- 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
- 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, ie. 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
Nuclear Fusion (Edexcel IGCSE Physics: Double Science)
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Nuclear Fusion
- Small nuclei can react to release energy in a process called nuclear fusion
- Nuclear fusion is defined as:
When two light nuclei join to form a heavier nucleus
- This process requires extremely high temperatures to maintain
- This is why nuclear fusion has proven very hard to reproduce on Earth
- Stars, including the Sun, use nuclear fusion to produce energy
- Therefore, fusion reactions are very important to life on Earth
- In most stars, hydrogen atoms are fused together to form helium and produce lots of energy
Two hydrogen nuclei are fusing to form a helium nuclei
- The energy produced during nuclear fusion comes from a very small amount of the particle’s mass being converted into energy
- Albert Einstein described the mass-energy equivalence with his famous equation:
E = mc2
- Where:
- E = energy released from fusion in Joules (J)
- m = mass converted into energy in kilograms (kg)
- c = the speed of light in metres per second (m/s)
- The amount of energy released during nuclear fusion is huge:
- The energy from 1 kg of hydrogen that undergoes fusion is equivalent to the energy from burning about 10 million kilograms of coal
Conditions for Fusion
- 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 wasteB Hydrogen nuclei require very high temperature to fuse togetherC Hydrogen is a rare element that would be difficult to get large amounts ofD 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
- 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
Fusion vs Fission
- Fusion and fission reactions should not be confused
- We are able to use fission reactions on Earth as a source of energy for producing electricity
- However, the temperatures required to maintain fusion reactions means that it is not yet a commercially viable method for generating electricity
- If however it were possible, then fusion would have a number of advantages over fission:
- Fusion reactions would be capable of producing even more energy than fission reactions
- The fuels required for fusion reactions are more plentiful than the Uranium and Plutonium used in fission reactors
- The products of nuclear fusion are much safer, with very little long term radioactive waste produced, and the risk of a dangerous accident at a fusion reactor is much lower than in a fission reactor
- The following table summarises some of the key differences between fusion and fission:
Comparison of Nuclear Fusion and Fission Table
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