The Physical World I

Spring 2005
MWF 11:30

Instructor: Dr. Bruce Law, CW 327, Tel: 532-1618.

REVIEW QUESTIONS OF Home Work 11

R2. The ``loose`` electrons in metals are free to conduct the heat throughout the metal via collisions.

R8. Heat is transferred from one place to another by convection via the flow of a gas or liquid due to a temperature difference.

R16. Radiant energy is a form of light which transfers heat.

EXCERISES OF Home Work 11

E1. No, the coat is not a source of heat, but merely keeps the thermal energy of the wearer from leaving rapidly.

E2. Your body temperature is at 98F hence you are warmer than both the air and water which have a temperature of 70F. Heat is therefore transferred from your body to both the air and the water. The air possesses a low heat capacity c and a low mass m therefore not much heat is required to be transferred from your body to the surrounding air in order to warm up this air -- the air therefore feels warm. By contrast the water possesses both a high heat capacity c and a high mass m therefore a lot of heat must be transferred from your body in order to warm up the water -- the water therefore feels cold. Recall that the heat energy
Q = mcDT where DT = difference in temperatures.

E3. When the temperature of the blocks are the same as the temperature of your hand, then no heat transfer occurs. Heat will flow between your hand and something being touched only if there is a temperature difference between them.

E4. Heat is flowing out of your hand down the metal nail to the ice. Your body replaces the heat lost from your hand more slowly than the rate at which heat flows down the nail because the nail is a good conductor while your body is a relatively poor conductor of heat. Therefore your hand feels cold.

E12. Convection causes the heat from the flame to rise therefore the temperature immediately above the flame is very high.

E13. Air is a poor conductor, whatever the temperature. Hence holding your hand in hot air for a short time is not harmful because very little heat is conducted by the air to your hand. However, if you touch the hot conducting surface of the oven, heat readily conducts to you and you will burn your hand.

E17. Hydrogen molecules will be the faster moving when mixed with oxygen molecules. They will have the same temperature, which means they will have the same average kinetic energy. Recall that KE = 0.5mv². Since the mass of hydrogen is considerably less than oxygen, the speed must correspondingly be greater.