Question

The first law of thermodynamics is based on law of conservation of:
A. Energy
B. Mass
C. Momentum
D. pressure

Answer 1

Hint: In this question, first we will define work and internal energy in thermodynamics. After this we will the statement for the first law of thermodynamics which describes the relation between heat supplied to the system, work done and change in internal energy of the system.

Complete answer:
Let us first the definition of internal energy and work done.

A.)Internal energy:
The concept of internal energy of a system is not difficult to understand. We know that every bulk system consists of a large number of molecules. Internal energy is simply the sum of the kinetic energies and potential energies of these molecules We know that in thermodynamics, the kinetic energy of the system, as a whole, is not relevant. Internal energy is thus, the sum of molecular kinetic and potential energies.
We denote the internal energy of a system by U.

B.)Work:
Work is energy transfer brought about by means (e.g. moving the piston by raising or lowering some weight connected to it).

Now, we will see the First law of thermodynamics.

First Law of Thermodynamics:

We know that the internal energy $\vartriangle U$ of a system can change through two modes of energy transfer : heat and work.
Let $\vartriangle $Q = Heat supplied to the system by the surroundings
$\vartriangle $W = Work done by the system on the surroundings
$\vartriangle $U = Change in internal energy of the system
The general principle of conservation of energy then implies that
$\vartriangle $Q = $\vartriangle $U + $\vartriangle $W (1)
The above equation means that the energy ($\vartriangle $Q) supplied to the system goes in partly to increase the internal energy of the system ($\vartriangle $U) and the rest in work on the environment ($\vartriangle $W).
Equation 1 is known as the First Law of Thermodynamics.

It is simply the general law of conservation of energy applied to any system in which the energy transfer from or to the surroundings is taken into account.

So, the correct answer is “Option A”.

Note:
Heat and work in thermodynamics are not state variables. They are modes of energy transfer to a system resulting in change in its internal energy, which is a state variable. And so, depends only on the initial and final states and not on the path taken by the gas to go from one to the other. However, $\vartriangle $Q and $\vartriangle $W will, in general, depend on the path taken to go from the initial to final states. The combination $\vartriangle $Q – $\vartriangle $W, is however, path independent.