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What is the difference between heat and temperature?

Temperature is a property of matter. If you touch an object that has a higher temperature than your body, the object feels hot. But our sense of temperature isn't always accurate, particularly when comparing the temperatures of two objects. You can accurately measure the temperature of an object with a thermometer (which relies on the expansion of liquids as their temperature rises) or one of several electrical devices, such as a thermocouple (which produces an electrical voltage that varies with temperature).

At the atomic scale, temperature is related to the average energy of the atoms or molecules that make up the matter we are studying. In substances made up of individual atoms (not bonded into molecules), the atoms can move, which means they have kinetic energy. In substances composed of molecules, each containing more than one atom, the molecules can move and also vibrate and rotate. Each of these types of motion provides a way for the molecules to possess kinetic energy. In addition each atom possess potential energy from the electrical forces between the electrons and nucleus and kinetic energy from the motion of the electrons. The sum of all these kinetic and potential energies is called the internal energy of a substance, often symbolised by the letter E.

If we take two pieces of matter with the same temperature and bring them into contact, because the average energies of the particles in each piece are the same (because their temperatures are the same) there will be no overall transfer of energy from one piece to the other. But if the temperature of one piece of matter is higher than that of the other piece, there will be a transfer of internal energy from the hotter to the colder piece. The energy that is transferred is called heat and is usually symbolised by the letter q.

It is not correct to say that an object 'contains' a certain amount of heat, but it is correct to say that an object has a certain internal energy. If some of that internal energy is transferred to the random motions of the particles in another object, then you can say a transfer of heat has occurred.

If you would like to make any comments about this article, please e-mail the author, Dr Ron Haines.