Question

hot liquid in a vacuum flask cools extremely slowly because some modes of heat transfer cannot take place in a vacuum. these modes are a. conduction and convection only. b. conduction and radiation only. c. conduction, convection and radiation. d. convection and radiation only.

Explanation:

Heat transfer has three modes: conduction (needs a medium to transfer heat through direct contact of particles), convection (needs a fluid medium to transfer heat through movement of fluid), and radiation (can occur in vacuum as it uses electromagnetic waves). A vacuum flask has a vacuum (no medium), so conduction (needs medium) and convection (needs fluid medium) can't occur in the vacuum part. Radiation can still happen. So the modes that can't take place in vacuum are conduction and convection only.

Answer:

A. conduction and convection only.