-g- Heat Transfer

Heat transfers by
……Convection and




Convection currents in water

Land And Sea Breeze

How does Heat Energy Move?

Hot objects have heat energy. Heat energy always moves from something hot to something colder. There is no such thing as cold energy, so an object can only get colder by heat energy moving away from it.

There are three ways that heat energy can move:

  1. Conduction.
  2. Convection.
  3. Radiation.


When you first pour boiling water onto a Pot Noodle, the plastic container feels cool on the outside. Soon, the heat energy has worked its way through the plastic and the container starts to feel hot on the outside. Heat energy has travelled through the solid plastic container. This process is called conduction.

Atoms in a substance are always vibrating. If the substance gets hotter,the atoms vibrate more. The heat energy is given to the atoms, which makes them move about faster. Note: the atoms don’t swap places, or move around they just vibrate more on the spot.

Have you ever danced next to someone really energetic? If so, you know that it makes you have to move about more – often just to get out of the way! It is like that for atoms passing heat energy on to each other.

Solids are better at conducting than liquids and gases because the atoms are closer together. If the atoms are too spaced out it makes it harder for the atoms to pass the energy along.

Metals are the best solids for conducting heat energy. In metals, there are free electrons that can move through the metal. These electrons are able to move from hot parts of the metal to colder parts, taking the heat energy with them. This is called electron diffusion.

The poorest conductors are gases as their molecules are too far apart to affect each other much. This means that air is a terrible conductor of heat energy.


Hot air rises in cold air. Hot water rises in cold water. This way of moving energy is called convection. When hot air rises, colder air has to move in to replace it. When hot water rises in a cup, colder water sinks to replace it.

This movement of a liquid or gas is called a convection current.

Convection cannot happen in solids, as the atoms aren’t able to move around.

When a liquid or gas heats up, the particles move around more. This makes the particles spread out, so they have more room to move. This lowers the density of the substance. The hotter it gets, the lower the density goes. It is this lower density that makes the hotter substance rise. The cooler substance has a higher density, which makes it sink.


“How to toast bread!” When a piece of bread is put in a toaster the wires inside the toaster glow red hot on either side of the bread.

How does the heat energy get to the bread?

Is it by conduction?

No, the heat energy cannot conduct through the air to the bread because air is a very bad conductor.

Is it by convection?

No, hardly any of the heat energy could have travelled to the bread by convection, as the hot air particles would rise out of the toaster.

The heat energy must have reached the toast some other way. It travelled as radiated heat. This heat energy movement is sometimes called heat waves, but strictly speaking, it is infrared radiation.

Hot objects radiate heat to their colder surroundings. The weird thing is that the surface colour of an object makes a difference.

Black and dull surfaces emit (give out) and absorb radiation well.

White and shiny surfaces do not emit radiation well and reflect radiation instead of absorbing it.

Which of these surfaces is best at emitting radiation?

Put them in order of best to worst:

Marathon runners are wrapped in foil blankets at the end of the race. The shiny surface is a poor emitter of radiation and so prevents them losing too much precious body heat.

So why do we paint radiators white so often?

I guess people think it looks better.

Which of these surfaces is best at absorbing radiation?

Put them in order of best to worst:

Solar panels are always coloured black. They then absorb the maximum amount of the Sun’s energy. This is very important considering the amount of sun we get in the UK. Wearing white in the summer should, in theory, be cooler than wearing black, as more of the incoming heat is reflected away.


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