Heat given to a system is 35 joules and work done by the system is 15 joules. The change in the internal energy of the system will be?
A. -50J
B. 20J
C. 30J
D. 50J

Hint:Before we start addressing the problem, we need to know about what data has been provided and what we need to solve. As part of the input, we have the total heat addition into the system and the work done by the system. Using these values, the change in the internal energy has to be found. So, in order to achieve it, we use the equation of the first law of thermodynamics, in which we get the value of internal energy.

Formula Used:
The formula to find the first law of thermodynamics is given by,
\[\Delta U = Q - W\]……… (1)
Where,
\[\Delta U\] is a change in the internal energy.
\[Q\] is heat added to the system.
\[W\] is work done by system.

Complete step by step solution:
According to the statement of the first law of thermodynamics we have,
\[\Delta U = Q - W\]
Now, substitute the values of Q and W as 35 joules and 15 joules, then,
\[\Delta U = 35 - 15\]
\[\therefore \Delta U = 20J\]
Therefore, the change in internal energy of the system is 20J.

Hence, option B is the correct answer.

Note:The first law of thermodynamics states that, when a system does some work or there is a change in the internal energy of the system, the energy expended for this work has to come from some source. Or in simple words energy can neither be created nor destroyed but can be transformed from one form to another. For example, when we are cooking vegetables on the gas stove, we are breaking the energy on the chemical bonds in the gas to release energy which is converted to heat energy used to boil the water and cook the vegetables.