Facts At Your Fingertips: Energy and forces – Heat
The atoms and molecules that make up matter are always on the move. The more energy they have, the faster they move.
We feel this energy as heat. Heat will always move from a warmer area to a colder one until both areas are the same temperature.
Cooling down
The molecules in a hot mug of coffee are moving very fast. The coffee transfers heat energy to the cooler air.
This makes the molecules in the coffee slow down, meaning that it gets cooler. After about an hour, the coffee and the air will be the same temperature. The colder the air around the mug, the faster the coffee will cool down.
Detecting heat
Heat energy escapes objects as infrared radiation. Infrared is invisible to our eyes, but special cameras can detect it.
Infrared cameras are used by firefighters to see whether there are people trapped in a building, and by the military to see at night.
Expanding air
When substances heat up, the molecules in them move faster and take up more space.
This means that substances expand as they are warmed and become less dense. To make a hot-air balloon float, a burner heats up the air in the balloon.
The air inside the balloon is now less dense than the colder air outside it. This causes the balloon to rise.
Measuring temperature
Temperatures are measured using thermometers. Simple thermometers work by measuring the length of a column of liquid.
As the temperature increases, the liquid expands and the column becomes longer.
Radiation
Warm objects give off heat in the form of infrared radiation. Earth is warmed by the heat given off by the Sun, which is the hottest object in the solar system.
Infrared lamps can be used to produce heat—the lamp shown here keeps the pigs warm at night.
Convection
Heat can be transferred from one place to another through convection, which is the movement of hot liquid or gas, such as air.
As air heats up, it becomes less dense and rises. Cooler air sinks and takes its place. This causes convection currents of moving air.
Insulators
Materials that do not allow heat to pass through them easily are called insulators.
Plastic, rubber, and wood are all insulators. Air is also a good insulator. Fur keeps animals warm by trapping a layer of air next to the skin. This stops heat from being removed from the body.
Conduction
As molecules move, they pass on some of their heat energy to neighboring molecules. This transfer of heat is called conduction. Solids conduct heat more easily than liquids or gases.
Metals are good conductors and are used to make saucepans and the base plates of irons.