GCSE Physics Edexcel P2 Topic 5

These are revision notes I made myself. I got A* in the Physics GCSE exam and 80 UMS (full marks) in the P2 exam using only these notes.

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P2 Topic 5: Nuclear fission and nuclear fusion
Atoms contain electrons, protons and neutrons which are called sub-atomic particles. Protons and neutrons
are found in the nucleus and thus are called nucleons and electrons surround the nucleus in shells.
All atoms of the same element have the same number of protons. This number is the atomic/proton number
of an element. The mass number of an atom is the number of neutrons and protons added together.
An atom is electrically neutral as it has an equal number of positive protons and negative electrons. However,
when they become ions it means they have either lost or gained electrons to become more stable.
Relative mass and charge of sub-atomic particles
Relative Mass Relative Charge
Proton 1 +1
Neutron 1 0
Electron 1/1840, effectively 0 -1
Positron* 1/1840, effectively 0 +1
* Not normal "matter" but is antimatter.
Although all atoms of the same element have the same number of protons, that does not mean they have the
same mass number. This is because they may have different numbers of neutrons. Atoms of a single element
that have different numbers of neutrons are called isotopes.
For example, lithium atoms can exist as both Lithium ­ 6 and Lithium ­ 7. However, Lithium-6 has 3 protons
and 3 neutrons, whereas Lithium-7 has 3 protons but 4 neutrons.
Ionising Radiation
Ionising Radiation is radiation that has enough energy to cause atoms to lose electrons and become ions.
A radioactive substance has an unstable nucleus. To become stable, it must decay by losing energy by
emitting certain types of ionising radiation. These types are alpha particles, beta particles and gamma rays.
You cannot predict when a nucleus will decay as it is a random process.
Alpha particles
These contain two protons and two neutrons, so are effectively Helium nuclei. They have a charge of +2.
Alpha particles are emitted from the nucleus of unstable nuclei at high speeds. However, each time they
ionise an atom they lose some energy. As they produce many ions in a short distance they lose energy
quickly.
This means they have a short penetration distance.
They are stopped by a few millimetres of air or card.
Beta particles
These are negative particles; electrons. They are much less ionising than alpha particles so lose energy much
less quickly. As a result, they have a further penetration distance. They can be stopped by a few millimetres
of aluminium or even smaller thickness of lead.
Gamma rays
These are high-frequency electromagnetic waves (from the electromagnetic spectrum) emitted by unstable
nuclei and thus travel at the speed of light. They do not have an electrical charge.
These are about ten times less ionising than Beta particles and therefore can penetrate matter very easily.
They can be stopped by a few centimetres of lead or many metres of concrete.
Ionisation

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Ionisation is when an atom loses or gains an electron to turn into an ion.
Alpha particles are capable of pulling a negatively charged electron out of its orbit (they have a large positive
charge).
Beta particles push the electrons out of orbit instead.
Gamma rays transfer energy to electrons and if they get enough energy they can break free from the atom.…read more

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Nuclear power
Nuclear reactors can transform energy contained in the nuclei of Uranium and Plutonium atoms into thermal
energy using nuclear fission. The fuel is made into pellets, which are inserted into hollow fuel rods several
metres long. These rods are placed inside the reactor core.
The rate of which nuclear energy is transformed into thermal energy is kept constant by controlling the chain
reaction; just keeping one neutron being absorbed by a uranium nucleus at a time keeps the reaction at a
constant rate.…read more

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After some time, radioactive waste builds up in the reactor core. This waste consists of daughter nuclei and
radioactive isotopes from when the materials in the core absorb neutrons.
Fusion
Nuclear fusion occurs when small nuclei combine to form larger ones. A good example is hydrogen nuclei
combining to form helium, which is a common reaction in Stars, such as our Sun.…read more

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If the nuclei are travelling fast enough, some can overcome the electrostatic repulsion and fuse together. The
higher the temperature, the faster the nuclei move and the more likely they are to collide. As a result, the
temperature inside a fusion reactor must be very high ­ about 150,000,000C! (Ten times hotter than the
Sun).
It is extremely difficult to sustain a very high temperature and pressure required for fusion.…read more

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