CBSE Class 10 Physics Nuclear Physics MCQs Set 14

Answer: (c) both uranium-235 and plutonium

Explanation:
1. Heavy nuclei such as U-235 and plutonium split into two nearly equal parts when struck by a neutron.
2. So both are fissionable.

Teacher's Note:
1. Fission releases a large amount of energy.
2. Ordinary uranium has only 0.7% of U-235.

Answer: (a) neutrons

Explanation:
1. Ordinary hydrogen has a nucleus of one proton and no neutron.
2. So it is not true that all atoms contain neutrons.

Teacher's Note:
1. Every neutral atom has equal numbers of protons and electrons.
2. Hydrogen-1 is the only stable atom without a neutron.

Answer: (d) Chadwick

Explanation:
1. James Chadwick discovered the neutron in the early 1930s.
2. Rutherford had earlier shown that the atom has a positively charged nucleus.

Teacher's Note:
1. A neutron is neutral, so it is a better bullet for splitting nuclei than the positive alpha particle.
2. Chadwick received the Nobel prize for the discovery.

Answer: (c) either cadmium rods or boron rods

Explanation:
1. Cadmium and boron are strong absorbers of neutrons.
2. Rods of these materials are used as control rods.
3. Graphite is used to slow neutrons down, not to absorb them.

Teacher's Note:
1. Lowering the control rods slows the reaction and raising them speeds it up.
2. Aluminium coats the uranium rods to prevent oxidation.

Answer: (b) the same number of protons

Explanation:
1. Isotopes of an element have the same atomic number, so the same number of protons.
2. They differ in neutron number and so in mass number.

Teacher's Note:
1. Isotopes of hydrogen are protium, deuterium and tritium.
2. Chemical properties are the same because the electron number is the same.

Answer: (b) ordinary hydrogen

Explanation:
1. The nucleus of ordinary hydrogen is a single proton.
2. Helium has two neutrons, deuterium one and U-235 has 143 neutrons.

Teacher's Note:
1. So the statement "all atoms contain neutrons" is false.
2. Deuterium has one proton and one neutron.

Answer: (d) Rutherford

Explanation:
1. Rutherford's alpha particle scattering experiment showed a tiny, heavy, positively charged nucleus.
2. In 1919 he also changed nitrogen into oxygen by bombarding it with alpha particles.

Teacher's Note:
1. Chadwick later found the neutron.
2. Bohr proposed the planetary model with quantised orbits.

Answer: (c) slow neutrons

Explanation:
1. Slow neutrons are captured more readily by U-235 nuclei and so cause fission more efficiently.
2. That is why graphite or heavy water moderators are used.

Teacher's Note:
1. Fission releases fast neutrons, which the moderator slows down.
2. The moderator does not absorb neutrons.

Answer: (c) two or three free fast neutrons

Explanation:
1. Each fission frees two or three neutrons in addition to energy.
2. These cause further fission, which makes a chain reaction possible.

Teacher's Note:
1. The average number is about 2.5.
2. A chain reaction needs more than one neutron per fission to go on.

Answer: (a) fusion

Explanation:
1. In a hydrogen bomb light nuclei, deuterium and tritium, join to form helium.
2. That is nuclear fusion.
3. A fission bomb is used to trigger it.

Teacher's Note:
1. Fusion needs a temperature of millions of degrees.
2. The sun gets its energy by fusion.

Answer: (a) 92 protons and 143 neutrons

Explanation:
1. Uranium has atomic number 92, so 92 protons.
2. Neutrons = mass number - atomic number = 235 - 92 = 143.

Teacher's Note:
1. U-238 has 146 neutrons.
2. The isotopes differ only in the number of neutrons.

Answer: (a) deuterium and tritium

Explanation:
1. Deuterium has one proton and one neutron and tritium has one proton and two neutrons.
2. All have one proton, so all are hydrogen.

Teacher's Note:
1. Deuterium oxide is called heavy water.
2. Tritium is radioactive.

Answer: (b) hydrogen; helium

Explanation:
1. Hydrogen, atomic number 1, is the lightest.
2. Helium, atomic number 2, with four times the mass of hydrogen, is next.

Teacher's Note:
1. The helium nucleus is an alpha particle.
2. Helium has two protons and two neutrons.

Answer: (c) cadmium or boron rods

Explanation:
1. Control rods of cadmium or boron absorb surplus neutrons.
2. Inserting them reduces the neutron supply and so the reaction rate.

Teacher's Note:
1. Graphite rods are moderators that slow the neutrons.
2. Water takes away the heat.

Answer: (b) steam; turbine

Explanation:
1. The heat of fission makes water into steam at high pressure.
2. The steam turns a turbine coupled to a generator.

Teacher's Note:
1. A reactor is basically a source of heat.
2. A condenser turns the used steam back to water.

Answer: (c) 9 × 10¹⁶ J

Explanation:
1. Einstein's equation is E = mc².
2. E = 1 × (3 × 10⁸)² = 9 × 10¹⁶ J.

Teacher's Note:
1. A tiny mass equals a huge amount of energy because c² is very large.
2. In fission and fusion a small part of the mass disappears and appears as energy.