CBSE Delhi-Set-3-2004
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Q1
T wo point charges 
are separated by a distance of 40 cm in air. (i) Calculate the electrostatic potential energy of the system, assuming the zero of the potential energy to be at infinity. (ii) Draw an equipotential surface of the system. Marks:1View AnswerAnswer:
Electrostatic potential is given by,
U=-45 J.
(ii) The equipotential surface for the system is given below,
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Q2
In the diagram below is shown a circular loop carrying crurent I. Show the direction of the magnetic field with the help of lines of force.
Marks:1View AnswerAnswer:
The direction of magnetic field is shown in figure below,
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Q3
Name the type of modulation scheme preferred for digital communication. Marks:1View AnswerAnswer:
The type of modulation scheme preferred for digital communication is pulse code modulation (PCM). -
Q4
Write the nuclear decay process for 
Marks:1View AnswerAnswer:
The required equation is given below:
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Q5
What is the name given to that part of electromagnetic spectrum which is used for taking photographs of earth under foggy conditions from great heights? Marks:1View AnswerAnswer:
Infrared rays. -
Q6
Microwaves are used in ‘Radar’. Why? Marks:1View AnswerAnswer:
Microwaves does not bend around the obstacles of any object. Also microwaves can be sent as a narrow beam of signals, this is because their wavelength is small. This is the reason why we use microwaves for radar. -
Q7
Explain with the help of diagram the terms (i) magnetic declination and (ii) angle of dip at a given place. Marks:2View AnswerAnswer:
(i) Magnetic declination: The angle between the geometric meridian and magnetic meridian at a given place is called magnetic declination at that place. It is denoted by symbol( 
).
(ii) Angle of dip: It is the angle made by the earth’s total magnetic field with the horizontal at a given place. It is denoted by symbol ( 
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Q8
The graph shows the variation of voltage, ‘V’ across the plates of two capacitors A and B versus increase of charge, ‘Q’ stored on them. Which of the two capacitors has higher capacitance? Give reason for your answer.
Marks:2View AnswerAnswer:
First of all consider the figure below,
Now we have the relation, Q=CV.
or C = Q/V. For capacitor A we can write, CA = Q/VA Similarly for capacitor B we can write, CB = Q/VB. Here since VA < VB. 
CA >CB -
Q9
Two circular coils, one of radius r and the other of radius R are placed coaxially with their centres coinciding. For R >> r, obtain an expression for the mutual inductance of the arrangement. Marks:2View AnswerAnswer:
Let us consider current I2 is flowing through the outer coil. Now the magnetic field at the centre of the coil is given by,
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Q10
An astronomical telescope, in normal adjustment position has magnifying power 5. The distance between the objective and the eye-piece is 120 cm. Calculate the focal lengths of the objective and of the eye-piece. Marks:2View AnswerAnswer:
Given, magnifying power of astronomical telescope m = 5
We have the relation,
m = f0 / feor f0=5.fe. ------ (I)
Also f0 + fe = 120cm
or, 5fe + fe = 120------(From I)
fe=20 cm.
Substituting the value of fe in equation (I) we get,
f0=100 cm.
