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1. Explain First Law of Thermodynamics ?
According to this law,
       ” the total energy supplied to the system is spend partially in increasing increasing internal energy of the system & the rest of energy is spend in doing work  on the surrounding ”. 
Mathematically it is written as : 
               ΔQ = ΔU + ΔW
 ΔQ =  total energy supplied(heat)
 ΔU = change in internal energy
 ΔW = work done on the surrounding.

Here Internal Energy is a state function. Hence it is perfect differential, while Heat & Work are path function, hence these are not perfect differential. 

See Also :
             (i) Heat, Internal Energy & Work
             (ii) Work Done during adiabatic process

Thermodynamics first law holds for law of Conservation of Energy? 
Let us suppose that a quantity dQ of heat is supplied to a body.  It is in general spent in three ways :
(i)  Partially, it is spent in raising the temperature of the body which is equivalent of increasing its internal kinetic energy. 
(ii)  A part of it is spent in doing internal work against molecular attractions, i.e. in increasing internal potential energy of the body.
(iii) A third part of it is spent in expanding the body against the external pressure, i.e. in doing external work.
If dUk & dUp are respectively the change in internal kinetic energy & internal potential energy & dW be the external work done then since “energy can neither be created nor destroyed but only convert from one form to another”,we have
               dQ = dUk + dUp + dW 
But dUk + dUp = dU, the increase in total internal energy of the body. 
Hence,    dQ = dU + dW

                ΔQ = ΔU + ΔW
This equation represent the differential form of first law of thermodynamics, which may therefore be stated.“In all transformations, the energy due to heat units supplied must be balanced by external work done plus increase in internal energy.”

Thus, first law holds for law of conservation of energy. 
Significance of First law? 

The first law of thermodynamics establishes an exact relationship between heat & work. According to it a definite quantity of heat will produce a definite amount of work and vice versa. It denies that work or energy can be created out of nothing. It means that it is impossible to construct a thermal machine which may operate without any expenditure of fuel and may thus create energy out of nowhere. A machine that would do this could run itself and is generally called a perpetual motion machine of the first kind.  
The first law rules out such mechanics and is therefore sometime defined in the form, 

“Perpetual motion machine of first kind is impossible “.

Three ideas are included in first law of thermodynamics :

(i) Heat is a form of energy in transit. 
(ii) Conservation of energy takes place in thermodynamic systems. 

(iii) Every system in equilibrium, possesses an internal energy which is a function of state, i.e., depends only upon initial & final state of system. 
See Also : (i) Quasi Static Process
Limitations of First Law? 

(i)  First law of thermodynamics is based on principle of conservation of energy of a system. Though it is applicable to every process in nature between the equilibrium states, it does not specify the condition under which a system can used its heat energy to produce a supply of mechanical work. 

(ii) It also does not say how much of heat energy can be converted into work. 
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Hope this article make you understand  First law of thermodynamics, still any query write to us in comment section.