The 3 Ways That Heat Travels: The Science of Keeping Warm

During the winter, you may have been told to ‘wrap up well to keep the cold out’, but did you know that this is scientifically wrong?

Keeping the cold out is impossible but keeping the warmth in can be done. Why? Because ‘cold’ doesn’t travel but heat does. Heat always goes from higher temperatures to lower temperatures.

Heat transfer by convection.

[photo:Gary Settles]

What is Heat?

Heat is a measure of how fast the particles are moving or vibrating in a substance. The particles in the iron bar at a temperature of 5oC are vibrating more slowly than particles of an iron bar at 25oC. This is because they have more energy.

Faster vibrating particles take up more space. This is the reason that things expand when they are heated. The same principles apply to liquids and gases.

1. Conduction

This is the way that heat travels through solids. If you heat one end of a piece of metal, the particles will vibrate faster. They will hit their neighbours harder and they in turn will vibrate more than before. This happens in turn all along the iron rod, transferring the heat along the whole length of the rod.

Conduction occurs fastest in metals. The atoms are packed tightly in a regular pattern so the vibrations are passed efficiently from one atom to the next. There are also ‘delocalised electrons‘ which move through the metal. These move faster because of the higher temperature and also help to transfer the heat.

2. Convection

This is the way that heat travels through fluids; a fluid being either a liquid or a gas. Where the fluid is heated, the particles move faster as they have more energy. This means that they move further apart.

Because there are fewer particles in a given volume, the heated part of the fluid is less dense so it rises. As the less dense area moves away from the heat source, it cools, becomes denser again and slowly sinks back down. This is called a convection current and it transfers heat throughout the fluid.

Convection cells.

Convection in a liquid [diagram: Eyrian]

3. Radiation

This is the way that heat travels through transparent solids, liquids and gases and is the only way that heat can travel through a vacuum such as space. Heat travels as infra-red radiation which is easily stopped — even by something as thin as paper.

On a sunny day, hold the back of your hand out and feel the infra red rays from the Sun warming your hand. Hold a piece of paper about 30cm from your hand so that it casts a shadow on it. You should notice that it does not feel anywhere near as warm as in the direct sunlight.

Insulation

In winter, heat can leave your body by all three types of heat transfer so you need to wear clothing that does not conduct heat very quickly. Air is a poor conductor of heat so clothes that have air trapped inside them are great for wearing in the winter.

You lose heat by conduction too. The warmth of your body heats the air between your skin and clothing. This convects upwards, taking your body heat with it through the neck opening of your clothes. Wearing a scarf slows down how fast this warm air can escape.

But that still leaves your head. Since your skin is warmer than the surrounding air, heat radiates away and some also convects away from the top of your head. Wearing a hat or balaclava will help to reduce both.

Ultimately, it is not possible to stop heat travelling, but if you wear clothes that slow down the heat loss as much as possible, you will be able to ‘keep the warmth in‘ and NOT ‘keep the cold out‘!

Taught science for 16 years at a secondary school in the East Midlands.

  • Epicrapbattlechem

    If you count convection as a heat transfer method, then cold will travel. Also all three forms of heat transfer are the same if you look closely enough. Lastly, to provide concrete ideas about heat, heat should be referred to via pushing, IE higher temperature materials collide with lower temperature materials and the collisions cause the lower temperature material to move faster. The collisions cause a force between the two materials where the higher temperature usually slows down and the lower temperature material speeds up from the pushing between the two materials that occurs.