Principle: It is based on the principle of temperature.
To understand this law, consider two systems A and B, separated by an adiabatic wall, each of these are in contact with a third system C by a conducting wall. The states of the system will change until A and B are in equilibrium with C. Now, let the adiabatic wall between A and B be replaced by a conducting wall and C is insulated from A and B by an adiabatic wall. It is found that A and B are in thermal equilibrium with each other. This is the basis of Zeroth law of thermodynamics. This law states that the two systems are in thermal equilibrium with a third system separately, then both are in thermal equilibrium with each other.
The zeroth law apparently suggests that when two systems A and B are in thermal equilibrium, then, there must be a physical quantity that has the same value for both. The thermodynamic variable whose value is equal for the two systems (in thermal equilibrium) is called temperature (T). If A and B are separately in equilibrium with C, then, TA = TC and TB = TC - This means that TA = TB. So, the systems A and B are in thermal equilibrium.
The first law of thermodynamics states the equivalence of heat and work. This law is based on the principle of conservation of energy.
The internal energy U of a system can change through two modes of energy transfer: heat and work.
Let ∆Q = Heat supplied to the system by the surroundings,
∆W = Work done by the system on the surroundings.
∆U = Change in internal energy of the system.
From the general principle of conservation of energy, i.e..
∆Q = ∆U + ∆W …(3)
The energy ∆Q, supplied to the system goes in partly to increase the internal energy of the system (∆U) and work (∆W). Equation (3) is called the first law of Thermodynamics. This is the general law of conservation of energy applied to any system in which the energy transfer from or to the surroundings.
We can write equation (3) as
∆Q – ∆W = ∆U …(a)
Now, the system go from an initial state to final state in a number of ways.
To change the state of a gas from (P1,V1) to (p2, V2), we first change the volume from V1 to V2, keeping the pressure constant. This means that we first go to state (P1 ,V2) Changing the pressure from P1 and P2, keeping volume constant to the state (P2,V2). We can also first keep the volume constant and then keep the pressure constant.
U is a state variable. ∆U depends only on the initial and final states and not on the path taken by the gas to .move from one to the other. However ∆Q and ∆W depends on the path in moving from initial to final states.
From equation (a), (∆Q – ∆W) is path independent for a system with ∆U = 0,
∆Q = ∆W
This implies that heat supplied to the system is completely used up by the system in doing work on the environment. Work done by the system against a constant pressure P is
∆W = P∆V
Here, ∆V is the small change in volume of gas.
Sis, equation (a) gives
∆Q = ∆U + P∆V …(b)
The second law of Thermodynamics states that the total entropy of an isolated system increases over time, or it remains same in ideal cases (where the system undergoes a reversible process or when it is in a steady-state). The first law of Thermodynamics tells the basic definition of internal energy and states the law of conservation of energy. The second law of Thermodynamics deals with the direction of natural process. It states that a natural process is not reversible. For e.g., heat flows from hot to cold region, and never the reverse, unless some external work is done on the system.
In Terms of Entropy
In a reversible process, a very small increase in the entropy (dS) of a system is defined to result from a small heat transfer to a closed system divided by the common temperature (T) of the system and the surroundings that supply of the heat :
dS = \(\frac { dQ }{ T } \) (closed system)
