Question

A system absorbs 50kJ heat and does 20kJ of work. What is the net change in the internal energy of the system?
(A) Increase by 30kJ
(B) Decrease by 30kJ
(C) Increase by the 70kJ
(D) Decrease by the 70kJ

Answer 1

Hint: Before attempting this question, one must have prior knowledge of the first law of thermodynamics and the formula derived from it.

Complete answer:
Considering all the data given in the question.
$q = 50kJ$( as in this case system absorbs heat, so it is positive)
$W = - 20kJ$( the work done is negative, as it done by the system)
$\Delta U = ?$(to be calculated)
Where,
$q = $heat absorbed/gained
$W = $work done
And $\Delta U = $change in internal energy of the system
According to 1st law of thermodynamics
$\Delta U = q + W = 50 - 20 = 30kJ$
Therefore, internal energy of the system increase by 30kJ
Hence the correct option is A

Note: If the work is done by the system, work done is negative and if the work is done on the system, work done is positive. If a system absorbs heat, the change in heat is positive and vice-versa. kJ is a unit to measure energy.