Question

What is the physical significance of the first law of thermodynamics?

Answer 1

The first law of thermodynamics states that during physical or chemical change the energy of an isolated system can neither be created nor destroyed; it is only allowed to convert from its one form to another.
Due to conservation of energy during reaction, the first law is commonly known as law of conservation of energy.

Complete answer:
First law of thermodynamics is solely based on the human experiences and has no theoretical proof yet.
Mathematically first law of thermodynamics is expressed as
${U_1} - {U_2} = q + w$
Physical significance of first law of thermodynamics includes
This law states that the sum of all the energy present in the universe remains the same.
Any system that is present in its isolated state has constant energy.
This law gives an idea about the working of machines which cannot do any work without the loss of energy.
It gives the relation between work and change in internal energy in the system.
$\Delta U = q - w$
Where $\Delta U$ is change in internal energy of system
$q$ Is amount of heat added to the system
$w$ Is work done by the system or work done to the system
From the above equation we see that total change in energy of a system is always the sum of heat added to the system and work done of the system.
For any adiabatic process in which there is no transfer of heat and matter takes place, internal energy of such a system becomes equal to its total work done.
$\Delta U = q - w$
Since, value of $q$ Is zero for adiabatic, above equation become
$\Delta U = 0 + w$
$\Delta U = {w_{\left( {ad} \right)}}$

Note:
Value of $w$ becomes positive when work is done to the system while it becomes negative when work is done by the system.
Similarly, when heat is supplied to the system value of $q$ becomes positive while during removal of energy from the system its value changes to negative.