First Law of Thermodynamics

First Law of Thermodynamics

Introduction : The thermodynamical system is governed by three important law as we have discussed earlier. the relation between heat and work or heat and internal energy or work or internal energy has been explained by first law of thermodynamics. Let us understand this laws in details.

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Eg 1: Consider a container filled with gas and attached with a moving , massless, frictionless piston. Lets heat is supplied to the system and piston is kept fixed., then no work is done on the system and gas do not compress. Thus  all the heat provided to system is used to increase the internal energy of the system. 

Mathematically , Q = ΔU 

Eg 2: Consider a container filled with gas and attached with a moving , massless, frictionless piston. Let system does some work  and no heat is added to the system ,then the work done  increase the volume and the  gas expands. Thus system will loose its energy which leads to decrease in  internal energy of the system.

Mathematically , ΔU = -W 

As internal energy can be changed by both heat as well as work , the equation above can be combined and written as,

Mathematically , ΔU = Q -W 

Or

Mathematically ,  Q =ΔU + W  ---   (A)

First Law of thermodynamics :  The heat supplied to the system is used to do some work and to change (increase) the internal energy of the system.                     

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Illustrative Example : 

 When we use a pressure cooker to cook food, we make use of thermodynamics first law. When the cooker is heated, the steam generated inside the cooker cannot escape out which leads to increase in pressure at fixed volume as the volume of cooker can not change (The process is isochoric). according to Gay lussac law  at constant volume, pressure is directly proportional to temperature. As the pressure increases there is a rise in temperature which increases the internal energy (ΔU)of the cooker and thus food cook quickly which save the time and energy. thus all the heat supplied is used to increase the internal energy (Q = ΔU)as no work is done in the system due to constant volume.

│<<<Heat and Work││Isothermal process>>>│

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