Energy is a fundamental property of a system. Some idea of its importance may be gained by considering its role in chemical reactions, where it determines what reactions may occur, how fast the reaction may proceed and in which direction the reaction will occur. Energy takes several forms: kinetic energy is that which a body possesses as a result of its motion; potential energy is the energy which a body has due to its position, whether gravitational potential energy or coulombic potential energy which is associated with charged particles at a given distance apart. All forms of energy are related, but in converting between the various types it is not possible to create or destroy energy. This forms the basis of the law of conservation of energy.

The internal energy U of a system is the sum of all the kinetic and potential energy contributions to the energy of all the atoms, ions and molecules in that system. In thermodynamics we are concerned with change in internal energy, A U, rather than the internal energy itself. (Notice the use of A to denote a finite change). We may change the internal energy of a closed system (one that cannot exchange matter with its surroundings) in only two ways: by transferring energy as work (w) or as heat (q). An expression for the change in internal energy is

If the system releases its energy to the surroundings AU is negative, i.e. the total internal energy has been reduced. Where heat is absorbed (as in an endothermic process) the internal energy will increase and consequently q is positive. Conversely, in a process which releases heat (an exothermic process) the internal energy is decreased and q is negative. Similarly, when energy is supplied to the system as work, w is positive; and when the system loses energy by doing work, w is negative.

It is frequently necessary to consider infini-tesimally small changes in a property; we denote these by the use of d rather than A. Thus for an infinitesimal change in internal energy we write equation (3.1) as dU = dw + dq (3.2)

We can see from this equation that it does not really matter whether energy is supplied as heat or work or as a mixture of the two: the change in internal energy is the same. Equation (3.2) thus expresses the principle of the law of conservation of energy but is much wider in its application since it involves changes in heat energy, which were not encompassed in the conservation law.

It follows from equation (3.2) that a system which is completely isolated from its surroundings, such that it cannot exchange heat or interact mechanically to do work, cannot experience any change in its internal energy. In other words the internal energy of an isolated system is constant - this is the first law of thermodynamics.

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