**Contents**show

# First law of thermodynamics

### The first law of thermodynamics is the thermodynamic

expression of **the conservation of energy**.

This law most simply stated by saying that “energy can not be

created or destroyed” or that “the

energy of the universe is constant”.

This law can be stated for a system (control mass) undergoing a

cycle or for a change of state of a system.

Stated for a system undergoing a cycle, the cyclic integral of the

work is proportional to the cyclic integral of the heat.

Mathematically stated, for a control mass undergoing a

cyclic process such as in Joule’s experiment and for consistent set of units

∫dQfrom system= ∫dWon system

or ∫dQfrom system– ∫dWon system = 0

The important thing to remember is that the first law states that

the energy is conserved always.

## **Sign convention **

The work done

by a system on the surroundings is treated as a positive quantity.

Similarly, energy transfer as heat to the system from

the surroundings is assigned a positive sign. With the sign convention one can write,

∫dQ = ∫dW

**Consequences of the**

first law

first law

Suppose a system is taken from state

1 to state 2 by the path 1-a-2 and

is restored to the

initial state by the path 2-b-1, then the system has undergone a cyclic

process 1-a-2-b-1. If the system is restored to the initial state by path

2-c-1, then the system has undergone the cyclic change 1-a-2-c-1. Let us apply

the first law of thermodynamics to the cyclic processes 1-a-2- b-1 and

1-a-2-c-1 to obtain

∫1-a-2dQ+ ∫2-b-1dQ – ∫1-a-2dW – ∫2-b-1dW =0

Subtracting,

we get

∫1-a-2dQ+ ∫2-c-1dQ – ∫1-a-2dW – ∫2-c-1dW=0

∫_{2b1}dQ- ∫_{2c1}dQ –( ∫_{2b1}dW – ∫_{2c1}dW)

=0

We know that the work is a path function and hence the term in

the bracket is non-zero.

Hence we find

∫_{2b1}dQ = ∫_{2c1}dQ

That is heat is

also a path function.

this law of thermodynamics changed the worlds machine history. The lawyer world