Heat, Temperature and Thermodynamics

Drafted with AI assistance and fact-checked on 28 October 2026; not yet read line by line by a human editor. Spotted a mistake? Tell us. Editorial standards & corrections

Heat and temperature

Heat is energy that flows from a hotter body to a colder one; temperature is the degree of hotness that decides the direction of that flow. Heat is measured in joules (1 calorie, the heat that warms 1 g of water by 1 degree C, is 4.186 J) and temperature in kelvin. Mercury suits thermometers because it is shiny, expands uniformly, does not wet glass and stays liquid from about -39 degrees C to 357 degrees C; alcohol thermometers are used in very cold places, and pyrometers measure very high temperatures such as those of furnaces and the Sun. A clinical thermometer reads from 35 to 42 degrees C and has a kink that stops the mercury from falling back.

Scales and conversion

Celsius marks the ice point at 0 and the steam point at 100, Fahrenheit at 32 and 212, and Kelvin at 273.15 and 373.15. Use C/5 = (F - 32)/9 = (K - 273)/5, or simply K = C + 273. Absolute zero, 0 K or -273.15 degrees C, is the lowest possible temperature, so kelvin readings are never negative. Celsius and Fahrenheit agree at -40, and normal body temperature, 37 degrees C, is 98.6 degrees F or about 310 K.

Thermal expansion

Most substances expand on heating, so rails are laid with gaps, bridges have expansion joints, electric wires sag in summer and an iron rim is heated before being fitted on a cart wheel. A bimetallic strip, two metals that expand differently, bends on heating and works thermostats, electric irons and fire alarms. For solids, area expansion is twice and volume expansion three times the linear expansion (β = 2α, γ = 3α). Water is the exception: between 0 and 4 degrees C it contracts on heating, so it is densest at 4 degrees C. Lakes therefore freeze from the top while fish survive in water at 4 degrees C below, and pipes burst in severe winters because water expands on freezing.

Specific heat and latent heat

Specific heat capacity is the heat needed to raise 1 kg of a substance by 1 K. Water's is very high, about 4200 J/kg K, so it is used in hot-water bottles and car radiators, and it keeps coastal climates mild while land heats and cools faster than the sea. Latent heat is absorbed or released during a change of state without any change of temperature: about 80 cal/g to melt ice and about 540 cal/g to boil water. So steam at 100 degrees C burns more severely than boiling water. Higher pressure raises the boiling point, which is how a pressure cooker cooks faster; low air pressure in the hills lowers it, so food takes longer to cook there. Dissolved salt raises the boiling point and lowers the freezing point of water.

Conduction, convection and radiation

In conduction heat passes from particle to particle without the particles moving along, mainly in solids; metals conduct well (silver and copper best), while wood, plastic, glass wool and air conduct poorly, so pans have plastic or wooden handles and woollens keep us warm by trapping air. In convection the heated liquid or gas itself moves: sea breezes blow from sea to land by day and land breezes from land to sea at night, and ventilators sit near the ceiling because warm air rises. Radiation needs no medium and travels at the speed of light, which is how the Sun's heat reaches us. Dark, rough surfaces absorb and emit radiation best and white, shiny ones reflect it, so white clothes suit summer and solar cookers are black inside. A thermos flask blocks conduction and convection with a vacuum and radiation with silvered walls.

Evaporation and cooling

Evaporation turns a liquid into vapour at its surface at any temperature below the boiling point, and it cools because the escaping molecules carry latent heat away. It is faster with a larger surface, a higher temperature and wind, and slower in humid air. Water stays cool in an earthen pot (matka) because it seeps through the pores and evaporates; sweating cools the body; humid days feel sticky because sweat cannot evaporate. Boiling, by contrast, happens throughout the liquid at a fixed temperature for a given pressure. Camphor, naphthalene, iodine and dry ice sublime, turning straight from solid to vapour.

Laws of thermodynamics

The zeroth law says two bodies each in thermal equilibrium with a third are in equilibrium with each other; it is the basis of the thermometer. The first law is the conservation of energy: heat supplied equals the rise in internal energy plus the work done (ΔQ = ΔU + ΔW), so no machine can work without an energy input. The second law says heat does not flow by itself from a colder body to a hotter one, and no heat engine turns all the heat it takes in into work, so no engine is 100% efficient. A refrigerator moves heat from its cold inside to the warmer room only because its compressor uses electrical energy. The third law says absolute zero cannot be reached in a finite number of steps.

Key facts

1 calorie = 4.186 J; heat is energy in transit (joule), while temperature (kelvin) decides which way heat flows.
C/5 = (F - 32)/9 = (K - 273)/5; -40 is the same on the Celsius and Fahrenheit scales; 0 K = -273.15 degrees C is absolute zero.
Body temperature 37 degrees C = 98.6 degrees F = about 310 K; a clinical thermometer reads 35 to 42 degrees C; a pyrometer measures furnace and solar temperatures.
A bimetallic strip works thermostats and fire alarms; β = 2α and γ = 3α; rails are laid with gaps to allow for expansion.
Water is densest at 4 degrees C: lakes freeze from the top and fish survive below; pipes burst in winter because water expands on freezing.
Latent heat: ice about 80 cal/g, steam about 540 cal/g, so steam burns are worse; a pressure cooker raises the boiling point, low pressure in the hills lowers it.
Sea breeze by day and land breeze by night (convection); the Sun heats the Earth by radiation; dark surfaces absorb and emit best; a thermos flask blocks all three modes.
A matka cools water by evaporation through its pores; humid days feel sticky; camphor, naphthalene, iodine and dry ice sublime.
Zeroth law: basis of the thermometer; first law: energy conservation (ΔQ = ΔU + ΔW); second law: heat does not flow from cold to hot by itself and no engine is 100% efficient; third law: absolute zero cannot be reached.

Quick revision

Heat in joule, temperature in kelvin; 1 cal = 4.186 J; C/5 = (F-32)/9 = (K-273)/5; -40 C = -40 F; absolute zero -273.15 C; 37 C = 98.6 F; clinical thermometer 35-42 C; β = 2α, γ = 3α; water densest at 4 C; latent heat ice 80 cal/g, steam 540 cal/g; pressure cooker raises boiling point; silver and copper best conductors; sea breeze by day; radiation needs no medium; evaporation cools; no engine is 100% efficient.

Practise this topic

All 12 lessons on General Science (Physics, Chemistry, Biology)