Question

A system is provided with 100 J of heat. Work done on the system is 20 J. What is the change in internal energy of the body?

Answer 1

Hint: -
Here we are given a system to which 100 J of heat is provided. Work done on the system is 20 J. This is a problem of thermodynamics. Here we need to be careful while using the sign conventions. We can use the first law of thermodynamics to solve this problem.

Complete Step by Step Solution:
According to First Law of Thermodynamics
$\Delta Q=\Delta U + \Delta W\\\$
Here $\Delta Q$ is the amount of heat given to the body.
$\Delta U$ is the change in the internal energy of the system
$\Delta W$ is the amount of work done.
First law tells us that heat given to a system can be utilized in two ways. A part of it can be used to increase the internal energy of the system and a part of it can be used to do the work.
Putting the values, we get,
100= $\Delta U\\\$+20
$\Rightarrow \Delta U\\\$=100-20
$\Rightarrow \Delta U\\\$=80 J
So, the correct answer is 80 J

Additional Information:
The First Law of physics states that heat could be a type of energy, and natural philosophy processes area units thus subject to the principle of conservation of energy. This implies that energy can’t be created nor be destroyed. It can, however, be transferred from one location to a different and reborn to and from different styles of energy.

Note:
Here the work done on the system comes out to be positive, if the case would have been opposite that work is done by the system then the work would have been taken in negative. So, we have to be careful in using sign conventions.