Hello students, we know that heat is the form of energy that always transfers from point of higher temperature to point of lower temperature. Heat given to system, increases its temperature. This changes the parameter like pressure, volume of the system. So that system does the work over surrounding or surrounding over system. During this work done, change in internal energy of system is an important measurable quantity so as to calculate the amount of work done.
In this article we are going to discuss the effects of heat give to system in the form of work done. The first law of thermodynamics is very important to study thermodynamic systems.
Let’s understand first law of thermodynamics in detail……………!
Consider the most basic example of a cylinder containing real gas. Cylinder has movable, mass less and frictionless piston fixed on its upper surface as shown in fig.
When heat it given to a gas contained in cylinder with piston, then temperature of system i.e. gas increases. Along with temperature an internal energy of gas is also increases from its initial value Ui to final value Uf. The change in internal energy of a system during thermodynamic process depends only on the initial and final states, not on the path leading from one to the other. During this increase in energy the piston starts moving upwards and the volume of gas is increased. Hence we can say that the work is done by the system over surrounding.
Let,
△Q = be the heat provided to the system
△U= Uf – Ui= Change in internal energy of system
△W= Work done by the system over surrounding or vice versa.
So if we use generalised form of law of conservation of energy, i.e. Energy can neither be created nor be destroyed and can be converted from one form to another form, total energy of universe is conserved.
Using the above statement we can relate, heat supplied, change in internal energy and work done as,
Heat supplied = Change in internal energy + Work done
∴ ∆Q = ∆U + ∆W
∴ ∆U = ∆Q – ∆W
The above equation in termed as first law of thermodynamics. Then we can state the law as, change in internal energy is equal to the difference of the heat supplied to the system and the work done by the system.
The work done during the process then can be written as,
∴ Work done,dW= Force×Displacement of piston
∴ dW = F × ∆h
∴ dW = P × A × ∆h
But, A × h = volume, dV, then the above equation can be written as,
∴ dW = P × dV
Where dV is small change in volume. To find the total work done we need to integrate the above equation from Vi to Vf
Using above method we can find the total work in the thermodynamic system.
Some important points to note:-
- If heat is given to system, Qin is positive then Work done by the system is positive i.e. when the volume of gas is increased, then work is done by system over surrounding, internal energy of system is increased. In this case work is taken as positive.
- If heat is released by system, Qout is negative work done on the system is negative i.e. When the volume of gas is decreased, then work is done by surrounding on system, internal energy decreased. In this case work is taken as negative.
- Both heat and work are considered only when there are some changes in system.
- Heat and work are associated with a process, not a state. They are both path-dependent functions
Applications of first law of thermodynamics…..!
- Steam engine
- Working of fans
- Cooking of food