Sec 3 Physics Thermal Processes notes

Energy transfer by heating always runs from the region of higher temperature to the region of lower temperature — never the other way. Put a hot spoon in cold tea and the spoon cools while the tea warms; the flow continues until both reach the same temperature, called thermal equilibrium, at which point the net transfer stops (individual particles still swap energy, but the swaps cancel).

The temperature difference is the driving force: a bigger difference means a faster transfer. That is why a cup of tea cools quickly at first and then seems to linger near room temperature — the slope flattens as it approaches equilibrium. Three routes carry the energy: conduction, convection and radiation, and a good answer names which route (or routes) each situation uses.

Conduction is the transfer of energy through a material without the material itself moving. Heat one end of a metal rod: the atoms there vibrate more energetically and jostle their neighbours, which jostle the next, and the energy ripples along the lattice like a bucket brigade. Every solid conducts this way.

Metals conduct far better than non-metals because of an upgrade: their outer electrons are free to drift through the lattice. Heated at one end, these fast-moving electrons spread through the metal and hand energy to atoms everywhere along the rod at once. Copper conducts roughly a thousand times better than wood — the free-electron courier service explains why saucepans are metal (in) and their handles are wood or plastic (out).

Poor conductors are insulators: wool, air, foam, glass. Their particles are locked in place with no free electrons, and much of the material is trapped air — itself a poor conductor as long as it cannot circulate. This is why blankets, double glazing and house insulation all work by holding air still.

Convection happens only in fluids (liquids and gases) because it needs the material itself to travel. Heat a patch of fluid: it expands, its density falls, and the cooler, denser fluid beside it sinks beneath it, pushing the warm patch upward. The rising fluid cools, densifies and sinks again — a circulating loop called a convection current.

Notice what convection requires: a fluid that can flow and expand, and gravity to tell "up" from "down". It cannot happen in a solid (nothing flows) and cannot happen in space (no gravity to sort densities — astronauts drink from pouches because convection does not stir their drinks).

Radiation transfers energy by infrared electromagnetic waves, which need no material medium: the Sun’s energy crosses 150 million kilometres of vacuum to reach us, and a vacuum flask’s vacuum only blocks the other two routes. Every object above absolute zero emits radiation; the hotter it is, the faster it emits.

Three factors set the rate. Surface colour and texture: dull, black, rough surfaces are the best emitters and absorbers; shiny, light, smooth surfaces are the worst — they reflect. Surface temperature: hotter objects radiate much faster. Surface area: more area, more emission. And one elegant symmetry: a surface that absorbs well emits well — good absorber, good emitter — which is why the black plate in a heater experiment warms first and cools first.

Applications stack all three processes. A vacuum flask: the vacuum stops conduction and convection; silvered walls reflect radiation back to the drink; the stopper blocks convection out of the neck; the case insulates by trapped air. A car radiator does the opposite on purpose: black, finned, and fan-cooled to maximise all three transfers out of the engine.

Every thermal-design question is the same puzzle in disguise: identify which route carries the energy, then block that route. Conduction is starved with insulators and trapped still air; convection is stopped with lids, seals and design; radiation is reflected with shiny surfaces. The vacuum flask simply runs the checklist; so do houses, coats and cool boxes.