# CBSE [All India]-Set-2-2010

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• Q1

Name the physical quantity whose S.I. unit is JC-1. Is it a scalar or a vector quantity?

Marks:1
##### Answer:

Since [Potential (V)=Work done (W)/Charge (q)], the physical quantity whose S.I. unit is JC-1 is electrostatic potential. It is a scalar quantity.

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• Q2

Show that the electric field at the surface of a charged conductor is given by , where is the surface charge density and is a unit vector normal to the surface in the outward direction. the infinitesimal plane sheet and is directed outward direction.

Marks:2
##### Answer:

We can consider an infinitesimally small part of the surface of the charged conductor as a plane sheet of uniform charge density . Let us consider a cylindrical Gaussain surface of cross section A, as shown in the figure below:

Now, by Gauss’ theorem,

Since the electric field is perpendicular to the infinitesimal plane sheet and is directed outward,

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• Q3

Find the ratio of energies of photons produced due to transition of an electron of hydrogen atom from its
(i) second permitted energy level to the first level, and
(ii) the highest permitted energy level to the first permitted level.

Marks:1
##### Answer:

(ii)Inthelimitofn=,andtheelectronisnolongerboundtothenucleustoformanatom.So,thereisnodefinedhighestenergylevel.

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• Q4

A converging lens is kept coaxially in contact with a diverging lens – both the lenses being of equal focal lengths. What is the focal length of the combination?

Marks:1
##### Answer:

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• Q5

A beam of electrons projected along + x-axis, experiences a force due to a magnetic field along + y-axis. What is the direction of the magnetic field?

Marks:1
##### Answer:

Therefore, the magnetic field is directed along the +z-direction.

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• Q6

Which of the following has the shortest wavelength: Microwaves, Ultraviolet rays, X-rays

Marks:1
##### Answer:

The wavelengths of the microwaves, ultraviolet rays and X-rays are of the order of 10-2, 10-8 and 10-10 metres respectively. Therefore, X-rays have the shortest wavelength.

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• Q7

Define self-inductance of a coil. Write its S.I. units.

Marks:1
##### Answer:

Self inductance of a coil is defined as the induced e.m.f. produced in the coil through which the rate of change of current is unity. Its S.I. unit is henry (denoted by H).

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• Q8

Two conducting wires X and Y of same diameter but different material are joined in series across a battery. If the number density of electron in X is twice that in Y, find the ratio of drift velocity of electrons in the two wires.

Marks:1
##### Answer:

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• Q9

When light travels a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a decrease in the energy carried by the light wave? Justify your answer.

Marks:1
##### Answer:

The frequency of light wave remains unchanged on travelling from one medium to another. The energy associated with light wave of frequency is , where h is the Planck’s constant. Therefore, the decrease in speed does not imply a decrease in the energy carried by the light wave on travelling from rarer medium to denser medium.

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• Q10

Draw a plot of the binding energy per nucleon as a function of mass number for a large number of nuclei, 2  A   240. How do you explain the constancy of binding energy per nucleon in the range 30 < A< 170 using the property that nuclear force is short-ranged?

Marks:2
##### Answer:

A plot of the binding energy per nucleon as a function of mass number for a large number of nuclei, 2  A  240 is given below.

The constancy of the binding energy in the range 30 < A< 170 is because of the fact that the nuclear force is short ranged. A nucleon inside a nucleus will be under the influence of only some of its neighbours, which come within the range of nuclear force. The binding energy of a nucleon is proportional to the maximum number of neighbouring nucleons within the range of nuclear force. If we increase A by adding nucleons, they will not change the B.E. of a nucleon inside. Since most of the nucleons in a large nucleus (in the range of 30 < A< 170) reside inside it and not on the surface, the change in the binding energy would be small.

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