Laws of Thermodynamics
If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.
When two systems are put in contact with each other, there will be a net exchange of energy between them unless or until they are in thermal equilibrium, that is, they contain the same amount of thermal energy for a given volume (say, 1 cubic centimeter, or 1 cubic inch.) While this is a fundamental concept of thermodynamics, the need to state it explicitly as a law was not perceived until the first third of the 20th century, long after the first three laws were already widely in use, hence the zero numbering. The Zeroth Law asserts that thermal equilibrium, viewed as a binary relation, is an equivalence relation.
First law of thermodynamics
In any process, the total energy of the universe remains the same.
It can also be defined as:
for a thermodynamic cycle the sum of net heat supplied to the system and the net work done by the system is equal to zero.
More simply, the First Law states that energy cannot be created or destroyed; rather, the amount of energy lost in a steady state process cannot be greater than the amount of energy gained.
This is the statement of conservation of energy for a thermodynamic system. It refers to the two ways that a closed system transfers energy to and from its surroundings - by the process of heating (or cooling) and the process of mechanical work. The rate of gain or loss in the stored energy of a system is determined by the rates of these two processes. In open systems, the flow of matter is another energy transfer mechanism, and extra terms must be included in the expression of the first law.
The First Law clarifies the nature of energy. It is a stored quantity which is independent of any particular process path, i.e., it is independent of the system history. If a system undergoes a thermodynamic cycle, whether it becomes warmer, cooler, larger, or smaller, then it will have the same amount of energy each time it returns to a particular state. Mathematically speaking, energy is a state function and infinitesimal changes in the energy are exact differentials.
Second law of thermodynamics
The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.
In a simple manner, the second law states that "energy systems have a tendency to increase their entropy" rather than decrease it.
A way of looking at the second law for non-scientists is to look at entropy as a measure of chaos. So, for example, a broken cup has less order and more chaos than an intact one. Likewise, solid crystals, the most organized form of matter, have very low entropy values; and gases, which are highly disorganized, have high entropy values. The entropy of a thermally isolated macroscopic system never decreases. However, a microscopic system may exhibit fluctuations of entropy opposite to that dictated by the Second Law. In a logical sense the Second Law thus ceases to be a "Law" of physics and instead becomes a theorem which is valid for large systems or long times.
Third law of thermodynamics
As temperature approaches absolute zero, the entropy of a system approaches a constant minimum.
In brief, this postulates that entropy is temperature dependent and leads to the formulation of the idea of absolute zero.
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