Question

Explain first law of thermodynamics.

Answer 1

First Law of Thermodynamics
We now know about internal energy, heat and work done for a system in thermodynamics, but how are these quantities related to each other. The laws of thermodynamics give us the guidelines for inter-conversion of energy from one form to the other. The history of thermodynamics dates back to the time when first heat engines were designed for the extraction of water from mines. The studies for improving the efficiency and output of these engines continued for a long time.
The first law of thermodynamics was conceived by a trial and error method that stretched over a period of fifty years. The first law of thermodynamics states that the total energy of an isolated system is constant. A close examination of this statement suggests that the first law of thermodynamics is nothing but conservation of energy. The basis for the development for the first law of thermodynamics is conservation of energy. As per the thermodynamic approach this law is stated as the change in internal energy of any closed system is given by the difference between the heat stored in a system and the work done by it. An equivalent way of stating the first law of thermodynamics is that a perpetual motion machine of first kind that is machines that can indefinitely do work without any energy source is impossible as it violates the conservation of energy and the first law of thermodynamics.
Mathematically the first law of thermodynamics is stated as:
$\Delta U=Q-W$
Where, $\Delta U$ is the change in internal energy of the system. Q is the heat given to the system and W is work done by the system. Internal energy being a state function is measured as the change since its absolute value cannot be measured and hence holds no value in absolute terms. The heat given to the system is always taken positive and the work done by the system is also positive. If heat is being extracted (given out) from the system then it is taken as negative and any work done on the system is also taken negative while calculating thermodynamic properties. The laws of thermodynamics have played a vital role in designing various engines and optimising their efficiency and output.



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Basic Thermodynamics
Thermodynamics is the sub-part of science that deals with the study of flow of heat. Heat energy associated with chemical reactions is converted into different usable forms based on the laws of thermodynamics. Some general terms like heat, energy and work done are often used in thermodynamics. Let us learn the basic thermodynamics and understand these terms.
1. Internal Energy It is the energy stored within the system. This energy represents the total energy of the system and may Include any form of energy, kinetic energy, potential energy, etc. We know about the energy transformations, energy can only be transferred and cannot be created or destroyed. Basic thermodynamics provides us the information regarding energy transformation associated with a system of chemical reaction. Internal energy of a system may change when heat passes into or out of the system, work is done on or by the system or matter enters or leaves the system.
2. Work
Basic thermodynamics defines work done by a system as the quantity of energy exchanged between a system and its surrounding. Work in thermodynamics is completely governed by external factors that are factors in the surrounding. These factors may be an external force, a change in temperature or pressure, chanae in volume etc.
3. Heat
Heat in thermodynamics is generally defined as the energy in transit. It is the energy by virtue of kinetic energy of the molecules of a substance. Heat and the thermodynamics together form a vertical of science that has helped process designers and engineers to optimize their processes and harness the energy associated with chemical reactions economically. Heat energy flows from higher temperature to lower temperature. The study of this flow and advances in thermodynamics has helped scientists to design various heat engines.