Thermal physics

📚The physics

The kinetic particle model. In a solid the particles are closely packed in a regular pattern, vibrating about fixed positions, held by strong forces — so solids keep their shape and volume. In a liquid they are still close but irregularly arranged and able to slide past one another — fixed volume, no fixed shape. In a gas they are far apart, moving fast and randomly, with almost no forces between them — so a gas fills its container and is easily compressed.

Brownian motion — the jittering of smoke particles seen under a microscope — is the evidence. The smoke specks are being knocked about by air molecules too small to see, which shows those molecules exist and are in constant random motion.

Changes of state need energy to break the forces between particles: melting and boiling absorb energy; freezing and condensing release it. During a change of state the temperature stays constant even though energy is still going in, because that energy is separating particles rather than speeding them up.

Evaporation is not boiling. Evaporation happens at any temperature, only at the surface, and only the fastest molecules escape — which is why the liquid left behind is cooler. Boiling happens at one fixed temperature and throughout the liquid. This distinction is asked about almost every year.

Gas pressure comes from particles colliding with the container walls. Heat the gas and the particles move faster, so collisions are harder and more frequent and the pressure rises. Squeeze the gas into a smaller volume and collisions become more frequent for the same speeds, so the pressure rises again.

Absolute temperature. At −273 °C the particles have as little kinetic energy as they can have — absolute zero. The Kelvin scale starts there: \( T(\text{K}) = \theta(^\circ\text{C}) + 273 \).

EXTENDED At constant temperature, \( pV = \) constant, so squeezing a gas to half its volume doubles its pressure.

Thermal expansion. Heating makes particles vibrate more and take up more room, so most materials expand. Gases expand most, then liquids, then solids — the reverse of how tightly the particles are held. This is why bridges have expansion gaps, why railway lines and overhead cables are laid with slack, and why a tight metal lid loosens under hot water.

Specific heat capacity \(c\) is the energy needed to raise the temperature of 1 kg of a substance by 1 °C:

Water’s value is unusually high, about 4200 J kg\(^{-1}\) °C\(^{-1}\), which is why the sea warms and cools far more slowly than the land.

The three ways energy moves. Conduction passes energy by particle vibration and, in metals, by free electrons — which is why metals conduct far better than non-metals, and why a metal handle feels colder than a wooden one at the same temperature. It needs a material, so it does not work in a vacuum. Convection works only in fluids: warmed fluid expands, becomes less dense, rises, and cooler fluid sinks to replace it, setting up a convection current. Radiation is infrared electromagnetic waves and is the only method that crosses a vacuum — which is how the Sun’s energy reaches us.

Surfaces matter for radiation. Dull black surfaces are the best emitters and the best absorbers; shiny silvery surfaces are the worst at both, and the best reflectors. That is why solar panels are black, why a vacuum flask is silvered inside, and why people in hot climates often wear white.