# CBSE Delhi-Set-2-2012

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• Q1

The horizontal component of the earth's magnetic field at a place is B and angle of dip is 60°. What is the value of vertical component of earth's magnetic field at equator?

Marks:1

On the equator,  and therefore the vertical component of earth's magnetic field is also zero.

• Q2

Name of physical quantity which remains same for microwaves of wavelength 1 mm and UV radiations of 1600 Å in vacuum.

Marks:1

Microwaves and UV radiations are a part of the electromagnetic spectrum. The physical quantity that remains same for both the radiations is their speed of propagation in vacuum, which is equal to  .

• Q3

Under what conditions does a biconvex lens of glass having a certain refractive index act as a plane glass sheet when immersed in a liquid?

Marks:1

A biconvex lens will act as a plane sheet of glass if it is immersed in a liquid that has the same refractive index as the material of the biconvex lens. In this case, the focal length of the lens becomes infinite.

• Q4

When electrons drift in a metal from lower to higher potential, does it mean that all the free electrons of the metal are moving in the same direction?

Marks:1

When an electric field is applied across a conductor, the electrons drift from lower to higher potential. The drift velocity of the electrons gets superimposed on the thermal velocity and as a result, each electron moves in the direction of the resultant of the drift and the thermal velocity. Thus, the resultant velocity and hence the direction of motion may be different for different electrons. The electrons may also collide with ions and may change its direction of motion.

• Q5

State de Broglie hypothesis.

Marks:1

According to the de Broglie hypothesis, a moving material particle (whether big or small) having a momentum ‘p’ will always have a wavelength  associated with it. Such waves are called matter waves or de Broglie waves having a wavelength,

• Q6

Show on a graph, the variation of resistivity with temperature for a typical semiconductor.

Marks:1

As the temperature of a semi conductor is increased, the number density of electrons and that of holes increases. Therefore, its resistivity decreases with increasing temperature. The following curve shows the variation of resistivity with temperature for a typical semiconductor:

• Q7

Why must electrostatic field be normal to the surface at every point of a charged conductor?

Marks:1

Let us prove this by contradiction.
Let the electric field at a point on the surface of the conductor be in a general direction, at an angle to the surface. The field can be resolved into a perpendicular component and a component parallel to the surface. If the electric field has a non-vanishing component along the surface, the free charges present on the surface would start moving under its influence. This is contrary to the situation since in the static situation, charges are not in motion. Therefore, the electrostatic field does not have a tangential component and must be normal to the surface at all points.

• Q8

Predict the direction of induced current in a metal ring when the ring is moved towards a straight conductor with constant speed v. The conductor is carrying current I in the direction shown in the figure.

Marks:1

According to the Lenz’s law, the induced current produced in the loop must oppose the cause of increase of magnetic flux when it moves towards the current carrying conductor with a constant speed v. Thus, the current induced in the loop will be in the clockwise direction.

• Q9

Draw a plot showing the variation of (i) electric field (E) and (ii) electric potential (V) with distance r due to a point charge Q.

Marks:2

Therefore, the electric field  shows an inverse square relationship whereas, the electric potential ‘V’ shows an inverse relationship with distancer’ as shown below:

• Q10

Define mutual inductance between two long coaxial solenoids. Find out the expression for the mutual inductance of inner solenoid of length l having the radius r1 and the number of turns n1 per unit length due to the second outer solenoid of same length and n2 number of turns per unit length.

Marks:2