Thermodynamics

Thermodynamics is the branch of physics that deals with heat, temperature, energy and work, and the ways energy is transferred and transformed. It explains why heat flows from hot objects to cold ones, how engines turn heat into motion, and why some processes cannot be reversed. Its principles apply across science and engineering, from the behaviour of gases to the operation of refrigerators and power stations.

The subject is founded on a few central laws. The zeroth law defines temperature through thermal equilibrium. The first law expresses the conservation of energy, stating that energy can change form but is never created or destroyed; the heat supplied to a system equals the work it does plus the change in its internal energy. The second law introduces entropy, a measure of disorder, and establishes that the entropy of an isolated system tends to increase, which is why heat does not flow spontaneously from cold to hot and why no engine can be perfectly efficient.

Heat itself moves by three mechanisms: conduction through direct contact, convection through the movement of fluids, and radiation through electromagnetic waves. Many engineering devices are analysed as thermodynamic cycles in which a working fluid is repeatedly heated, expanded, cooled and compressed. The Otto cycle, for example, models the petrol internal-combustion engine, describing how a fuel-air mixture is compressed, ignited and expanded to produce work. Such cycles let engineers calculate efficiency and design better engines.

Frequently asked questions

What is entropy?

Entropy is a measure of the disorder or the unavailable energy in a system. The second law of thermodynamics states that the total entropy of an isolated system tends to increase over time.

What are the three ways heat is transferred?

Heat moves by conduction through direct contact between materials, by convection through the motion of liquids or gases, and by radiation as electromagnetic waves.

What does the Otto cycle describe?

The Otto cycle is the idealised thermodynamic cycle of a spark-ignition petrol engine, describing the compression, combustion and expansion of a fuel-air mixture.