We talked earlier about Work as a transfer of energy from one object to another, but there is another type of energy transfer called heat. Heat is the flow of thermal energy from a hotter object to a cooler object. As the hotter object loses energy it cools, while the colder object gains energy and warms, so that eventually the two objects are the same temperature (when they are in thermal equilibrium), and the flow of heat stops. Note that heat is the flow of energy, not the energy itself. Physicists do not say that a fire or a hot cup of coffee have a lot of heat. Molecules in an object are constantly in motion— vibrating in solids, and also rotating and wandering around in liquids and gases—and the energy associated with this motion is what we call thermal energy. The higher the temperature of an object, the more thermal energy it has.
There are three ways in which heat can flow:
Conduction
Conduction occurs within an object or between two objects which are in contact with each other. It can occur in solids or fluids.
Convection
When water in a pot is warmed, it becomes less dense, and rises. At the top of the pot, the water cools and sinks again. These convection rolls help mix the water until it is all at a uniform temperature.
Convection occurs within a fluid such as air or water, where warm parts of the fluid flow towards colder places and vice versa: heat is carried directly from one region to the other by molecules, instead of passed from molecule to molecule as in conduction.
For example, hot air is less dense and so tends to rise while colder air sinks: this creates convection rolls which can warm a room more easily or, on a larger scale, the Earth’s atmosphere (where convection causes winds and weather).
Radiation
All objects radiate electromagnetic radiation depending on their temperature: mostly infrared light, although hotter objects (like molten iron, for instance) radiate visible light as well. For example, the Sun and the Earth both radiate energy and absorb each other’s energy as well, but the Earth absorbs more radiation from the Sun than vice versa, and so heat is transferred from the hotter Sun to the cooler Earth. Radiation is the only type of heat which can travel through a vacuum, although it can travel through other media as well. If you’ve ever felt a warm fire or the warm Sun when the air is cold in winter, you’ve felt their radiated heat.
