In fission a heavy nucleus splits into smaller ones, and in fusion light nuclei join to form a heavier one. In both cases, careful measurement shows the total mass afterwards is a little less than before. This missing mass is called the mass defect.
Mass and energy are two forms of the same thing, linked by Einstein's relation E = mc squared. Because the speed of light c is so large, and it is squared, even a tiny loss of mass releases a huge amount of energy. That is why nuclear reactions give out far more energy per kilogram of fuel than burning coal or oil, which only rearranges electrons.
Fission powers nuclear reactors, while fusion is what makes the Sun shine, joining hydrogen into helium and losing mass in the process.
In SPM you should relate the energy released to the mass defect through E = mc squared and distinguish fission from fusion.
Common misconceptions
- Mass is destroyed in a nuclear reaction -> Mass is converted into energy; the two are equivalent, and the total mass-energy is conserved.
- Nuclear and chemical reactions release similar energy -> Nuclear reactions release vastly more per kilogram because they convert mass, not just rearrange electrons.
- Fission and fusion are the same process -> Fission splits a heavy nucleus; fusion joins light nuclei; both can release energy.
The physics behind it
In a nuclear reaction the total mass of the products is slightly less than the total mass of the starting materials. This missing mass is the mass defect, and it is converted into energy according to Einstein’s relation: energy released equals mass defect multiplied by the speed of light squared.
The speed of light c is about 3.0 × 10⁸ m s⁻¹, and because it is squared, even a tiny mass loss gives an enormous energy. If a mass defect of 0.001 kg is converted, the energy released = 0.001 kg × (3.0 × 10⁸ m s⁻¹)² = 0.001 × 9.0 × 10¹⁶ = 9.0 × 10¹³ J, where energy is in joules (J).
This is why nuclear reactions give out far more energy per kilogram of fuel than burning coal or oil, which only rearranges electrons. In fission a heavy nucleus splits into smaller ones; in fusion light nuclei join to form a heavier one.
Both show a mass defect, and both release energy.
See it in daily life
The energy that reaches you from the Sun comes from nuclear fusion. Deep in its core, hydrogen nuclei join to form helium, and the small mass lost in each step is released as energy that eventually leaves as sunlight and warmth.
Every plant that grows and every solar panel that charges is, in the end, powered by mass turning into energy inside a star.
Closer to home, a nuclear power station uses fission. The nucleus of a heavy element is split, releasing energy that heats water into steam; the steam then turns a turbine and a generator to produce electricity.
A very small mass of fuel yields a great deal of energy, which is why so little fuel is needed compared with a coal station.
The same physics underlies both the gentle warmth of the Sun and the intense output of a reactor: a small mass defect converted through Einstein’s relation.
How this comes up in SPM
In Paper 2 you are commonly asked to relate the energy released in a nuclear reaction to the mass defect through Einstein’s relation, and to calculate the energy released from a given mass defect. Recall the relation yourself, as there is no formula sheet, and always keep units on each line.
Questions often ask you to distinguish between fission and fusion, and to state that both involve a loss of mass. You may be asked to describe a fission chain reaction, or to explain why nuclear fuel releases so much more energy than a chemical fuel of the same mass.
This sits at the end of nuclear physics, next to radioactive decay and half-life, so be ready to link mass defect to the energy that powers reactors and stars, and to convert a mass in kilograms into an energy in joules.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)