First Law of Thermodynamics?

 A special case of law of conservation of energy with particular reference to heat and work. • Definition: Heat and work are interconvertible. • When a system undergoes a thermodynamic cycle then the net heat supplied to the system from surroundings is equal to the net work done by the system on its surroundings.

First law for a process When a system executes a process the change in stored energy of the system is numerically equal to the net heat interactions minus the net work interactions during the process.

 dE =δQ-δW So there exists a property of a closed system such that a change in its value is equal to the difference between heat supplied and the work done during the change of state. The property, E is called energy of the system. The energy resides within the system and it increases or decreases with change of state. Further

, δQ = dE + δW

. First law for an isolated System The energy of an isolated system is always constant. An isolated system is one in which there is no interaction of the system with the surroundings, so for an isolated system:

 δQ = 0, δW = 0 and Hence

 dE = 0 Or E = constant

According to law of conservation of energy no engine can continuously produce mechanical work without some other form of energy disappearing simultaneously as shown in fig a. Such an imaginary machine is known as PMM1. • The converse of this is also true i.e. no machine can continuously consume work without some other form of energy appearing simultaneously as shown in fig b. • Such a machine is impossible to be constructed as it violates the first law of thermodynamics.

A process is cyclic if initial and final states of the

system executing the process are identical.

• For a process, Fist law states:

Q1-2 = E2

-E1 +W1-2

Q2-3 = E3

-E2 +W2-3

Q3-4 = E4

-E3 +W3-4

Q4-1 = E1

-E4 +W4-1

Then for the cycle.

Q1-2 + Q2-3 +Q3-4 +Q4-1 =W1-2 +W2-3 +W3-4 +W4-1

This equation indicates that when a closed system undergoes a cyclic change then the cyclic integral of heat is equal to cyclic integral of work. = 0 Hence for a cycle , if the system returns back to its original conditions the sum of heat and work effects is equal to zero. Further, both δQ and δW are path functions , but their difference (δQ- δW) is a point function

Stored energy a property of the system •

 Hence from here we say that value of does not depend on the path, so it is a function of state only, therefore this quantity is a point function and this is called the total energy of the system and given by E. Further energy is the property of the system and hence (δQ-δW) is an exact differential and is denoted by dE. So, δQ-δW=dE E is sum of all the energies but is absence of K.E., P.E. And other energies, it is taken equal to internal energy only and then, δQ-δW=dU