Question

1. 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.

1. a swimmer coming out of a pool of water soon feels cold on a sunny day. the swimmer experienced coldness because

a. water evaporates from the skin.
b. convection occurs in the air.
c. air is a bad conductor of heat.
d. water is a good conductor of heat.

Explanation:

Question 3

To solve this, we recall heat transfer modes: conduction needs a medium (molecules collide), convection needs a fluid medium (movement of fluid), radiation doesn't need a medium (electromagnetic waves). In a vacuum, no medium, so conduction (needs medium) and convection (needs fluid medium) can't occur. Radiation can occur. So the modes that can't take place are conduction and convection.

When a swimmer comes out, water on skin evaporates. Evaporation is a cooling process (liquid to gas, requires heat, taken from skin). Convection (air movement) isn't the main reason here, air's conductivity or water's conductivity aren't relevant. So the cause is water evaporating from skin.

Answer:

A. conduction and convection only

Question 4