Keeping food and drink warm
Imagine you are hot soup in a bowl. As you lie in the bowl there are three ways you lose heat. At your surface that is in contact with the air, you lose heat by conduction and radiation. Your hot surface warms the air directly above you. This makes the air lighter in weight than the air over the rest of the table. This lightweight air rises like an invisible balloon and takes some of your heat with it. Cooler air from around the bowl flows over your surface and also receives heat then floats away. At the same time some heat is also leaving your surface by radiation. Rays of heat move away in straight lines in all directions.

While your upper surface is losing heat by convection and radiation, your sides touching the soup bowl are losing heat by conduction. This heat moves through the material of the soup bowl. Eventually it reaches the outer surface of the soup bowl. At the sides of the soup bowl, the heat changes into waves and is lost by radiation. Anyone putting their hands very close to the sides but not touching them can feel their skin warm slightly as the rays of heat strike them.

The bottom of the bowl touches the table. When heat reaches the bottom of the bowl by conduction it keeps on moving by conduction into the material from which the table is made. It could be stopped if a cork tablemat were put between the bowl and the table. Cork is an insulator. The surface of the table would be protected from the hot bowl and you, the soup, would keep warmer for longer.

With all this heat you are losing, you may think that you will freeze. This will not happen because you will only cool down to the temperature of your surroundings and most rooms have a temperature of 15°C or more. However, if someone wished to keep you for a long time they might put you in the icebox in a fridge. Here you would start to lose heat again by conduction, convection and radiation until you had become a block of frozen soup. Alternatively if they wanted to keep you warm for a picnic they could put you in a thermos flask where you would keep warm until mealtime.

How does a mug with thick sides keep a drink warm?
A warm drink loses heat through the sides of the mug by conduction and then radiation. If the sides are thin, the heat passes through quickly by conduction. If the sides are thick it takes longer for the heat to pass through by conduction and the drink stays warmer for longer.

How does a mug with hollow sides keep a drink warm?
The hollow walls are filled with air. Heat cannot pass through the air by conduction. It can normally pass through the air by convection but the air is trapped inside the sides so heat cannot move by convection either, so the drink stays warm.

Sometimes when you buy hot drinks the plastic cup has a lid. How does this keep the drink warm?
The drink loses some of its heat at the surface by convection. In this process the air receives heat from the surface, becomes warm and lightweight, and then rises. Cooler air takes its place and the process is repeated. After a while, a large amount of heat can be lost into the air. The lid traps the air close to the surface and does not let it rise. This stops the drink losing heat by convection.

What is a tea cosy?
A tea cosy is a cover made of cloth. It is put over a teapot full of hot tea to stop the pot losing heat. The cloth is made of fibres, which have been knitted together. There are air spaces between the fibres, which stop heat passing through the tea cosy by conduction. The fibres themselves are also poor conductors of heat. In the past people also used to knit egg cosies to keep boiled eggs warm at the breakfast table.

Why are fish and chips wrapped in newspaper?
Newspaper is a very cheap insulating material. It is made from tiny wood fibres which are squashed together. Wood is a poor conductor of heat. There are also tiny air spaces in the paper, which stop heat being conducted through it. A few sheets of paper are used to wrap the fish and chips to make a thick insulating wall around them.

Why are many foods put in polystyrene cartons?
Polystyrene is a plastic and plastics are poor conductors of heat. The polystyrene used in food cartons has bubbles in it. The gas, which forms the bubbles, is made at the same time as the plastic. It acts like air and stops heat passing through the carton walls by conduction.

Was the vacuum flask invented to keep foods hot?
No. It was invented to keep liquids cool. James Dewar invented the flask to help him with his investigations. He studied how gases could be cooled down until they turned into a liquid. The temperatures at which gases in the air turn into a liquid are very low. For example, oxygen must be kept below - 183°C to stay as a liquid. The flask was invented not to keep heat in the liquid but to keep heat from the air away from the liquid. If a liquid gas were put in a bowl, heat from the air would rapidly enter it and make it boil away. The walls and stopper in the flask prevent heat entering the flask so the liquid gases do not boil away. After Dewar invented the flask other people saw how it could be useful in keeping drinks hot or cold. The vacuum flask is also known as the thermos flask.