Heat Transfer

Explained

Three ways energy moves, and what each one needs

Conduction, convection and radiation are usually listed as three methods of heat transfer, which makes them sound like three versions of the same thing. They are not. They differ in what has to be present for them to happen at all, and starting there makes each one easier to explain.

Conduction needs particles that stay put. Convection needs particles that can move around. Radiation needs nothing at all.

That is why conduction works best in solids, convection happens only in fluids, and radiation is the only one that crosses the vacuum between the Sun and the Earth.

Conduction

Heat a solid at one end and the particles there vibrate faster. Because the particles are held closely together, they collide with their neighbours and pass some of that vibration along, and the effect travels through the material.

Note what is not happening. The particles do not travel from the hot end to the cold end. They stay where they are and vibrate more, so energy moves through the solid while the solid itself stays put.

Metals conduct far better than anything else, and the reason is a second mechanism they alone possess. Metals contain free electrons, which are not attached to any one atom and can move through the whole structure. At the hot end they gain kinetic energy, travel to the cooler parts, and collide with ions there, transferring energy far faster than vibration alone could.

So a question asking why metals are the best conductors wants free electrons, not close packing. Every solid has closely packed particles; only metals have delocalised electrons, so only that answer explains the difference.

Non metal solids conduct slowly. Liquids conduct more slowly still, and gases hardly at all, because their particles are far apart and collide rarely. That is the basis of almost every insulator.

Convection

Convection happens in liquids and gases, where particles are free to move from place to place, and it moves the fluid itself rather than passing energy along a fixed line of particles.

Warm a fluid from below and the particles there move faster and spread further apart. The fluid expands, so the same mass now occupies a larger volume, so its density falls. Less dense fluid rises, cooler denser fluid sinks to take its place, and the cycle repeats as a convection current.

Keep density in that chain. Heating causes expansion, expansion lowers density, lower density causes rising. Answers that jump straight from hot to rising have skipped the reason.

It cannot happen in a solid, and that is worth stating explicitly, because the particles in a solid are fixed in position and cannot flow. A solid can expand when heated, but it cannot circulate.

This is why a radiator heats a whole room from one wall, why the heating element in a kettle sits at the bottom, and why a freezer compartment is placed at the top of a fridge.

Radiation

All objects emit infrared radiation, which is an electromagnetic wave, and the hotter an object is the more it emits.

Because it is a wave rather than a movement of particles, it travels through a vacuum. Energy from the Sun reaches the Earth across ninety three million miles of empty space, and it can only be doing so by radiation.

Dark matt surfaces absorb and emit infrared well. Shiny light surfaces reflect it and are poor emitters, which is why a vacuum flask is silvered inside and why survival blankets are made of reflective foil.

Sorting a question quickly

When a question describes a situation and asks which method is at work, ask what is between the source and the destination.

A solid in contact with both means conduction. A liquid or gas free to circulate means convection. A gap, a vacuum, or nothing in contact at all means radiation.

Most real situations use more than one. A saucepan on a hob conducts through the metal base, sets up convection currents in the water, and radiates from its hot outer surface, and a question asking for a full explanation expects all three to be named.