Thermal, Chemical, and Nuclear Energy
Thermal energy is the total kinetic and potential energy of the particles in a substance, chemical energy is the energy stored in the bonds between atoms, and nuclear energy is the energy stored in the nuclei of atoms; each can be released and transferred to other stores. In brief: thermal energy depends on the temperature, mass and material. Chemical energy is released in reactions such as combustion and respiration. Nuclear energy is millions of times more concentrated than chemical energy: fission of 1 kg of uranium-235 releases roughly 8 × 1013 J, while burning 1 kg of coal releases about 3 × 107 J.
Heating a house, burning petrol and running a nuclear power plant all involve these three stores. This guide describes thermal, chemical and nuclear energy, explains how each is released, compares them in a table, works through a comparison of energy per kilogram and corrects common mistakes.
What is thermal energy?
All matter is made of particles that move (vibrate in solids, move freely in gases) and attract each other. The sum of the kinetic and potential energies of all the particles is the internal (thermal) energy. When a substance is heated, its particles move faster, and the temperature rises. Thermal energy depends on the amount of substance, the temperature and the material: a bath of warm water holds more thermal energy than a cup of hotter water, because it has more particles. Thermal energy spontaneously moves from hotter to colder places. The energy needed to raise the temperature of mass m by ΔT is Q = mcΔT, where c is the specific heat capacity (water: 4,180 J/kg·K).
What is chemical energy?
Chemical energy is stored in the arrangement of atoms in molecules, in the form of electrical potential energy of electrons and nuclei. In a chemical reaction, bonds break and form. If the products have lower energy than the reactants, the energy is released, usually as heat and light (exothermic reactions): combustion of fuels, explosives, respiration. Food, wood, petrol and batteries are stores of chemical energy. Typical values: petrol about 46 MJ/kg, wood about 16 MJ/kg, carbohydrate about 17 MJ/kg (4 kcal/g), fat about 38 MJ/kg (9 kcal/g).
What is nuclear energy?
Nuclear energy is stored in the nuclei of atoms, held by the strong nuclear force. It is released in two ways:
- Fission: a heavy nucleus such as uranium-235 splits into two lighter nuclei, after absorbing a neutron, and releases about 200 MeV per nucleus, plus neutrons that can continue a chain reaction. Used in nuclear power stations.
- Fusion: light nuclei, such as hydrogen isotopes, join to make a heavier nucleus, releasing energy. It powers the Sun, and is the aim of fusion energy research.
- Radioactive decay: unstable nuclei emit radiation and release smaller amounts of energy.
The energy comes from a small decrease in mass, according to E = mc2. Because c2 is very large (9 × 1016 m2/s2), a tiny mass change gives a very large energy.
Three stores compared
| Store | Where is it? | How is it released? | Typical use |
|---|---|---|---|
| Thermal | Motion and bonds of particles | Cooling, heating other objects | Central heating |
| Chemical | Bonds between atoms | Chemical reactions | Fuels, food, batteries |
| Nuclear | Nuclei of atoms | Fission or fusion | Power stations, the Sun |
Worked example: comparing energy per kilogram
- Thermal: to heat 1.0 kg of water by 50 K requires Q = mcΔT = 1.0 × 4,180 × 50 = 209,000 J = 0.21 MJ.
- Chemical: burning 1.0 kg of petrol releases about 46 MJ. Ratio to item 1: 46 ÷ 0.21 = about 220. So 1 kg of petrol could heat that water by 50 K about 220 times (in an ideal case with no losses).
- Nuclear (fission of U-235): about 8 × 1013 J per kg. Ratio to petrol: 8 × 1013 ÷ 4.6 × 107 = about 1.7 × 106, which means that fission of 1 kg of uranium-235 releases about 1.7 million times as much energy as burning 1 kg of petrol.
- Mass change check: E = mc2, so for 8 × 1013 J, m = 8 × 1013 ÷ 9 × 1016 = 8.9 × 10−4 kg, which is less than 1 gram of mass converted out of 1 kg of uranium.
How were these energies discovered?
James Joule measured the mechanical equivalent of heat in the 1840s. Antoine Lavoisier studied combustion and respiration in the 1780s. Henri Becquerel discovered radioactivity in 1896. Lise Meitner and Otto Frisch explained nuclear fission in 1939 after the experiments of Otto Hahn and Fritz Strassmann, and the first controlled chain reaction was achieved by Enrico Fermi’s team in 1942. The first commercial nuclear power plant for electricity began operation in 1954 in the Soviet Union.
Which mistakes are common?
Mistake 1: Confusing heat and temperature
Temperature measures the average kinetic energy of particles. Thermal energy depends on the total number of particles as well.
Mistake 2: Saying that chemical reactions create energy
They transfer the energy that was already stored in bonds.
Mistake 3: Thinking that nuclear energy comes from burning
Fission and fusion are nuclear processes, not chemical combustion, and they produce no carbon dioxide directly.
Mistake 4: Saying that mass is destroyed in nuclear reactions
A small part of the mass is converted to energy, and the total mass-energy is conserved.