The energy is always conserved for a system which is
- (a)isolated only
- (b)non-isolated only
- (c)both isolated and non-isolated
- (d)none of above
Correct — A, isolated only. The law of conservation of energy says the total energy of an isolated system stays constant — an isolated system exchanges neither matter nor energy with its surroundings, so its total energy has nowhere to go. A non-isolated system can gain or lose energy across its boundary as heat or work, so the energy of that system alone is not conserved. Energy is still conserved overall, but only once you enlarge the system to include the surroundings — so only for an isolated system is the system's own energy always conserved.
- (b)non-isolated only — Backwards — a non-isolated system freely exchanges energy with its surroundings, so its own total energy changes; it is not the case that energy is conserved for non-isolated systems only.
- (c)both isolated and non-isolated — A non-isolated system's energy is not conserved on its own, because energy crosses the boundary as heat or work, so 'both' is wrong; the guarantee holds only for an isolated system.
- (d)none of above — Energy conservation does hold for isolated systems, so 'none of the above' is incorrect.
Thermodynamic systems are classified by what can cross their boundary: an open system exchanges both matter and energy, a closed system exchanges energy but not matter, and an isolated system exchanges neither. The conservation-of-energy law guarantees a constant total energy only when nothing crosses the boundary — that is, for an isolated system.
The universe as a whole is the ultimate isolated system, so energy is conserved globally. For any smaller system you must ask whether energy can enter or leave: if it can (non-isolated), the system's own energy changes. So 'always conserved for the system itself' points uniquely to the isolated case.
- An isolated system exchanges neither matter nor energy with the surroundings, so its total energy is constant.
- A closed system exchanges energy (heat or work) but not matter.
- An open system exchanges both matter and energy.
- By the first law of thermodynamics the change in internal energy equals heat added minus work done; for an isolated system both are zero, so the internal energy stays constant.
Only the isolated system keeps its total energy constant (option A).
- Assuming energy is conserved for every system — it is guaranteed only when nothing crosses the boundary.
- Confusing 'closed' with 'isolated'; a closed system still exchanges energy with its surroundings.
Asked as for which type of system the total energy is conserved, or through the open/closed/isolated definitions.
No directly related past PYQ was found.
- practice — not a real PYQ
A system that exchanges neither matter nor energy with its surroundings is called
- (a)Open
- (b)Closed
- (c)Isolated
- (d)Adiabatic
Answer(c) Isolated — nothing crosses its boundary.
- practice — not a real PYQ
For an isolated system, the change in internal energy is
- (a)Equal to the heat added
- (b)Equal to the work done
- (c)Positive
- (d)Zero
Answer(d) Zero — with no heat and no work crossing the boundary, the internal energy stays constant.