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45:- (d) The minimum energy of a photon required to excite an electron from the valence band to the conduction band is the forbidden energy gap of that semiconductor material. The relationship between the wavelength of the photon and the energy gap is given by


= energy gap = minimum energy required to move an electron from the valence band to the conduction band, in electron volts.= Critical wavelength of the photon (micrometers).

1.24 = constant that arises from the Planck’s constant.

Substituting the given data,

=  = 2.48 electron volt.

46:- (c) If the operation of the transformer is at the same rms value of the voltage, then

Eddy current loss,= f²

Thus eddy current loss in not affected. And the total loss is 179.3+100=279.3W. This does not tally with any of the given answers. Assume, the before that the flux density value is not charge. Then

= 200 x =240 W

= 100 x )² = 144 W.

Total loss = 384 W.

47:- (c) Cogging and crawling are phenomena associated with squirrel cage induction machine, the former during starting and later at a fraction of its rated speed.

48:- (d) A – 1, B – 4, C – 3, D - 2.

49:- (b) Coil span factor for harmonic is

= Cos  n

Where  is the angle by which the coil is short of full pitch?

= Cos = 0

Thus fifth harmonic component gets eliminated by using a coil pitch of 4/5.

50:- (a) 1, 3 and 5 are Correct.

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