Physics-1998-Set-I

Q1) Name the physical quantity whose SI unit is Coulomb/Volt. (Marks 1)
Ans1) Capacitance

Q2) Write the frequency limit of visible range of electromagnetic spectrum in kHz. (Marks 1)
Ans2) 8 x 1011 HHz to 4 x 1011 KHz

Q3) How does the conductance of a semi-conducting material change with rise in temperature? (Marks 1)
Ans3) Increases.

Q4) The force experienced by a particle of charge e moving with velocity in a magnetic field is given by. = e( x ) Of these, name the pairs of vectors which are always at right angles to each other. (Marks 1)
Ans4) = e( x )
| and |

Q5) Two wires A and B are of same metal, have the same area of cross-section and have their lengths in the ratio 2 : 1. What will be the ratio of currents flowing through them respectively when the same potential difference is applied across length of each of them?
(Marks 1)

Ans5) Resistance length
RA/RB = 1/2
Current is inversely proportional to resistance
… IA/IB = 1/2

Q6) Calculate the rms value of the alternating current shown in the figure. (Marks 1)

Ans6) Irms = i2 = ((22 + (-2)2 +22)/3) = (12/3) = 2A

Q7) The image of an object formed by a lens on the screen is not in sharp focus. Suggest a method to get clear focusing of the image on the screen without disturbing the position of the object the lens or the screen. (Marks 1)
Ans7) The reason for such image is spherical aberration. Clearer focusing can be got by using the other central position of the lens & blocking the inter portion or using other proton & blocking central portion.

Q8) Two points A and B are placed between two parallel plates having a potential difference V as shown in the figure.

Will these protons experience equal or unequal force? (Marks 1)
Ans8) Both A and B will experience equal force as field between the plates is uniform.

Q9) Define the terms ‘threshold frequency’ and ’stopping potential’ for photo-electric effect. Show graphically how the stopping potential, for a given metal, varies with frequency of the incident radiations. Mark threshold frequency on this graph. (Marks 2)
Ans9) Threshold frequency: It is the maximum frequency of the incident radiation below which no emission of photoelectrons takes place.
Stopping potential : It is the maximum negative potential required to stop the fastest photoelectrons i e, photoelectric current becomes zero.

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