the horizontal position, because it increases the area to receive the waves.

Q.16.4 Explain why are light waves able to travel through vacuum, whereas sound waves cannot?
Ans.Light waves are electromagnetic in nature having wavelength (400nm to 760 nm),sound waves produce when the particles of any medium are disturbed .In vacuum there is no medium no particles so they will not propagates from the vacuum and light will.

Q.16.5 Explain the condition under which radiation of electromagnetic waves takes place from certain source.
Ans. transmitting antenna is coupled with an alternating source of potential called Oscillator, Charges Like electrons are accelerated up and down to the antenna. This will fluctuates the electric flux, which generates the magnetic flux, This will produce the E.M waves.

Q.16.6 Can a diode be used for amplifying a weak signal?
Ans.No diode is not used for the amplification but a special type of diode named “Tunnel Diode” is used as an amplifier.

Q.16.7 Are radio waves a form of light?
Ans. As the radio waves and light waves are electromagnetic in nature so we can say that it is a form of light but they has lower frequency than that of light so they are not visible.

Q.16.8 Can electromagnetic wave be propagated through a piped vacuum.
Ans.Yes it can travel through a piped vacuum because the electromagnetic waves don’t require any medium for their propagation.

Q.16.9 Why does a semi conductor acts as an insulator at O K and why does its conductivity rises with the temperature? Or Discuss the effect of temperature on semi conductors.
Ans.In semi-conductor at OK the valence band is completely filled and conduction band is empty. Therefore it behaves like a perfect insulator. But at room temperature the electrons gain energy and move through the forbidden gap to the conduction band so we can say that there conductivity increases with the temperature.

Q.16.10 Explain the role of forbidden band in solids.
Ans.It can be classified in to three roles
In conductors there is no forbidden gap because the valence band and conduction band overlaps each other. In insulator there is large forbidden gap because the valence band and conduction band are separated from each other. In semi conductors there is small forbidden gap because the valence band and conduction band are at small distance.

Q.16.11 Why is light not seen in an ordinary diode but LED emits light?
Ans. As the silicon is opaque to light the energy is radiated only in the form of heat and when we use gallium arsenide phosphate crystal it emits small amount of heat and larger amount of Photon so in LED we see the light.