Completed

The whole specification for IGCSE Physics has been completed (Yay!)
Good luck and happy studying.

7c) Particles

Geiger and Marsden's experiment with gold foil and alpha particles:
Experiment - Model with gold foil where nuclei of helium atoms (positively charged). If the model is correct all alpha particles would go straight through foil
Results - Some of the alpha particles bounced back meaning that inside the atoms there most be small positively charged nuclei which repels the passing alpha particles. So the model of the atom says that: most of the mass must be concentrated at the centre, nucleus must be small compared to atoms as very few alpha particles are deflected and negative charge (electrons) must be around the nuclear material.

Nuclear fission:
Is the splitting of a large nucleus (for example Uranium-235) into smaller fragments which is an artificial process so doesn't occur naturally- this releases energy.

Process steps:

1. The 'fuel' that is split in a nuclear reactor which is usually a stable isotope such as uranium-235 or platonium-239 
2. If a slow moving neutron gets absorbed by the nucleus the nucleus can split
3. Every time a U-235 nucleus splits up, it results in a small number of neutrons and chain reactions can occur if they go on to hit another U-235, causing it to split and release more smaller neutrons and so on. 
4. When a U-235 splits in two it will form two new daughter nuclei, usually Barium and Krypton- they are lighter elements than uranium
5. The new nuclei are usually radioactive
6. Each nucleus splitting gives out a lot of energy which is in the form of kinetic energy of the fission products - the daughter nuclei and (three) neutrons
7. This energy can then be converted into heat energy in the reactor by collisions with other atoms 

Nuclear Reactor:

1. free neutrons in the reactor start the fission process
2. The atoms produced the collide with other atoms causing the temperature to rise in the reactor
3. The moderator (water or graphite) slows neutrons so that they can successfully collide with uranium nuclei and chain reaction is controlled a steady output of heat
4. The control rods limit the rate of fission by absorbing excess neutrons- this also maintains a steady output of energy 
5. carbon dioxide or another gas is pumped through the reactor to carry away the heat generated 
6. The gas is passed through a heat exchanger and the water is heated turned to steam which turns the turbines and generator, generating electricity which can then be used by the National Grid to homes 

Nuclear waste? It's hard to find a suitable site to dispose and has to be stored until radioactivity levels are lower

(Extra: Nuclear Fusion is when two small nuclei are joined together, at the moment this can only occur naturally- an example of this would be the sun. This method doesn't produce waste and can be recycled)