5.1.4Nuclear Physics
Fission and Fusion — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 6 of 17 1 quiz
Lesson summary
5.1.4 Fission and Fusion — IGCSE PHYSICS notes: Describe nuclear fission and fusion, compare them and explain why each releases energy.Source — fission and fusion change the nucleus to release energy stored in it.Amount — they release far more energy per kilogram of fuel than che…
- Nucleon numbers
- Proton numbers
- Energy transfer
- Chain reaction
- Control rods
- Power station
Part 1 of 8Energy from nuclei
Source
- Source fission and fusion change the nucleus to release energy stored in it.
Amount
- Amount they release far more energy per kilogram of fuel than chemical reactions.
Form
- Form the energy appears mainly as kinetic energy of the products and as gamma radiation.
Part 2 of 8Nuclear fission
Nuclear fission
Nuclear fission the splitting of a large, unstable nucleus into two smaller nuclei.
Reactants
- Reactants a neutron collides with an unstable nucleus such as uranium-235.
Products
- Products two smaller daughter nuclei, two or three neutrons and gamma rays.
Fuels
- Fuels large nuclei such as uranium and plutonium are used in nuclear power stations.
1
A slow-moving neutron is absorbed by a uranium-235 nucleus.
2
The nucleus becomes unstable and splits into two smaller nuclei.
3
Two or three neutrons and energy are released.
4
The neutrons can cause further fissions.
Part 3 of 8Fission equation
23592U + 10n → 9236Kr + 14156Ba + 3 10n + energy.
Nucleon numbers
- Nucleon numbers the totals are equal on both sides: 235 + 1 = 92 + 141 + 3.
Proton numbers
- Proton numbers the totals are equal on both sides: 92 + 0 = 36 + 56 + 0.
Energy transfer
- Energy transfer energy moves from the nuclear store of the parent nucleus to the kinetic store of the fast-moving products.
Mass
- Mass the products have fractionally less mass than the original nucleus, because the missing mass is converted into energy.
Part 4 of 8Chain reaction and control
Chain reaction
- Chain reaction released neutrons are absorbed by other U-235 nuclei, causing further fissions in a self-sustaining process.
Control rods
- Control rods made of boron or cadmium, they absorb neutrons, so lowering them slows the reaction.
Moderator
- Moderator water or graphite slows neutrons so they are more likely to be absorbed.
Power station
- Power station the heat makes steam to turn a turbine that drives a generator.
Weapon
- Weapon in a nuclear weapon the chain reaction is uncontrolled.
Part 5 of 8Nuclear fusion
Nuclear fusion
Nuclear fusion two light nuclei joining to form a heavier nucleus.
Stars
- Stars in most stars hydrogen nuclei fuse into helium, releasing a huge amount of energy.
Conditions
- Conditions extremely high temperature and pressure, because the positive nuclei repel each other, so fusion is hard on Earth.
Equation
- 21H + 11H → 32He + energy.
Mass
- Mass the fused nucleus has fractionally less mass than the two original nuclei, and the missing mass is released as energy.
Energy
- 1 kg of hydrogen undergoing fusion gives as much energy as burning about 10 million kg of coal.
Part 6 of 8Fission compared with fusion
| Feature | Fission | Fusion |
|---|---|---|
| Process | Large nucleus splits | Small nuclei join |
| Fuel | Uranium-235, plutonium-239 | Hydrogen isotopes |
| Conditions | Neutron absorption at ordinary temperatures | Extremely high temperature and pressure |
| Waste | Radioactive waste | Much less long-lived waste |
| Use | Power stations today | Stars and experimental reactors |
Part 7 of 8Mass and energy (Extended)
E
E m c2
What the symbols mean: E = energy (J) · m or Δm = mass (change) (kg) · c = 3.00 × 108 m s−1
- a tiny loss of mass gives a large amount of energy because c2 is huge.
Iron
- Iron it is the most stable nucleus, so splitting heavier nuclei or joining lighter ones both release energy.
Fission waste
- Fission waste products are radioactive and some stay dangerous for thousands of years, so they need secure storage.
Fission advantage
- Fission advantage no carbon dioxide is produced while running.
Part 8 of 8More from the specification
More from the specification
Nuclear energy release mechanisms
- Nuclear energy release mechanisms comparing binding energy per nucleon curves provides a theoretical means to calculate energy released during fission and fusion.
Atomic Structure and Relative Charges
- Atoms consist of a central nucleus containing positively charged protons and uncharged neutrons (relative charge 0), surrounded by orbiting negatively charged electrons (relative charge -1); uncharged neutral atoms have an equal number of protons and electrons.
- (relative charge +1)
Nuclide Notation and Subatomic Particle Numbers
- In standard nuclide notation ᴬ_Z X, the atomic number Z represents the number of protons,
- and the mass number A represents total nucleons; subtracting Z from A gives the number of neutrons.
- (protons + neutrons)
Nuclear Fission vs Nuclear Fusion Reactions
- Nuclear fission occurs when a heavy, unstable nucleus absorbs a neutron and splits into smaller daughter nuclei, releasing neutrons and large kinetic energy; nuclear fusion occurs when light nuclei (such as hydrogen isotopes) fuse under high temperature and pressure to form a heavier helium nucleus.
- (such as uranium-235)
Nuclear model
- Model of the atom with a small dense nucleus surrounded by electrons.
Charge
- Quantity of electricity in coulombs, Q = It.What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
Electron
- Negatively charged particle in an atom.
Nuclide
- Nucleus with a specific number of protons and neutrons.
Relative charge
- Charge compared with the fundamental charge 1.6 × 10−19 C.
Alpha particle
- Helium nucleus, charge +2.
Nuclide notation
- AZX.
Alpha scattering
- Experiment firing alpha particles at gold foil that showed the nuclear atom.
Nuclide equation
- An equation showing nuclei and particles in a nuclear reaction.
Key terms
| Term | Meaning |
|---|---|
| Nuclear fission | Splitting of a large unstable nucleus into two smaller nuclei. |
| Nuclear fusion | Joining of two light nuclei to form a heavier nucleus. |
| Chain reaction | Self-sustaining series of fissions caused by released neutrons. |
| Control rod | Neutron-absorbing rod that slows the chain reaction. |
| Moderator | Material that slows neutrons. |
| Daughter nuclei | The smaller nuclei produced in fission. |
Avoid these mistakes
1
Saying fission joins nuclei — fission splits them.
2
Thinking control rods speed up the reaction — they absorb neutrons and slow it.
3
Forgetting that fusion needs extremely high temperatures.
4
Confusing nuclear fission with radioactive decay.
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