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Articles from Britannica Encyclopedias for elementary and high school students. First law of thermodynamics, thermodynamic relation stating that, within an isolated system, the total energy of the system is constant, even if energy has been converted from one form to another. This law is another way of stating the law of conservation of energy.
The laws of thermodynamics are deceptively simple to state, but they are far-reaching in their consequences. The first law asserts that if heat is recognized as a form of energy, then the total energy of a system plus its surroundings is conserved; in other words, the total energy of the universe remains constant.
The first law of thermodynamics for closed systems was originally induced from empirically observed evidence, including calorimetric evidence. It is nowadays, however, taken to provide the definition of heat via the law of conservation of energy and the definition of work in terms of changes in the external parameters of a system.
The first law asserts that if heat is recognized as a form of energy, then the total energy of a system plus its surroundings is conserved; in other words, the total energy of the universe remains constant. The first law is put into action by considering the flow of energy across the boundary separating a system from its surroundings.
