6.4 Nuclear Physics (Nuclear Model and E=mc2)

Question 1

Describe Thompson’s plum pudding model

Answer 1

Neutral atom made of a uniform sphere of positive charge with tiny electrons embedded in it.

Question 2

What experiment disproved the plum pudding model?

Answer 2

Rutherford’s alpha-scattering experiment.

Question 3

How did the alpha scattering experiment work?

Answer 3

Alpha particles are fired at a thin sheet of gold foil under a vacuum (so that alpha doesnt get stopped by air in the apparatus).

The deflected particles are detected on all sides by a ring of scintillators, which release photons when a particle hits them.

Question 4

What did Rutherford observe during his alpha scattering experiment?

Answer 4

He saw the majority of particles passing straight through the foil, with only a slight deflection of, on average, only a degree.

A small proportion of the alpha particles were deflected by more than 90 degrees.

Question 5

What did Rutherford’s observations lead him to conclude?

Answer 5

The atom is mostly empty space, and the mass must be concentrated at some point within the atom (the nucleus).

The nucleus is small, dense and positively charged.

The nucleus is surrounded by orbiting negative electrons which make the overall charge of the atom neutral.

[This is known as the Rutherford model]

Question 6

What is an isotope?

Answer 6

Atoms of an element with the same number of protons but a different number of neutrons.

Question 7

How do isotopes of the same element differ reaction-wise?
What is this due to?

Answer 7

They undergo the same chemical reactions, but undergo different nuclear reactions.
This is due to their electron configurations being identical, whereas the stability of the nuclei vary.

Question 8

What does a nucleon refer to?

Answer 8

A subatomic particle that resides in the nucleus of the atom.

Question 9

What is the Z number (bottom)?

Answer 9

Proton number.

Question 10

What is the A number (top)?

Answer 10

The mass / nucleon number. (sum of protons and neutrons in nucleus).

Question 11

How can an estimate for the mass of an atom be found?
Why is this only an estimate?

Answer 11

By multiplying the mass/nucleon number by the atomic mass unit.
This is an estimate because an atom is often heavier than its constituent parts.

Question 12

How can we experimentally determine the radius of a gold nucleus?

Answer 12

• Fire an alpha particle at it. There will be electrostatic repulsion between the alpha and gold.
• At the closest point between them, the alpha’s kinetic energy will have been turned into electric potential energy.
• Using coulomb’s law, we can find the distance r.

Question 13

What equation relates the radius of an atom and its nucleon number?

Answer 13

r = r0 x A^1/3

r: radius
r0: constant (for all atoms)
A: nucleon number

Question 14

What is the constant r0 equal to?

Answer 14

1.2fm.

Question 15

Which is more dense? An atom or a nucleus.

Answer 15

Nucleus, because the atom includes a lot of empty space.

Question 16

Define atomic mass unit.

Answer 16

One twelfth the mass of carbon-12.

Question 17

Name the four fundamental forces.

Answer 17

• Gravitational force
• Electromagnetic force
• Weak nuclear force
• Strong nuclear force

Question 18

Describe the gravitational force.

Answer 18

The force that acts between bodies with mass. It is always attractive and has an infinite range, but it is very weak.

Question 19

Describe the electromagnetic force.

Answer 19

The force that acts on particles with charge, and comprises all electrostatic and magnetic forces as well as radiation pressure. It has an infinite range.

Question 20

Describe the weak nuclear force.

Answer 20

The force that is responsible for beta decay, and acts to change quark types over very small distances.

Question 21

Describe the strong nuclear force.

Answer 21

The force that acts between all nucleons and quarks, holding the nucleus together. It does this by counteracting the repulsive electrostatic forces between protons in the nucleus, but not so much as to cause the nucleus to collapse.
It is attractive at small distances and repulsive at incredibly small distances. It has a limited range.

Question 22

At what distances does the strong force attract / repel?

Answer 22

Below 0.5fm - Repulsive
Up to 3.0fm - Attractive

Question 23

What are hadrons

Answer 23

A class of subatomic particles that are composed of quarks, including protons and neutrons.

Question 24

Do hadrons experience the strong force?

Answer 24

Yes

Question 25

What are the classes of hadrons?

Answer 25

Baryon (three quarks)
Mesons (two quarks)

Question 26

What is the equation for mass energy equivalence?

Answer 26

ΔE = Δmc²

ΔE: Change in energy
Δm: Change in mass
c: Speed of light in a vacuum

Question 27

Scientists want to measure the mass of something. Why should it be measured at rest, rather than when it has been given lots of energy and is moving around quickly?

Answer 27

Any object, given high enough energy, can acquire a very high mass. The only true measure of mass is when it is at rest.

Question 28

What forms of energy does the mass energy equivalence apply to?

Answer 28

All forms of energy. (kinetic, thermal, gpe, potential from strong nuclear force, …)

Question 29

What is the binding energy?

Answer 29

The minimum energy required to completely separate a nucleus into its constituents. (This can be the ΔE in E=mc2)

Question 30

What is the binding energy per nucleon?

Answer 30

The minimum energy required to remove a nucleon from the nucleus.

Question 31

I have the binding energy of a nucleus. How do I get the binding energy per nucleon?

Answer 31

Divide it by the number of nucleons.

Question 32

Why is the mass of a helium nucleus slightly lower than its constituent parts?

Answer 32

The binding energy has increased, and therefore the mass must decrease to obey mass-energy conservation.

Question 33

What is the mass defect?

Answer 33

The difference between the mass of a completely separated nucleus, and the nucleus itself. (This can be the Δm in E=mc2)

Question 34

Why is Iron-56 the most stable isotope?

Answer 34

It has the maximum binding energy per nucleon.

Question 35

[Graph: Binding Energy per nucleon against Nucleon number]
Can elements to the right of iron yield energy through fusion?

Answer 35

No. This is because the binding energy will decrease. Fission must be used.

Question 36

[Graph: Binding Energy per nucleon against Nucleon number]
Can elements to the left of iron yield energy through fission?

Answer 36

No. This is because the binding energy will decrease. Fusion must be used.

Question 37

Two small nuclei H-2 and H-3 are combined. What is this called? What happens?

Answer 37

Nuclear Fusion
- Mass has been decreased, going from the mass of the two smaller nuclei to the resulting He nucleus.
- This is due to a significant increase in the strong attraction between nucleons, compared to the small increase in electrostatic repulsion.
- The mass defect causes energy to be released.

Question 38

I have decided to break apart a large nucleus, with more nucleons than iron. What is this called? What happens?

Answer 38

Nuclear Fission
- The binding energy per nucleon decreases for nuclei with more nucleons than iron
- Breaking apart larger nuclei will release the binding energy and reduce the mass.
- This is because the large proton number causes a large repulsive force, and a relatively feeble strong attraction because of the increased average distance between nucleons.