3.8.1.6 Mass and energy and Induced fission pt 1.

Question 1

What is the mass energy equivalence equation? ;)

Answer 1

E = mc^2

Question 2

What are examples of mass-energy equivalence?

Answer 2

The fusion of hydrogen into helium in the centre of the Sun.
The fission of uranium in nuclear power plants.
Nuclear weapons.
High-energy particle collisions in particle accelerators.

Question 3

What is mass defect?

Answer 3

The difference between an atom’s mass and the sum of the masses of its protons and neutrons.

∆m = Zm_p + Nm_n - m_total

Question 4

What is binding energy?

Answer 4

The amount of energy required to separate a nucleus into its constituent protons and neutrons.

Question 5

What type of reaction is the formation of a nucleus?

Answer 5

Exothermic - mass and energy are proportional, so the total energy of a nucleus is less than the sum of energies of its constituent nucleons.

Question 6

What is nuclear fusion?

Answer 6

The fusing together of two small nuclei to produce a larger nucleus.

Question 7

Why is it difficult for fusion to happen?

Answer 7

Both nuclei must have high kinetic energy - this is because nuclei must be able to get close enough to fuse.
This is difficult to achieve due to electrostatic repulsion (protons in nuclei repel each other as they are positively charged) and the strong force (acts at very short distances within nuclei, so nuclei must get very close together).
It takes a lot of energy to overcome these forces, so fusion can only be achieved in extremely hot environment e.g. the core of a star

Question 8

What is nuclear fission?

Answer 8

The splitting of a large atomic nucleus into smaller nuclei.

Question 9

How does nuclear fission happen?

Answer 9

When a nucleus is struck by a neutron, it splits into two or more daughter nuclei. Neutrons are also ejected from the nucleus, which can collide with other nuclei, leading to a chain reaction, which occurs until all of the material has undergone fission, or the reaction is halter by the moderator.

Question 10

What is the binding energy per nucleon?

Answer 10

The binding energy of a nucleus divided by the number of nucleons in the nucleus.

A higher binding energy per nucleon indicates higher stability

Question 11

What are the key features of the average binding energy against number of nucleons in the nucleus graph?

Answer 11

Low A; Nuclei have lower binding energy per nucleon so less stable. Means that lightest elements have weaker electrostatic forces and are more likely to undergo fusion.
High A; general binding energy per nucleon is high and gradually decreases with A. Means that heaviest elements are most unstable and likely to undergo fission.

Question 12

Which elements do not fit the binding energy per nucleon trend?

Answer 12

H-4, C-12, O-16
H-4 is a particularly stable nucleus so has a high binding energy per nucleon.
C-12 and O-16 can be considered to be 3 and 4 helium nuclei respectively.

Question 13

Which element is the most stable?

Answer 13

Fe-56
it has the highest binding energy per nucleon

Question 14

Why does fusion occur at low values of A?

Answer 14

Attractive nuclear forces dominate over repulsive electrostatic forces between protons.

Question 15

Why is the mass of the nucleus created in fusion less than the sum of the original nuclei?

Answer 15

Mass defect is equal to the binding energy that is released since the nucleus that is formed is more stable.

Question 16

Why does fission occur at high values of A?

Answer 16

Repulsive electrostatic forces between forces dominate, which tend to break apart the nucleus.

Question 17

Compare fusion and fission

Answer 17

• Fusion occurs at low values of A , fission occurs at high values of A
• Mass of nucleus created in fusion is less than the reacting particles, in fission, an unstable nucleus is converted to more stable nuclei with smaller total mass.
• Fusion releases more energy per kg than fission (energy released is the difference in binding energy caused by difference in masses)
• at small A (fusion), gradient is much steeper compared to large values of A (fission) - corresponding to a larger binding energy per nucleon released by fusion.

Question 18

What is induced fission?

Answer 18

When a stable nucleus splits into small nuclei from bombardment of a slow-moving neutron.

Question 19

What are thermal neutrons?

Answer 19

Neutrons involved in induced fission.
They have low energy and speed, meaning they can induce fission - this is important as neutrons with too much energy will rebound away from the nucleus and fission does not take place.

Question 20

What is critical mass?

Answer 20

The minimum mass of fuel required to maintain a steady chain reaction

Question 21

What happens if you go over or under the critical mass?

Answer 21

At critical mass - means that a single fission reaction follows the last.
Less than - subcritical mass leads to the reaction eventually stopping.
More than - supercritical mass leads to a runaway reaction and eventually an explosion.