What the Third Law Means

The third law means a few things, and again all of these formulations result in the same outcome depending upon how much you take into account:

Formulation 3 contains the least restraints, merely stating that entropy goes to a constant. In fact, this constant is zero entropy (as stated in formulation 2). However, due to quantum constraints on any physical system, it will collapse into its lowest quantum state but never be able to perfectly reduce to 0 entropy, therefore it is impossible to reduce a physical system to absolute zero in a finite number of steps (which yields us formulation 1).

The Second Law & Entropy:

The Second Law of Thermodynamics can be restated to talk about entropy, which is a quantitative measurement of the disorder in a system. The change in heat divided by the absolute temperature is the entropy change of the process. Defined this way, the Second Law can be restated as:

In any closed system, the entropy of the system will either remain constant or increase.

By "closed system" it means that every part of the process is included when calculating the entropy of the system.

Thermodynamic Processes:

A system undergoes a thermodynamic process when there is some sort of energetic change within the system, generally associated with changes in pressure, volume, internal energy (i.e. temperature), or any sort of heat transfer.

There are several specific types of thermodynamic processes that have special properties:

• Adiabatic process - a process with no heat transfer into or out of the system.

Definition: An adiabatic process is a thermodynamic process in which there is no heat transfer (Q) into or out of the system. In other words Q = 0.

An adiabatic process is generally obtained by surrounding the entire system with a strongly insulating material or by carrying out the process so quickly that there is no time for a significant heat transfer to take place.

Applying the first law of thermodynamics to an adiabatic process, we obtain:

delta- U = - W

Since delta- U is the change in internal energy and W is the work done by the system, what we see the following possible outcomes:

• A system that expands under adiabatic conditions does positive work, so the internal energy decreases.
• A system that contracts under adiabatic conditions does negative work, so the internal energy increases.

There is often, though not always, a change in temperature associated with the change in internal energy.

The compression and expansion strokes in an internal-combustion engine are both approximately adiabatic processes. What little heat transfers outside of the system is negligible and virtually all of the energy change goes into moving the piston.

• Isochoric process - a process with no change in volume, in which case the system does no work.

· Definition: An isochoric process is a thermodynamic process in which the volume remains constant. Since the volume is constant, the system does no work and W = 0.

· This is perhaps the easiest of the thermodynamic variables to control, since it can be obtained by placing the system in a sealed container which neither expands nor contracts.

· Applying the first law of thermodynamics to this situation, we find that:

· delta- U = Q

· Since delta- U is the change in internal energy and Q is the heat transfer into or out of the system, we see that all of the heat either comes from internal energy or goes into increasing the internal energy.

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