Why is a thermos flask evacuated? Choose the correct statement about the role of vacuum between the walls of a thermos (vacuum) flask.

Introduction / Context:
Thermos flasks reduce heat transfer so that contents remain hot or cold. The evacuated space between the double walls is the key design feature limiting two of the three heat-transfer modes.

Given Data / Assumptions:

• Two closely spaced walls with the interspace evacuated.
• Reflective coatings reduce radiation (though not mentioned explicitly, typical in flasks).

Concept / Approach:
Conduction and convection both require matter. By evacuating the interspace, the flask essentially eliminates gas conduction and internal natural convection. Radiation remains the principal mode, which is mitigated with low-emissivity surfaces.

Step-by-Step Solution:
Conduction: k_medium ~ 0 in vacuum → negligible conduction.Convection: no fluid in the gap → no convective currents.Radiation: minimized via reflective surfaces.Net effect: vacuum is a poor (bad) conductor, reducing heat leak.

Verification / Alternative check:
Heat-leak calculations for vacuum insulation show orders-of-magnitude reduction versus air-filled gaps.

Why Other Options Are Wrong:
(a) Opposite of true; vacuum conducts neither well nor at all. (c) With no fluid, convection cannot occur. (d) Radiation is not increased by vacuum; it is unchanged by the medium, but surfaces are designed to reduce it. (e) Vacuum does not increase wall conductivity.

Common Pitfalls:
Assuming radiation is eliminated by vacuum; only conduction and convection are.

Final Answer:
Vacuum is a bad conductor of heat