The vacuum between the double walls of a thermos flask reduces heat loss through

Analyze the question: The thermos flask (vacuum flask) has a double wall with a vacuum in between. We need to know which heat transfer methods the vacuum stops.

Explain the concept: There are three modes of heat transfer:

• Conduction: heat passes through matter (solids, liquids, gases) by particle-to-particle collisions. It needs a material medium.
• Convection: heat is carried by the movement of fluids (liquids or gases). It needs a material medium.
• Radiation: heat travels as electromagnetic waves. It needs no medium and can cross a vacuum.

Solution:

1. A vacuum is a space with essentially no particles.
2. With no particles, there is nothing to conduct heat by collisions, so conduction is prevented.
3. With no air or fluid, there are no currents to carry heat, so convection is prevented.
4. Radiation can still travel through a vacuum. That is why the flask's walls are silvered (shiny) to reflect radiant heat back. The vacuum itself does not stop radiation.

So the vacuum reduces heat loss by conduction and convection.

Answer: Option B, convection and conduction.

Why the others are wrong:

• A: Radiation is not stopped by a vacuum, so it is reduced only by the silvering.
• C: Radiation is the one mode the vacuum cannot prevent.
• D: Convection is also stopped, so "conduction only" is incomplete.

Exam tip: Remember "Vacuum kills the medium." Conduction and convection need a medium, so the vacuum stops both. Radiation does not, which is why the Sun's heat reaches Earth through space. For a thermos flask, remember: vacuum → conduction and convection; silvered walls → radiation; stopper/cork → conduction and convection through the top.