﻿ A photocell is illuminated by a small bright source placed 1 m away. When the same source of light is placed 1 /2 m away, the number of electrons emitted by photocathode would | Nuclei

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# Nuclei

#### Multiple Choice Questions

1.

The binding energy per nucleon of deuteron (21H) and helium nucleus (42He) is 1.1 MeV and 7 MeV respectively. If two deuteron nuclei react to form a single helium nucleus, then the energy released is

• 13.9 MeV

• 26.9 MeV

• 23.6 MeV

• 19.2 MeV

C.

23.6 MeV

As given,
The binding energy per nucleon of a deuteron (1H2 ) = 1.1 MeV
∴ Total binding energy = 2× 1.1 = 2.2 MeV

The binding energy per nucleon of helium (2He4 ) = 7 MeV
∴ Total binding energy = 4× 7 = 28 MeV
Hence, energy released in the above process = 28 - 2× 2.2 = 28 - 4.4 = 23.6 MeV

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2.

When 3Li7 nuclei are bombarded by protons, and the resultant nuclei are 4Be8 , the emitted particles will be

• neutrons

• alpha particles

• beta particles

• gamma photons

D.

gamma photons

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3.

A nucleus disintegrates into two nuclear parts which have their velocities in the ratio 2:1. The ratio of their nuclear sizes will be

• 21/3:1

• 1:31/2

• 31/2:1

• 1:21/3

B.

1:31/2

Law of conservation of momentum gives
m1 v1 = m2 v2

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# 4.A photocell is illuminated by a small bright source placed 1 m away. When the same source of light is placed 1 /2 m away, the number of electrons emitted by photocathode would decrease by a factor of 4 increase by a factor of 4 decrease by a factor of 2 increase by a factor of 2

B.

increase by a factor of 4

On doubling the distance the intensity becomes one fourth i.e. only one-fourth of photons now strike the target in comparison to the previous number. Since the photoelectric effect is a one photon-one electron phenomenon, so only one-fourth photoelectrons are emitted out of the target hence reducing the current to one fourth the previous value.
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5.

A nuclear transformation is denoted by X(n, α) . Which of the following is the nucleus of element X?

B.

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6.

Starting with a sample of pure 66Cu, 7/8 of it decays into Zn in 15 minutes. The corresponding half-life is

• 10 minutes

• 15 minutes

• 5 minutes

• 7.5 minutes

C.

5 minutes

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7.

If radius of  nucleus is estimated to be 3.6 Fermi then the radius nucleus

• 6 fermi

• 8 fermi

• 4 fermi

• 5 fermi

A.

6 fermi

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8.

An α-particle of energy 5 MeV is scattered through 180° by a fixed uranium nucleus. The distance of the closest approach is of the order of

• 1 Å

• 10−10 cm

• 10−12 cm

• 10−15 cm

C.

10−12 cm

At closest approach, all the kinetic energy of the α-particle will converted into the potential energy of the system, K.E. = P.E.

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9.

Two spherical conductor B and C having equal radii and carrying equal charges in them repel each other with a force F when kept apart at some distance. A third spherical conductor having same radius as that of B but uncharged brought in contact with B, then brought in contact with C and finally removed away from both. The new force of repulsion, between B and C is

• F/4

• 3F/4

• 3F/8

• F/8

C.

3F/8

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10.

The intensity of gamma radiation from a given source is I. On passing through 36 mm of lead, it is reduced to l/8. The thickness of lead which will reduce the intensity to l/2 will be

• 6 mm

• 9 mm

• 18 mm

• 12 mm

D.

12 mm

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