What determines the direction of spontaneous change? It is not the total energy of
the isolated system. The First Law of thermodynamics states that energy is conserved
in any process, and we cannot disregard that law now and say that everything tends
towards a state of lower energy: the total energy of an isolated system is constant.
Is it perhaps the energy of the system that tends towards a minimum? Two arguments
show that this cannot be so. First, a perfect gas expands spontaneously into a
vacuum, yet its internal energy remains constant as it does so. Secondly, if the energy
of a system does happen to decrease during a spontaneous change, the energy of its
surroundings must increase by the same amount (by the First Law). The increase in
energy of the surroundings is just as spontaneous a process as the decrease in energy
of the system.
When a change occurs, the total energy of an isolated system remains constant but
it is parcelled out in different ways. Can it be, therefore, that the direction of change is
related to the distribution of energy? We shall see that this idea is the key, and that
spontaneous changes are always accompanied by a dispersal of energy.
What determines the direction of spontaneous change? It is not the total energy ofthe isolated system. The First Law of thermodynamics states that energy is conservedin any process, and we cannot disregard that law now and say that everything tendstowards a state of lower energy: the total energy of an isolated system is constant.Is it perhaps the energy of the system that tends towards a minimum? Two argumentsshow that this cannot be so. First, a perfect gas expands spontaneously into avacuum, yet its internal energy remains constant as it does so. Secondly, if the energyof a system does happen to decrease during a spontaneous change, the energy of itssurroundings must increase by the same amount (by the First Law). The increase inenergy of the surroundings is just as spontaneous a process as the decrease in energyof the system.When a change occurs, the total energy of an isolated system remains constant butit is parcelled out in different ways. Can it be, therefore, that the direction of change isrelated to the distribution of energy? We shall see that this idea is the key, and thatspontaneous changes are always accompanied by a dispersal of energy.
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