Question

In a process, $701J$ of heat is absorbed by a system and $394J$ of work is done by the system. What is the change in internal energy for the process?

Answer 1

Internal energy of a body or a system is the kinetic and potential energy associated with the unordered vibrational and random motion of the molecules. It is the sum of the heat exchanged and work done on or by the system.
Formula used
Internal energy formula:
According to first law of thermodynamics,
$\Delta U = Q + W$
Where, $\Delta U$ is change in internal energy, $Q$ is heat and $W$ is work done.

Complete step-by-step answer
We will start with the definition of the internal energy of a system or body.
Internal energy of a body or system: The internal energy of a body or a system is the kinetic and potential energy associated with the unordered and random motion of the molecules.
Now, what important points we can observe from the question:
Heat is absorbed by a system, then the heat absorbed will be positive and if work is done by the system, then the work done by the system will be negative.
Given data in the question,
Heat absorbed by the system, $Q = + 701J$
Work done by the system, $W = - 394J$
So, now putting the values of the heat absorbed and the work done by the system in the formula.
$\Delta U = + 701 - 394 = 307J$
So, the internal energy for the process is $307J$ .

Note:
There are some sign conventions in thermodynamics, like, when heat is absorbed by the system, i.e, endothermic, the heat is positive and when heat is released from the system, i.e, exothermic, the heat is negative and when work is done by the system, the work is negative and when work is done on the system, the work is positive.