Question

The total energy of an isolated system is conserved under which of the following?
(A) Only when the forces involved are conservative
(B) Only when friction can be neglected
(C) Only when non-conservative forces are involved
(D) Always
(E) Technically never

Answer 1

Hint: This question is related to the first law of thermodynamics. We can also understand this based on the definition of an isolated system. An isolated system is completely isolated from its surroundings and no transfer of energy or mass is possible between the system and surroundings.

Complete step-by-step answer:
The law of conservation of energy or the first law of thermodynamics states that the energy can neither be created nor be destroyed, but it can be transformed from one form to another. In other words, the total energy of an isolated system is always conserved. For example, when you are driving a car or a motorcycle, the chemical energy that was stored either in the fuel or in the battery is getting converted into mechanical energy. Similarly, when an explosion happens, the chemical energy of the fuel is converted into light energy and heat energy.
We have many such experiences to prove that the total energy of an isolated system is conserved over time, always.

So the correct answer is option D.

Note: An isolated system is a physical system which is completely isolated from its surroundings and such that there occurs no transfer of energy or mass. If you take a cup of tea and keep it open on the table, then after sometime you can find that the tea is cold. Here, if the cup is considered as your system, then the energy is conserved, because at the time of keeping the cup on the table and after some time, the heat energy in the cup is different. But, what we should keep in mind is that the tea cup placed over the table is not an isolated system. It can transfer energy and even mass from itself to its surroundings.