The first theory about how a hot object differs from a cold object was formed in the 18th century. The suggested explanation was that when an object was heated, an invisible fluid called “caloric” was added to the object. Hot objects contained more caloric than cold objects. The caloric theory could explain some observations about heated objects (such as that the fact that objects expanded as they were heated) but could not explain others (such as why your hands got warm when you rub them together).
In the mid-19th century, scientists devised a new theory to explain heat. The new theory was based on the assumption that matter is made up of tiny particles that are always in motion. In a hot object, the particles move faster and therefore have greater kinetic energy. The theory is called the kinetic-molecular theory and is the accepted theory of heat. Just as a baseball has a certain amount of kinetic energy due to its mass and velocity, each molecule has a certain amount of kinetic energy due to its mass and velocity. Adding up the kinetic energy of all the molecules in an object yields the thermal energy of the object.
When a hot object and a cold object touch each other, the molecules of the objects collide along the surface where they touch. When higher kinetic energy molecules collide with lower kinetic energy molecules, kinetic energy is passed from the molecules with more kinetic energy to those with less kinetic energy. In this way, heat always flows from hot to cold and heat will continue to flow until the two objects have the same temperature. The movement of heat from one object to another by molecular collision is called conduction.
Heat is the energy that flows as a result of a difference in temperature. We use the symbol Q for heat. Heat, like all forms of energy, is measured in joules.
The temperature of an object is a measurement of the average kinetic energy of all the molecules of the object. You should note the difference between heat and temperature. Heat is the sum of all the kinetic energies of all the molecules of an object, while temperature is the average kinetic energy of the molecules of an object. If an object was composed of exactly three molecules and the kinetic energies of the three molecules are 50 J, 70 J, and 90 J, the heat would be 210 J and the temperature would be 70 J.
The terms hot and cold refer to temperature. A hot object has greater average kinetic energy but may not have greater total kinetic energy. Suppose you were to compare a milliliter of water near the boiling point with a bathtub full of water at room temperature. The bathtub contains a billion times as many water molecules, and therefore has a higher total kinetic energy and more heat. Nonetheless, we would consider the bathtub colder because its average kinetic energy, or temperature, is lower.
