Conceptual Physics Practice Problems: Heat Transfer, Part II
1. Differentiate between conduction, convection and radiation. Which type of heat transfer
is most prevalent in solids? Liquids and gases?
Conduction is heat transfer via vibration/movement on the
atomic and sub-atomic scale. Convection involves heat
transfer through the fluid movement of a mass of matter,
often due to differences in density. Radiation is the
transmission of heat energy via electromagnetic radiation
(light, be it visible or invisible). Solids tend to transfer
heat via conduction most readily due to the closeness of
the particles. Convection is the major mode of heat
transfer for liquids and gases. All matter radiates energy.
2. If you wish to save money (and fuel) and you’re going to leave your warm house for a
hour or so on a very cold day, should you turn your thermostat down a few degrees, turn
it off altogether, or let it remain at the room temperature you desire? Explain.
Turn the thermostat off. Your house will cool most rapidly when the difference in
temperature between your home and the outside is the greatest. (This question is based
upon Newton’s Law of Cooling)
3. Why is the insulation in an attic commonly thicker than the insulation in the walls of a
house?
Insulation is thicker because most heat loss in a home is via convection (the major mode
of heat transfer for gases), where warm air rises and is carried upwards. Keeping the
insulation the thickest in the attic slows the transfer of heat in this manner.
4. A 159.89 g sample of Thermite, a mixture of rust and aluminum, will produce 851.5 kJ of
heat. Use this information to answer the questions that follow.
(a) Determine the amount of heat energy produced through the reaction of 100 g of
thermite.
Approximately 532 kJ of energy is produced.
(b) After observing the thermite demonstration, describe how the thermite reaction
illustrates all three modes of heat transfer.
Glowing metal illustrates emission of radiation (light). The clay pot becomes
extemely warm due to conduction of heat from the thermite to the pot. The air
around the apparatus warms as heat is transferred via convection between the
molten metal and the air immediately above it.