Sec 3 Combined Science (Physics) Thermal Processes notes

Whenever two regions sit at different temperatures, energy transfers spontaneously from the hotter one to the cooler one — never the reverse without help. The transfer continues until both reach the same temperature: thermal equilibrium. That is a hot drink cooling to room temperature, a cold spoon warming in soup, and your thermometer settling at a steady reading — it stops changing exactly when it reaches equilibrium with its surroundings.

Three processes do this carrying: conduction, convection and radiation. A single situation usually mixes all three — a lit match warms your fingers by radiation across the gap, conduction up the metal wire, and convection as the hot smoke rises.

Conduction is the transfer of energy through matter without the matter moving along — the main channel in solids, whose particles cannot swap places. Two mechanisms: (1) one particle is heated and vibrates more vigorously, jostling its neighbours and passing energy particle-to-particle down the line; (2) in metals only, free electrons drift through the lattice, pick up energy in the hot region, and deliver it further away — which is why metals are such good thermal conductors and non-metals such poor ones.

Poor conductors are insulators: wood, plastic, wool, air. Trapped air is the star insulator — it conducts badly (particles far apart) and cannot convect if it is locked in small pockets (foam, double glazing, feathers).

Convection happens only in fluids — liquids and gases — because it needs the matter itself to flow. Heat a fluid and it expands: the same mass now occupies more volume, so its density falls. The warmed fluid floats up through the cooler, denser fluid around it; cooler fluid sinks and slides in beneath to take its place. The circulating loop is a convection current, and it shuttles energy around the fluid bulk.

This is why kettle elements and room heaters live at the bottom: heating from below lets warm fluid rise through cold fluid and sets up a strong current. Sea breezes, boiling water’s rolling motion and hot-air balloons are all the same density story.

Radiation is energy carried by infrared waves, and it is utterly different from the other two: it needs no medium, travelling even through a vacuum — that is how the Sun's energy reaches Earth. Every object emits it, and the hotter the object, the faster it emits.

The rate depends on the surface: dull black surfaces are the best emitters and best absorbers; shiny, light surfaces are the worst at both — they reflect radiation instead. Larger surface area also means faster emission. A dull-black can of hot water cools faster than a shiny one; a white house in the tropics stays cooler because its walls absorb less sunshine.

The vacuum flask defeats all three processes at once: a vacuum between the double walls (no particles → no conduction or convection), silvered walls (poor absorber/emitter → radiation reflected back), a stopper (blocks convection up the neck) and a poor-conductor case. Keeping things cold uses the same tricks in reverse — stop the outside heat getting in.

Coastal weather runs on convection twice a day: the sea breeze by day (warm land air rises, cool sea air flows in) and the land breeze by night (the sea stays warmer, so the flow reverses — warm sea air rises and air flows from the land out to sea). House design joins in: light shiny walls reflect radiation, insulation traps poorly-conducting air, and ventilation flushes warm air out.