Heat Transfer

Topics Conduction Convection Radiation Solar power Emission Absorption Reflection Solar power

Concept of Heat Put spoon into coffee spoon warms up and coffee cools down as they reach thermal equilibrium We say energy has been transferred from one substance to another. Energy transfers that occur only because of a temperature difference T is called heat flow or heat transfer and such energy transferred in this way is called HEAT.

Concept of Heat Heat refers to a transfer of energy from one body to another Heat DOES NOT refer to the amount of energy “contained” in a body or system. Heat is transferred from higher temp to a lower temp object

Conversion of Units Experimentation has shown: 1 cal = 4.186 Joules 1 kcal = 1000 cal = 4186 J 1 BTU = 252 cal = 1055 J The calorie is not a “fundamental” SI unit in the metric system. The fundamental unit for energy is the Joule, not the calorie.

Heat capacity for water c for water is 1 cal/(g oC) This means it takes a transfer of heat energy of 1 cal to raise 1 gram of water 1 oC. 1 cal/(g oC) = 4190 J/(kg oC) = 4190 J/(kg K)

Conduction Why do pots and pans have wooden handles? Why can we hold a burning wooden stick but not an iron poker in a fire in our bare hands. Water is a poor conductor. Can boil water in test tube with ice in it. Air is a poor conductor, snow is a good insulator. Fishnet underwear, double pane windows.

Convection Why will smoke rise and settle off and helium rise to top of atmosphere? Why can you hold your hand a few inches above a pressure cooker when it is blowing steam? What happens to the temperature of air in a pump when you pump up your tires? Shift in winds, shore to ocean, ocean to shore

Convection

Convection

Radiation Everything Emits Radiation! Everything has a Temperature f ~T Everything absorbs Radiation The earth emits radiation at lower frequency than visible light. The atmosphere is warmed primarily by the earth – that is why it is warm at night. Black objects emit and absorb radiation better than white objects. Glass traps infrared radiation

Radiation

Radiation

Radiation

Newton’s law of cooling The rate of loss of heat from an object is proportional to the temperature difference between the object and it’s surroundings. The rate of cooling or warming is proportional to the difference in temperatures and emissivities of surfaces.

Greenhouse Effect

Thermos Bottle

Class Problem Suppose in a restaurant your coffee is served about 5 or 10 minutes before you are ready for it. In order that it be as hot as possible when you drink it, should you pour the room temperature cream in: right away or when you are ready to drink the coffee?

Class Problem Pour the cream in right away. In so doing, you lighten the color of the coffee. When the coffee is black, it is a better radiator and will cool faster than when it is a lighter color. Perhaps you can think of some other reasons for pouring the cream right away.

Class Problem Which body glows with radiant energy that has its source in nuclear processes in the interior of the body? a) Sun b) Earth c) Both d) Neither

Class Problem Answer: c. Both glow with the same type of radiation, electromagnetic waves, with energy that originates in the interior from nuclear processes. The sun is heated by fusion of hydrogen nuclei to make helium. The earth is heated by radioactive decay of heavy elements deep underground. Both bodies are powered by nuclear processes, and both radiate the produced energy-- but in vastly different quantities and at different frequencies. (earth’s glow is in the infrared)

Class Problem Why is it significantly colder on a winter night under a clear sky than under a cloudy sky?

Class Problem Why is it significantly colder on a winter night under a clear sky than under a cloudy sky? We all know that energy from the glowing sun affects temperature here on earth. Like the sun, the earth glows- but only in the infrared. This is terrestrial radiation- lower in both frequency and intensity than solar radiation. On a clear night, terrestrial radiation escapes through the atmosphere, which lowers the temperature of the earth's surface and the air near it. But on a cloudy night, much terrestrial radiation is absorbed by the clouds and reradiated back to the earth, countering a nightly lowering of temperature.

Class problems 1. Your feet feel warmer on a rug than on your tile floor because your rug a) for the same mass has more internal energy than your tile. b) is a better insulator than your tile. c) is usually warmer than your tile. d) all of these e) none of these 2. At the same temperature, which move with the greater speed in the air? a) very light molecules b) heavier molecules c) All will have equal average speeds. 3. The higher the temperature of an object, a) the shorter the wavelengths it radiates. b) the longer the wavelengths it radiates. c) makes no difference in the wavelengths it radiates

Class problems 4. Objects that radiate relatively well, a) reflect radiation relatively well. b) absorb radiation relatively well. c) both of these D) neither of these 5. If a solid object radiates more energy than it absorbs, its a) temperature decreases. b) internal energy decreases. c) internal energy and temperature decrease. d) None of the above choices are true. 6. An object will normally be a net radiator of energy when its temperature is a) lower than its surroundings. b) higher than its surroundings. c) neither of these

Class problems 7. Wrapping a hot potato in aluminum foil significantly reduces the rate at which it loses energy by a) convection. b) conduction. c) radiation. 8. If you double the pressure of an ideal gas while keeping the temperature constant, the average kinetic energy of the molecules a) is doubled. b) Increases by less than twice. c) remains unchanged. d) Increases by more than twice. 9. One of the main reasons one can walk barefoot on red-hot coals of wood without burning the feet has to do with a) low thermal conductivity of the coals. b) mind over matter techniques. c) low temperature of the coals.

Class problems 10. When a volume of air is compressed, its temperature a) decreases. b) increases. c) neither increases nor decreases. 11. Hydrogen and oxygen molecules in a gas sample have the same temperature. This means the hydrogen molecules, on the average, have the same a) speed and the same kinetic energy. b) kinetic energy, but more speed. c) speed, but less kinetic energy. d) speed, but more kinetic energy. e) kinetic energy, but less speed.