Glossary — Thermal physics and matter

Absolute zero — the lowest temperature that is possible, at which the volume of an ideal gas would fall to zero. It is 0 K, or about −273 °C.

Avogadro constant — the number of particles in one mole of a substance, $N_A = 6.02 \times 10^{23}\,\text{mol}^{-1}$.

Avogadro's law — equal volumes of any gas at the same temperature and pressure contain the same number of molecules.

Boltzmann constant — the constant $k = 1.38 \times 10^{-23}\,\text{J}\,\text{K}^{-1}$ that links the average kinetic energy of a gas molecule to the temperature. It is related to the molar gas constant by $R = N_A k$.

Boyle's law — for a fixed mass of gas at constant temperature, the pressure multiplied by the volume is constant, so $p_1V_1 = p_2V_2$.

Change of state — a change from one state of matter to another, such as melting, boiling, freezing or condensing. The temperature stays the same during the change.

Charles's law — for a fixed mass of gas at constant pressure, the volume divided by the temperature in kelvin is constant, so $\dfrac{V_1}{T_1} = \dfrac{V_2}{T_2}$.

Gas laws — the three laws that link the pressure, the volume and the temperature of a fixed mass of gas: Boyle's law, Charles's law and the pressure law.

Heating curve — a graph of temperature against time for a substance heated at a steady rate. The sloping parts show the substance warming up, and the flat parts show a change of state.

Ideal gas equation — the equation $pV = nRT$, or $pV = NkT$, that links the pressure, the volume, the amount of gas and the temperature. The temperature must be in kelvin.

Ideal gas model — a model of a gas as a very large number of identical molecules in random motion, with a negligible volume and negligible forces between them except during elastic collisions.

Internal energy — the total kinetic energy plus the total potential energy of all the particles in a system.

Kelvin — the SI unit of temperature. The kelvin scale starts at absolute zero, and $T(\text{K}) = \theta(^\circ\text{C}) + 273$. A change of 1 K is the same size as a change of 1 °C.

Kinetic theory — the theory that explains the pressure and the temperature of a gas from the motion and the collisions of its molecules.

Molar gas constant — the constant $R = 8.31\,\text{J}\,\text{K}^{-1}\,\text{mol}^{-1}$ that appears in $pV = nRT$. It is related to the Boltzmann constant by $R = N_A k$.

Molar mass — the mass of one mole of a substance, in $\text{kg}\,\text{mol}^{-1}$. It is written $M$ in $c_{\text{rms}} = \sqrt{\dfrac{3RT}{M}}$.

Mole — the amount of a substance that contains the Avogadro constant of particles, $6.02 \times 10^{23}$. Its symbol is mol.

Molecule — a group of atoms joined together. In a gas, the molecules move at random and collide with one another and with the walls of the container.

Plateau — the flat part of a heating curve, where a substance changes state and the temperature stays the same.

Pressure — the force per unit area on a surface. A gas exerts a pressure because very many molecules collide with the walls of the container each second.

Pressure law — for a fixed mass of gas at constant volume, the pressure divided by the temperature in kelvin is constant, so $\dfrac{p_1}{T_1} = \dfrac{p_2}{T_2}$.

Root-mean-square speed — the square root of the mean of the squares of the speeds of the molecules in a gas, written $c_{\text{rms}}$. It appears in $pV = \tfrac{1}{3}Nm(c_{\text{rms}})^2$ and is larger than the average speed.

Specific heat capacity — the energy needed to raise the temperature of 1 kg of a substance by 1 K. In symbols $E = mc\Delta\theta$, and the unit is $\text{J}\,\text{kg}^{-1}\,\text{K}^{-1}$.

Specific latent heat — the energy needed to change the state of 1 kg of a substance with no change in temperature. In symbols $E = mL$, and the unit is $\text{J}\,\text{kg}^{-1}$. There are two kinds: the specific latent heat of fusion and the specific latent heat of vaporisation.

Specific latent heat of fusion — the specific latent heat for melting or freezing.

Specific latent heat of vaporisation — the specific latent heat for boiling or condensing.

System — the object, or the group of objects, that is being studied.

Temperature — a measure of the average random kinetic energy of the particles in a system. It does not measure the total energy stored.

Vapour pressure — the pressure of the vapour above a liquid. A liquid boils when its vapour pressure equals the pressure acting on it.