Fusion & Stars | HL IB Physics Topic Questions 2025

1a

2 marks

The binding energy per nucleon of Helium−4 $({}_{2}^{4}{He})$ is 7.1 MeV.

Determine the energy required to completely separate the nucleons of the atom of helium. Give your answer in MeV.

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1b

2 marks

Match the processes with the correct definition by drawing a line:

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1c

4 marks

Complete the following sentences:

Helium is formed inside main sequence stars due to the process of nuclear ......................... .

For this process to occur, both nuclei must have high ......................... energy.

A high energy is required because the protons inside the nuclei are ......................... charged and a very high energy is needed to overcome the ......................... force of repulsion.

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1d

2 marks

Main sequence stars can remain stable for millions of years due to the equilibrium of internal forces.

Explain how a main sequence star can remain in equilibrium.

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1a

5 marks

In the research into nuclear fusion, one of the most promising reactions is between deuterium ${}_{1}^{2}H$ and tritium nuclei ${}_{1}^{3}H$ in a gaseous plasma.

Although deuterium can be extracted relatively easily from seawater, tritium is more difficult to produce. It can, however, be produced through the induced fission of lithium−6 ${}_{3}^{6}{Li}$ nuclei.

These reactions can be represented in the following nuclear equations:

${}_{1}^{2}H + {}_{1}^{3}H \to {}_{2}^{4}{He} + {}_{0}^{1}n + energy$

${}_{3}^{6}{Li} + X \to {}_{1}^{3}H + Y + energy$

The masses of the nuclei involved are given in the following table:

Nucleus	Mass / u
neutron	1.008665
deuterium	2.014102
tritium	3.016049
helium−4	4.002604
lithium−6	6.015122

(i)

Determine the nature of particles X and Y.

[1]

(ii)

Calculate the energy released from the fission of 1.0 kg of lithium-6.

[2]

(iii)

Calculate the energy released from the fusion of 1.0 kg of a 50:50 mixture of deuterium and tritium (D-T).

[2]

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1b

4 marks

Calculate the mass of deuterium and lithium-6 required to produce a power output of 500 MW from a fusion plant for a year.

Assume an efficiency of one-third for the conversion of fusion energy into electrical energy.

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1c

6 marks

Over the coming decades, nuclear fusion reactors are expected to become commercially viable for the first time. Meanwhile, nuclear fission reactors have been in operation for decades already.

(i)

Explain, with reference to the operating conditions of both types of reactor, why this discrepancy exists.

[4]

(ii)

The average energy released per fission of a uranium-235 nucleus is about 200 MeV.

Compare the energy generated from the fusion of 1 kg of D-T and the fission of 1 kg of uranium-235.

[2]

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1d

4 marks

The aim of current fusion research programs is to achieve fusion 'ignition', which occurs when enough fusion reactions take place for the process to become self-sustaining.

(i)

Suggest how D-T fusion could be thought to be self-sustaining once 'ignition' is achieved.

[2]

(ii)

Discuss two problems associated with sustaining D-T fusion reaction for the production of energy on a commercial scale.

[2]

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1a

4 marks

Under the right conditions, four hydrogen nuclei can fuse to make a helium nucleus in a process known as the proton–proton cycle.

Nuclei	Mass/ u
${}^{1}H$	1.0078
${}^{4}{He}$	4.0026

Show that 4 × 10^–12 J of energy is released as a result of the fusion of four hydrogen nuclei.

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1b

2 marks

Fusion occurs naturally in the core of stars.

Explain why very high densities of matter and very high temperatures are needed to bring about and maintain nuclear fusion in stars.

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1c

2 marks

While on the main sequence, the Sun maintains a constant luminosity of 3.86 × 10²⁶ W. It is predicted that the Sun will spend a total of 10¹⁰ years in this phase of its evolutionary cycle.

Show that the Sun will convert a total mass of 2 × 10²⁹ kg of hydrogen into helium during its time on the main sequence.

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1d

3 marks

One day, the Sun will leave the main sequence and move on to the next stage of its evolutionary cycle.

Discuss what will happen to the Sun.

In your answer, you should outline

the conditions that will initiate this change
the nuclear processes that will occur
the physical changes that the Sun will undergo.

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	nuclear mass
${}_{1}^{1}p$	1.007825u
${}_{1}^{2}H$	2.014102u

mass
radius
surface temperature
core temperature

star colour	surface temperature / T_☉
blue	> 5.5
blue-white	1.7 − 5.5
white	1.3 − 1.7
yellow-white	1.0 − 1.3
yellow	0.8 − 1.0
orange	0.6 − 0.8
red	< 0.6

	Trappist-1	Sun
*Luminosity L / L_☉*	5.26 × 10⁻⁴	1
*Surface temperature T / T_☉*	0.4	1
Radius of star / R_☉	R	1
Distance between Earth and Sun / m	−	1.5 × 10¹¹
Distance between planets and Trappist-1 / m	1.6 × 10⁹ to 9.0 × 10⁹	−

Name	Colour	Luminosity / L_Sun
Aljanah	orange	62
Deneb	blue-white	196 000
Fawaris	blue	155
Sadr	yellow-white	33 000

Fusion & Stars (HL IB Physics)

Topic Questions