Question

A system absorbs $ 20kJ $ heat and does $ 10kJ $ of work. The internal energy of the system___________
A) Increases by $ 10kJ $
B) Decreases by $ 10kJ $
C) Increases by $ 30kJ $
D) Decreases by $ 30kJ $

Answer 1

Hint: Thermodynamics is a branch of science that mainly deals with the heat and work quantities. According to the first law of thermodynamics, the amount of heat given to a system will be utilized to change the internal energy of a system and the amount of work done.
 $ U = q - W $
q is amount of heat given
U is internal energy of a system
W is work done.

Complete answer:
Thermodynamics is a branch of science that mainly deals with the heat, work done and other forms of energy like electrical, mechanical. There were some basic laws in thermodynamics which were used to determine the amount of work done and other terms.
First law of thermodynamics explains that the amount of heat energy given to the system will be utilized to change internal energy and work done.
Given that the work done is $ 10kJ $ and the amount of work done is $ 20kJ $ .
Thus, from the above formula the change in internal energy will be
Substitute the above values in the formula
 $ U = 20 - 10 = 10kJ $
Thus, the internal energy is increased by $ 10kJ $ as the positive value is observed.
Option A is the correct one.

Note:
System is the limited area or bounded within limits. Outside the system there will be surroundings. The work done on the system is positive in the above case thus, the internal energy increases by $ 10kJ $ , if the same amount is observed with a negative value, it indicates the decrease of internal energy.