What is the difference between absolute, Gauge and vacuum pressures?

To answer this question let us know the various types of pressures

1) Atmospheric pressure

2) Absolute pressure

3) Gauge pressure

4) Vacuum pressure

Atmospheric pressure:

The atmospheric pressure exerts a normal pressure upon all surfaces with which it is in contact, and is known as atmospheric pressure.

It varies with altitude and it can be measured by means of a barometer.As such it is also called "Barometric pressure".

Absolute pressure: -

When pressure is measured above absolute zero it is called absolute pressure. All values of absolute pressure are positive. The lowest absolute pressure which can probably exist corresponds to absolute zero or complete vacuum.

Gauge pressure: -

When pressure is measured either above or below atmospheric pressure as a arbitrary datum then it is called as gauge pressure. All pressure gauges read zero when open to atmosphere.
Pressure gauges read only the difference between, pressure of a fluid to which they are connected & atmospheric pressure.

Vacuum pressure:-

If pressure of a fluid is below atmospheric pressure it is designated as vacuum pressure. Its gauge value is the amount by which it is below the atmospheric pressure. A gauge which measures vacuum pressure is vacuum gauge.

All about Second Law of Thermodynamics?

Why second law of thermodynamics introduced? 

We know that some processes occur spontaneously but if we try to reverse the direction of process, the process do not occur spontaneously and further some external energy is required to move the given system away from state of equilibrium. 

The question is that "why such reversed processes do not occur spontaneously?" could not be answered by first law because the total energy of system would remain constant in the reversed process as it did in the orginal path and ther is no voilation of first law. Therfore there must be some other natural principle in addition to first law which determines the direction in which a process can take place in an isolated system. This principle is "second law of thermodynamics" .

Second law infers us that "the entropy of universe tends to maximum". 

Second law of thermodynamics in terms of entropy:

  "The entropy of an isolated system is fully conserved in every reversible process i.e. for every reversible process the sum of all changes in entropy taking place in an isolated system is zero. If the process is not a reversible one, then the sum of all changes in entropy taking place in an isolated system is greater than zero. In general we can say that in every process taking place in an isolated system the entropy of system either increases or remains constant."  

Condition for equilibrium of an isolated system

“If an isolated system is in such a state that its entropy is maximum, any change from that state would evidently lead to decrease in entropy and hence will not happen. Thus the necessary condition for equilibrium of an isolated system is that its "Entropy shall be maximum."

 Other forms of second law of thermodynamics

Kelvin-Planck statement : It is impossible to construct a device which, operating in a cycle has the sole effect of extracting heat from a single reservoir and performing equivalent amount of work.

Clausius statement : It is impossible for heat to flow from a cooler body to another hotter body  without the aid of external energy.

 "Study on heat engines is based on the above law"