Question

Explain first law of Thermodynamics and give its limitations.

Answer 1

This law is given by hemlholtz and robert mayer. this law is known as "Law of conservation of energy ". According to this law . "Energy can either be . Created nor destroyed although it can be converted from one form into another.OR "Total energy of the universe remains constant "or "Energy of an isolated system is constant :".The internal energy of the system can be changed in two ways :
1. By allowing the heat to flow into the system or out of the system.
2. By doing work on the system or by the system . Consider a system whose internal energy is $U_1$ Now if the system absorbs $q$ amount of heat , then the internal energy of the system increases and becomes $U_1+q$
If works is done on the system then its internal energy further increases and becomes $U_2$ thus
$U_2=U_1+q+w$
or $U_2-U_1=q+w$
or $\Delta U=q+w$
The above relationship is mathematical statement of first law of thermodynamics.
Limitations:
1. The first law of thermodynamics merely indicates that in any process there is a transformation between the various forms of energy involved in the process but provides no information regarding the feasibility of such transformation.
2. First law does not provide any information regarding the direction . A process will take place whether it is a spontaneous or non-spontaneous process.
3. Practically it is not possible to covert the heat energy into an equivalent amount of work.
4. No restriction on the direction of the flow of heat : the first law establishes definite relationship between the heat absorbed and the work performed by a system. the first law does not indicate whether heat can flow from a cold end to a hot end or not. for example , we cannot extract heat from the ice by cooling it to a low temperature. Some external work has to be done.