Physics questions, chapter 6

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Nuclear Physics

46 marks. Use Practice to check each answer as you go, or Exam to answer everything against the clock first. Take c = 3.00 × 10⁸ m s⁻¹ and 1 amu = 1.66 × 10⁻²⁷ kg.

Section A: objective questions

14 marks

Choose the best answer for each question.

Question 1

What is radioactive decay?

The splitting of a heavy nucleus into two equal parts by a neutron
The joining of two light nuclei at a very high temperature
The process in which an unstable nucleus emits radiation to become more stable
The emission of light by a hot object

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An unstable nucleus emits radioactive radiation (α, β or γ) to become more stable (textbook p. 200). Options A and B describe fission and fusion.

For the teacher: Remembering, low difficulty, standard 6.1.1

Textbook p. 200

Question 2

An alpha particle is the same as

a helium nucleus
a fast-moving electron
a hydrogen atom
a high-frequency electromagnetic wave

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An alpha particle is a helium nucleus of 2 protons and 2 neutrons. A fast electron is a beta particle; a gamma ray is an electromagnetic wave.

For the teacher: Remembering, low difficulty, standard 6.1.1

Textbook p. 200

Question 3

Radium-226 (²²⁶₈₈Ra) decays by emitting an alpha particle. What is the daughter nucleus?

²²⁶₈₉Ac
²²²₈₆Rn
²²²₈₇Fr
²³⁰₉₀Th

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An α-particle takes away 4 nucleons and 2 protons: A = 226 − 4 = 222 and Z = 88 − 2 = 86, which is radon-222.

For the teacher: Applying, medium difficulty, standard 6.1.1

Textbook p. 200

Question 4

A nucleus of ²⁴₁₁Na undergoes beta decay. What are the nucleon number and proton number of the new nucleus?

Nucleon number 24, proton number 10
Nucleon number 20, proton number 9
Nucleon number 23, proton number 11
Nucleon number 24, proton number 12

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In β-decay a neutron changes to a proton: the nucleon number is unchanged (24) and the proton number rises by 1 (12). The nucleus is magnesium-24.

For the teacher: Applying, medium difficulty, standard 6.1.1

Textbook p. 201

Question 5

What happens to a nucleus when it emits a gamma ray?

Its proton number decreases by 2 and its nucleon number by 4
Its proton number increases by 1 and its nucleon number stays the same
It loses energy but its nucleon number and proton number do not change
Its nucleon number stays the same and its proton number decreases by 1

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Gamma rays carry away excess energy only. They have no mass and no charge, so A and Z stay the same.

For the teacher: Understanding, low difficulty, standard 6.1.1

Textbook p. 201

Question 6

A sample of a radioactive substance is heated to a much higher temperature. What happens to its rate of decay?

It increases
It stays the same
It decreases
It stops

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Radioactive decay is random and spontaneous and is not influenced by temperature, pressure or other physical factors.

For the teacher: Understanding, low difficulty, standard 6.1.1

Textbook p. 200

Question 7

The half-life of a radioactive substance is the time taken for

all the nuclei in the sample to decay completely
the mass of the sample to become twice its initial value
the radiation to travel a distance of one metre
the number of undecayed nuclei to decrease to half of the initial number

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Half-life is the time for the number of undecayed nuclei (and the activity) to fall to half of its initial value.

For the teacher: Remembering, low difficulty, standard 6.1.2

Textbook p. 202

Question 8

A radioactive source has an activity of 800 s⁻¹ and a half-life of 3 minutes. What is its activity after 12 minutes?

50 s⁻¹
100 s⁻¹
200 s⁻¹
400 s⁻¹

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n = 12 ÷ 3 = 4 half-lives. A = (½)⁴ × 800 = 800 ÷ 16 = 50 s⁻¹.

For the teacher: Applying, medium difficulty, standard 6.1.4

Textbook p. 206

Question 9

Two rock samples contain uranium-238 and its final product lead-206. Which rock is the oldest?

The one with the lowest ratio of lead-206 to uranium-238
The one with the most uranium-238
The one with the highest ratio of lead-206 to uranium-238
The one with the largest mass

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The longer the uranium has been decaying, the more lead-206 has formed, so a higher ratio of lead-206 to uranium-238 means an older rock.

For the teacher: Understanding, medium difficulty, standard 6.1.2

Textbook p. 203

Question 10

Which statement describes nuclear fission?

Two light nuclei join to form a heavier nucleus
A heavy nucleus splits into lighter nuclei and releases energy
An unstable nucleus emits only an electron
A nucleus absorbs energy and becomes lighter

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Fission is the splitting of a heavy nucleus (such as uranium-235) into two or more lighter nuclei with the release of a large amount of energy.

For the teacher: Understanding, low difficulty, standard 6.2.1

Textbook p. 209

Question 11

In a nuclear reaction the total mass of the products is less than the total mass of the reactants. The missing mass

is lost completely and cannot be accounted for
is carried away by the emitted neutrons only
stays in the nucleus as extra mass
is converted into energy according to E = mc²

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The mass defect is converted to energy: E = mc² (textbook p. 211).

For the teacher: Understanding, low difficulty, standard 6.2.2

Textbook p. 211

Question 12

A nuclear reaction has a mass defect of 2.0 × 10⁻²⁹ kg. What is the energy released? [c = 3.00 × 10⁸ m s⁻¹]

1.8 × 10⁻¹² J
6.0 × 10⁻²¹ J
6.7 × 10⁻³⁸ J
1.8 × 10⁻²⁰ J

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E = mc² = 2.0 × 10⁻²⁹ × (3.00 × 10⁸)² = 2.0 × 10⁻²⁹ × 9.0 × 10¹⁶ = 1.8 × 10⁻¹² J.

For the teacher: Applying, medium difficulty, standard 6.2.3

Textbook p. 212

Question 13

What is the function of the control rods in a nuclear reactor?

To slow down the fast neutrons so that fission can occur
To absorb excess neutrons to control the rate of the reaction
To produce steam to turn the turbine
To stop radiation leaking out to the surroundings

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Control rods (boron or cadmium) absorb neutrons. The moderator slows neutrons; concrete walls stop radiation leaking.

For the teacher: Remembering, low difficulty, standard 6.2.4

Textbook p. 216

Question 14

Which sequence shows the energy changes in a nuclear power station?

Nuclear → electrical → heat → kinetic
Heat → nuclear → kinetic → electrical
Nuclear → heat → kinetic → electrical
Kinetic → heat → nuclear → electrical

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Fission gives heat that makes steam, the steam turns the turbine (kinetic energy), and the generator produces electrical energy (textbook p. 214).

For the teacher: Understanding, low difficulty, standard 6.2.4

Textbook p. 214

Section B: structured questions

32 marks

Answer all questions. Show your working.

Question 15

Complete each decay equation by identifying A, B and C.

[2 marks] (a)²²⁸₈₈Ra → A + ⁰₋₁e

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Nucleon number 228 [1 mark]1
Proton number 89, so A is ²²⁸₈₉Ac [1 mark]1

Tick each point you got.

[1 mark] (b)²⁰⁹₈₄Po → ²⁰⁵₈₂Pb + B

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B is an alpha particle, ⁴₂He [1 mark]1

Tick each point you got.

[1 mark] (c)C → ¹₁p + ⁰₋₁e

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C is a neutron, ¹₀n [1 mark]1

Tick each point you got.

For the teacher: Applying, low difficulty, standard 6.1.1

Textbook p. 200

Question 16

Diagram 1 shows the uranium-238 decay series, which ends with stable lead-206.

Uranium-238 decay series from uranium-238 to lead-206 with the half-life of each step — Diagram 1

[1 mark] (a)State the half-life of radium-226.

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1 622 years [1 mark]1

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[4 marks] (b)Calculate the number of alpha particles and beta particles emitted in the decay of ²³⁸₉₂U to ²⁰⁶₈₂Pb.

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Fall in nucleon number = 238 − 206 = 32 [1 mark]1
Alpha particles = 32 ÷ 4 = 8 [1 mark]1
Proton number falls by 8 × 2 = 16 to 76, but lead has 82 [1 mark]1
Beta particles = 82 − 76 = 6 [1 mark]1

Tick each point you got.

[2 marks] (c)The ratio of uranium-238 to lead-206 is 5 : 1 in rock A and 7 : 1 in rock B. Which rock is older? Explain.

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Rock A [1 mark]1
It has more lead-206 in proportion to uranium-238, so more uranium has decayed [1 mark]1

Tick each point you got.

For the teacher: Analysing, medium difficulty, standard 6.1.2

Textbook p. 202

Question 17

Diagram 2 shows how the activity of a radioactive source changes with time.

Graph of activity against time in days: 640 at 0, 320 at 4, 160 at 8 and 80 at 12 days — Diagram 2

[1 mark] (a)Determine the half-life of the source.

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Activity halves from 640 to 320 s⁻¹ in 4 days, so T½ = 4 days [1 mark]1

Tick each point you got.

[3 marks] (b)Calculate the activity after 20 days.

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n = 20 ÷ 4 = 5 [1 mark]1
A = (½)⁵ × 640 [1 mark]1
= 20 s⁻¹ [1 mark]1

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[2 marks] (c)How long does it take for the activity to fall to 12.5% of its initial value?

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12.5% = (½)³, so 3 half-lives [1 mark]1
3 × 4 = 12 days [1 mark]1

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For the teacher: Applying, low difficulty, standard 6.1.3

Textbook p. 205

Question 18

A uranium-235 nucleus absorbs a neutron: ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₅Cs + ⁹³₃₇Rb + 2 ¹₀n + energy. The mass defect is 0.19585 amu.

[1 mark] (a)Name this type of nuclear reaction.

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Nuclear fission [1 mark]1

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[3 marks] (b)Calculate the energy released, in joules.

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m = 0.19585 × 1.66 × 10⁻²⁷ = 3.25 × 10⁻²⁸ kg [1 mark]1
E = mc² = 3.25 × 10⁻²⁸ × (3.00 × 10⁸)² [1 mark]1
= 2.93 × 10⁻¹¹ J [1 mark]1

Tick each point you got.

[2 marks] (c)Express your answer to (b) in MeV. [1 MeV = 1.6 × 10⁻¹³ J]

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2.93 × 10⁻¹¹ ÷ 1.6 × 10⁻¹³ [1 mark]1
= 183 MeV [1 mark]1

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For the teacher: Applying, high difficulty, standard 6.2.3

Textbook p. 212

Question 19

Diagram 3 shows a nuclear power station.

Nuclear power station: reactor, steam generator, turbine, generator, condenser and cooling tower — Diagram 3

[1 mark] (a)What nuclear reaction takes place in the reactor?

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Nuclear fission of uranium-235 [1 mark]1

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[2 marks] (b)Describe the chain reaction in the reactor.

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Fission of a uranium-235 nucleus releases (fast) neutrons [1 mark]1
These neutrons hit other uranium nuclei and cause further fission, which repeats [1 mark]1

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[2 marks] (c)State the function of the control rods and the material they are made of.

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Absorb excess neutrons to control the rate of the reaction [1 mark]1
Boron or cadmium [1 mark]1

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[2 marks] (d)State the energy changes from the reactor to the electrical output.

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Nuclear energy to heat energy [1 mark]1
Heat to kinetic energy (turbine) to electrical energy (generator) [1 mark]1

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[2 marks] (e)A coal-fired power station and a nuclear power station both generate electricity. State one advantage and one disadvantage of using nuclear energy.

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Advantage: large amount of energy from a small amount of fuel, or low carbon footprint [1 mark]1
Disadvantage: high cost, expert technology needed, or radioactive waste must be managed carefully [1 mark]1

Tick each point you got.

For the teacher: Evaluating, low difficulty, standard 6.2.4

Textbook p. 214

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