CBSE DelhiSet12005
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Q1
An electric dipole of the dipole moment 20x10^{6} C.m is enclosed by a closed surface. What is the net flux coming out of the surface? Marks:1Answer:
The net charge on the electric dipole is zero, hence net flux coming out of the closed surface is zero. 
Q2
An electron beam projected along +Xaxis, experiences a force due to a magnetic field along the +ve Yaxis. What is the direction of the magnetic field? Marks:1Answer:
The magnetic field will be in the direction of +z direction. 
Q3
The power factor of an a.c circuit is 0.5. What will be the phase difference between voltage and current in this circuit? Marks:1Answer:
According to the given condition,
cos= 0.5
= cos^{1}(0.5) = 60^{o}

Q4
Electrons are emitted from a photosensitive surface when it is illuminated by green light but electron emission does not take place by yellow light. Will the electrons be emitted when the surface is illuminated by (i) red light, and (ii) blue light. Marks:1Answer:
(i) Electrons will not be emitted with red light. (ii) Electrons will be emitted by blue light. 
Q5
What should be the length of the dipole antenna for a carrier wave of frequency 3x10^{8} Hz? Marks:1Answer:
Length of the dipole antenna = /2
= c/2v
= [(310^{8})/(2310^{8})]
= 0.5 m 
Q6
Define ‘electric line of force’ and give its two important properties. Marks:2Answer:
Electric line of force may be defined as a curve which is drawn in such a manner that a tangent to it at any point gives the direction of the electric field at that point. Its two important properties are given as follows: 1. Two lines of forces can never intersect each other. 2. It is a continuous curve that starts from positive charge and ends on a negative charge. 
Q7
(a) Why does the electric field inside a dielectric decrease when it is placed in an external electric field?
(b) A parallel plate capacitor with air between the plates has a capacitance of 8 pF. What will be the capacitance if the distance between the plates be reduced by half and the space between them is filled with a substance of dielectric constant K = 6? Marks:2Answer:
(a) If we place the dielectric in an external magnetic field then polarization of dielectric takes place which induces a field in the opposite direction. This effect results in decrease of net electric field inside the dielectric.
(b) Let C_{0} be the capacitance of capacitor in air. C_{0} is given by,
C_{o} = (_{o}A)/(d) ..........(I)
Now change in the capacitance according to the given condition can be given by the relation,
C = k[(_{o}A)/(d/2)]C = 2KC_{0}  (fromI) C = 2 x 6 x 8 = 96 pF. 
Q8
There point charges of +2 C, 3 C and 3 C are kept at the vertices, A, B and C respectively of an equilateral triangle of side 20 cm as shown in the figure. What should be the sign and magnitude of the charge to be placed at the midpoint (M) of side BC so that the charge at A remains in equilibrium? Marks:2Answer:
The force exerted on +2 C charge by the charge at B is given by,
Similarly the force exerted by charge at point C is given by, The resultant of force F_{1} and F_{2} is given by 
Q9
Draw VI graph for ohmic and nonohmic materials. Give one example for each. Marks:2Answer:
(i) The VI graph for ohmic material is given below,
Metal is an example of ohmic material. (ii) The VI graph for nonohmic material is given below, Semiconductor is an example of nonohmic material. 
Q10
Define the term ‘Magnetic Dip’ and ‘Magnetic Declination’ with the help of relevant diagrams. Marks:2Answer:
(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
(ii) Angle of dip: It is the angle made by the earth’s total magnetic field with the horizontal at a given place.