Question
Answers

Which of these is true? The internal energy of an isolated system:
A. keeps on changing
B. remains constant
C. Zero
D. none of these

Answer
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Hint: Since we know from the law of conservation of energy that energy is neither created nor destroyed, it can only be transferred. Then, either the energy must remain in the system or escape the system, according to the nature of the system. An isolated system is a type of a thermodynamic system.

Complete step by step answer:
Since we know from the law of conservation of energy that energy is neither created nor destroyed, it can only be transferred. Since the system is isolated, it implies that there is no loss or gain of energy and mass to the surroundings i.e. it obeys laws of conservation of both mass and energy. Then we can say that the energy remains in the system only, as there is no loss of energy.

An example of an isolated system is thermos-flasks.

Hence, the correct answer is option B.

Additional Information
A system is said to be isolated when it does not interact with its surroundings. In thermodynamics, an isolated system is differentiated from the surroundings by a rigid wall, which neither allows the movement of mass or energy to pass through it.
The other types of thermo-dynamical systems are
1. Closed systems, which allow the transfer of energy but not mass. Obeys only law of conservation of energy; and
2. Open systems, which allow the transfer of both energy and mass. Doesn’t obey the law of conservation of energy or mass.

Note:
An isolated system obeys laws of conservation of both mass and energy. Such a system however is impractical in real life. Hence we try to make a system isolated as much as possible for the experiments.