Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Heat (physics), in physics, transfer of energy from one part of a substance to another, or from one body to another by virtue of a difference in temperature. Heat is energy in transit; it always flows from a substance at a higher temperature to the substance at a lower temperature, raising the temperature of the latter and lowering that of the former substance, provided the volume of the bodies remains constant. Heat does not flow from a lower to a higher temperature unless another form of energy transfer, work, is also present. See also Power.

Until the beginning of the 19th century, the effect of heat on the temperature of a body was explained by postulating the existence of an invisible substance or form of matter termed caloric. According to the caloric theory of heat, a body at a high temperature contains more caloric than one at a low temperature; the former body loses some caloric to the latter body on contact, increasing that body's temperature while lowering its own. Although the caloric theory successfully explained some phenomena of heat transfer, experimental evidence was presented by the American-born British physicist Benjamin Thompson in 1798 and by the British chemist Sir Humphry Davy in 1799 suggesting that heat, like work, is a form of energy in transit. Between 1840 and 1849 the British physicist James Prescott Joule, in a series of highly accurate experiments, provided conclusive evidence that heat is a form of energy in transit and that it can cause the same changes in a body as work.