Question

If work done by the system is 300 joule when 100 cal, heat is supplied to it. The change in internal energy during the process is?
(A) -200 J
(B) 400 J
(C) 720 J
(D) 120 J

Answer 1

Hint: The First Law of Thermodynamics states that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy.

Complete step by step answer:
> When a force causes a body to move, work is being done on the object by the force. Work is the measure of energy transfer when a force (F) moves an object through a distance (d). So when work is done, energy has been transferred from one energy store to another, and so: energy transferred = work done.
>According to first law of thermodynamics,
\[\Delta U = Q + W\]
Where \[\Delta U\]= change in internal energy
Q= Heat added
W= work done by system.
It is Given given in the question that,
Q = 100 cal = 420 J
W = -300 J
Now, substituting the values of Q and W in the above equation.
\[\begin{gathered}
\Delta U = Q + W \\
\Delta U = 420 + ( - 300) \\
\Delta U = 120J \\
\end{gathered} \]
Hence the correct option is (D).

Note: Keep in mind that if work is done on the system, its sign is positive, whereas, if work is done by the system, its sign is negative. Don’t confuse between them while you are answering this question. Using the wrong sign may alter the results.