ThermodynamicsQ1.1.1 First Law of ThermodynamicsThe first law of thermodynamics states that energy cannot be created or destroyed--in other words it has always been here. Let there be a system which absorb ∆Q amount of heat and as a consequence of this it performs ∆W amount of work. In this process the initial equilibrium states “i” of the system changes to a final equilibrium state “f” in a particular way and ∆Q-∆W is computed. Now this system is changed from the same initial state “i” to the final state “f” but along a different path.
This procedure is repeated many times. It is observed that ∆Q-∆W comes out the same in all cases inspite of the fact that ∆Q and ∆W separately depend on the path taken. ∆Q-∆W depends only on the initial and final states. ∆Q is the energy added to the system and W is equal to the energy that has been extracted, from the system by the performance of work. The difference ∆Q-∆W which is retained within the system is the change in the energy of the system.
It follows that the initial energy change of a system is independent of the path and is therefore equal to Ui internal energy of the system in state “f” minus the internal energy in state “i” or Uf – Ui therefore it follows that ∆Q-∆W=Uf-Ui=∆UThe change in internal energy of a system in any process is equal to net heat flows into system minus the total work W done by the systemApplication of the First Law of ThermodynamicsIsobaric Process: Isobaric process is that process which takes place at constant pressure. In such a process the heat transferred and the works done are both non-zero.
∆Q=P (V2-V1) +∆UThis is the form of 1st law of thermodynamics in an isobaric process. Isochoric process: Isochoric process is defined as that process in which the volume of the system remains constant. U2-U1=∆Q (Isochoric process)Isothermal Process: If the temperature of the system remain constant throughout the process. It is called an isothermal process. ∆Q=∆W (Isothermal process)Adiabatic Process: The process in which no heat flows into or out of the system is called an adiabatic process. ∆U= -∆W1.2 Second Law of Thermodynamics. Any device which converts heat into mechanical energy is called heat engine. The essentials of a heat engine are the furnace, or hot body, the working substance and a condenser or cold body.
In a steam engine, the steam absorbs heat from the furnace, converts some of it into work by applying pressure to the piston and rejects the rests to the condenser. The point to note is that the furnace is at a higher temperature than the condenser and this conversion of heat into work is possible when the working substance falls in temperature as shown in figure in appendix.
We can generalise it by saying that the two bodies must be maintained at different temperature for the working of a heat engine. A continuous supply of work has never yet been obtained from a single supply of heat otherwise we could build a ship which would use far more heat in the ocean water without needing any fuel. This lead to the first way of stating the second law of thermodynamics due to Kelvin.