Law of conservation of energy.

Physical principle that the total amount of energy in a system remains constant, although energy can be changed from one form to another or transferred from one object to another. (Other formulation: energy can be converted from one form to another, but can be neither created nor destroyed).

We will defi ne energy as the capacity to do work or to produce heat.

The law of conservation of energy can be seen in these everyday examples of energy transference: water can produce electricity. Water falls from the sky, converting potential energy to kinetic energy. This energy is then used to rotate the turbine of a generator to produce electricity. In this process, the potential energy of water in a dam can be turned into kinetic energy which can then become electric energy.

Laws of defi nite and multiple proportions.

Joseph Proust (1754–1826), showed that a given compound always contains exactly the same proportion of elements by mass. For example, Proust found that the substance copper carbonate is always 5.3 parts copper to 4 parts oxygen to 1 part carbon (bymass). The principle of the constant composition of compounds, originally called “Proust’slaw, ” is now known as the law of defi nite proportion. Proust’s discovery stimulated John Dalton (1766–1844), an English schoolteacher, to think about atoms as the particles that might compose elements. Daltonreasoned that if elements were composed of tiny individual particles, a given compoundshould always contain the same combination of these atoms. This concept explained whythe same relative masses of elements were always found in a given compound. But Dalton discovered another principle that convinced him even more of the existence of atoms. He noted, for example, that carbon and oxygen form two different compounds that contain different relative amounts of carbon and oxygen.