“First Law of Thermodynamics” MHT-CET Class 12 Physics Crash Course | Chapter Thermodynamics
In this session G.S. Khairnar discussed the theory and MCQ’s of “First Law of Thermodynamics”. The session will be useful for all the aspirants of MHT-CET 2022. The session will be in Marathi and English.
First Law of Thermodynamics:-
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.
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,
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,
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
First Law of Thermodynamics:-
Let’s solve some MCQ’s….!
Q.1) The first law of thermodynamics is analogous to….
a) law of conservation of mass
b) law of conservation of momentum
c) law of conservation of energy
d) none of these
Answer
c) law of conservation of energy
Q.2) Which among the following is correct form of first law of thermodynamics?
Q.3) When heat is given to system contained gas, its internal energy increases. This is because…
a) kinetic energy of gas molecules increases
b) potential energy of gas molecules increases
c) momentum of gas molecules increases
d) both ‘a’ and ‘c’
Answer
a) kinetic energy of gas molecules increases
Q.4) When the volume of gas is increased, then work is done by system over surrounding, internal energy of system is increased, magnitude of work done is….
a) only negative
b) initially negative and becomes positive during expansion of gas
c) only positive
d) initially positive and becomes negative during expansion of gas
Answer
c) only positive
Q.5) When the volume of gas is decreased, then work is done by surrounding on system, internal energy decreased, magnitude of work done is….
a) only negative
b) initially negative and becomes positive during compression of gas
c) only positive
d) initially positive and becomes negative during compression of gas
Answer
a) only negative
Q.6) 6 KJ of energy incident on system with movable piston and contains the gas, increases the volume by 0.25 m3. If the pressure exerted by gas on the piston is 200 KPa , internal energy gain by system is….
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