You extend an impromptu invitation to a friend for dinner. The only food you have is a couple of frozen steaks. You wish to defrost the steaks before grilling them. You defrost one by using your microwave oven, and you defrost the other by placing it in a bowl of very warm water. In each case, decide whether heat is transferred by conduction, convection, radiation, or some combination of these mechanisms. (Select all that apply.)

Microwave:

a. conduction.

b. convection.

c. radiation

Water:

a. conduction.

b. convection.

c. radiation

Microwave: Radiation

Water: Conduction, convection & radiation.

Explanation:

When we heat a food using microwave then the water content of the food is only heated by the microwave. Microwaves make the molecules of water vibrate frequently with a frequency closer to the frequency of microwaves and this increases the kinetic energy of the molecules which produces heat in the water molecules this heat then propagates to the whole food by conduction and convection, but the heat enters the food only via radiation.

Now when the food item is kept into warm water then the molecules closer to the food heat the food by conduction through direct energy transfer by lattice vibrations and when they become cooler than the mass of water all around then due to density difference they settle down and their place is occupied by warmer molecules around in the fluid leading to convection.

Radiation of energy from a mass occurs continuously irrespective of the medium present there. So the heat of water also enters the food by radiation of energy from the water molecules.