When work done by the system was 10 J, the increase in internal energy of the system was 30 J. The heat ‘q’ supplied to the system was:
A. 10 J
B. 20 J
C. 30 J
D. None of these

Hint: To answer this question we should recall the First law of Thermodynamics. That is, “Energy of the universe is always conserved or total energy of an isolated system is always conserved”. Now write the famous equation based on this law.

Complete step by step answer:
Now, let’s discuss the stepwise answer to this question -
From the first law of thermodynamics, we can write
$\Delta $u = q + w
Or,
q = $\Delta $u − w
(q = internal energy + work done)
Where,
q = heat supplied to the system
w = work done by the system = -10 J
$\Delta $u = change in internal energy = +30 J
As the work is done by the system and the internal energy is increased.

Now, we will insert the values in the above equation,

q = 30 − (−10)
q = 40 J

Hence, we can say that the heat supplied to the system is 40J.

Therefore, we can conclude that the correct answer to this question is option D.

Note: We should know that The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes: The first law makes use of the key concepts of internal energy, heat, and system work.
This is used extensively in the discussion of heat engines.