The Conditions for Fusion (Edexcel GCSE Physics)
Revision Note
The Conditions for Fusion
Since protons have a positive charge, they repel each other
In order to overcome this repulsion, the protons must have very high kinetic energy in order to be travelling towards each other at very high speeds
The fusion of two protons is only possible through high temperature and pressure
In order to make the molecules of a gas travel at such speeds, the gas has to be heated to millions of degrees Celsius – a temperature that is usually only reached at the centre of a star
In regular conditions, i.e. on Earth, the possibility of collisions between nuclei which result in fusion is very low
In order to increase the number of collisions (and hence fusions) that occur between nuclei, high densities (and hence pressures) are also needed
The conditions for fusion are:
Very high temperature of fuel
Very high kinetic energy / speed of nuclei to overcome repulsion
Very high density / pressure to increase the possibility of suitable collisions
Energy from Fusion
The main reasons why fusion is not currently used as a source of power on Earth are the difficulties in achieving (and maintaining)
High temperatures
High pressures
Whilst physicists have been able to attain the temperatures and pressure needed, there are difficulties in containing them, which inevitably means that only a small amount of fusion can take place
Such a small rate of fusion is not useful for current energy needs
Creating the temperatures needed for fusion requires a great deal of energy
Hence, physicists are still a long way from the point where they will produce more energy from fusion than the energy needed to start it
Worked Example
An example of a hydrogen fusion reaction which takes place in stars is shown here.
Which of the following is a valid reason as to why hydrogen fusion is not currently possible on Earth?
A. Hydrogen fusion produces dangerous radioactive waste
B. Hydrogen nuclei require very high temperature to fuse together
C. Hydrogen is a rare element that would be difficult to get large amounts of
D. Hydrogen fusion does not produce enough energy to be commercially viable
Answer: B
Hydrogen nuclei have positive charges
So two hydrogen nuclei would have a repulsive force between them
There exists a repulsive force between protons that has to be overcome for hydrogen fusion to happen
Hydrogen ions are protons, and their positive charge makes them repel one another
High temperatures are required to give the nuclei enough energy to overcome the repulsive force
The answer is not A because the products of the hydrogen fusion shown in the reaction is helium
Helium is an inert gas
The answer is not C because hydrogen is a very abundant element
It is the most common element in the universe
The answer is not D because hydrogen fusion would produce a huge amount of energy
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