# Edexcel IGCSE Physics: Double Science 复习笔记：7.3.4 Nuclear Fusion

### 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

• 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

• 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

• 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