Emission of Radiant Energy

Transcription

1 Lecture 18 Chapter 16 Heat Transfer by Radiation Greenhouse Effect & Global Warming Chapter 17 Phase Change Emission of Radiant Energy All objects radiate; higher the temperature, the higher the frequency. At room temperature, the radiated light is at frequencies too low for our eyes to see. Special cameras are sensitive to this infrared radiation. 98º 75º Attics in this house were kept warm for growing marijuana.

2 Reflection of Radiant Energy White and silver objects reflect light, black objects and holes don t. White tubes look black inside. Hole in a box with white interior looks black because almost none of the light entering the hole reflects back out. Black objects are also the best emitters of radiation. White objects emit less radiation, and perfectly reflective objects don t emit at all. (Space blanket.) Controlling Heat Transfer Thermos bottle eliminates conduction and convection by having doublewalled sides with vacuum. Silvered interior walls minimize heat transfer by radiation.

3 Radiation If you are in sunlight, Sun s radiation will warm you. The intensity of solar radiation is 1000 W/m 2. In general, you will not be perfectly perpendicular to the Sun s rays, and will absorb energy at a rate that depends on your angle to the sun s rays. Solar Power Example You receive average solar power of 300 W/m 2. If you convert the solar power to electrical power with 8% efficiency, how much collecting area do you need to produce 2 kw electrical power? Electrical power from sun = (0.08)*300W/m 2 = 24 W/m 2 Area needed = 2000 W/(24 W/m 2 ) = 83 m 2

4 Seasons This angle effect is also responsible for the seasons. Greenhouse Effect Glass is transparent to sunlight (short-wavelength). Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat. Physics 1 (Garcia) SJSU

5 Earth s Greenhouse Effect Earth s atmosphere acts as a greenhouse, trapping solar energy. Most of the trapping is due to carbon dioxide and water vapor, which is why they re called greenhouse gasses.

6 Global Temperature Variations Temperatures increased from 1910 to Temperatures then cooled for 40 years until they started rising again in the 1980 s. Physics 1 (Garcia) SJSU Greenhouse Carbon Dioxide Over past 1000 years temperatures nearly constant until CO 2 emissions increased starting with the industrial revolution. Physics 1 (Garcia) SJSU Industrial revolution begins

7 Cars & Carbon Dioxide One gallon of gasoline has about 5.2 lb of carbon. At 26 miles per gallon, that s 0.2 lb of carbon per mile. Anthropogenic* Global Warming Rising temperatures are due to human production of greenhouse gases. Temperature *Caused by humans Physics 1 (Garcia) SJSU

8 Consequences of Global Warming Melting of Polar Ice Caps Weather modifications Species extinctions Solutions to Global Warming Many simple, small changes together could make a significant difference. Efficient Appliances Building Insulation Efficient Lighting Physics 1 (Garcia) SJSU

9 Phases of Matter Four Phases of Matter: Solid Liquid Gas Plasma Water Ice Steam Change of phase occurs when we pass from one phase to another, such as water (liquid) boiling to change into vapor (gas). Plasma Evaporation Evaporation is a change of phase from liquid to gas that takes place at the surface of a liquid. A random molecule at the surface acquires enough energy to escape the attraction force among the molecules (which holds the liquid together). GAS LIQUID

10 Evaporative Cooling Because only the most energetic molecules can escape the surface, evaporation removes internal energy from the liquid, that is, evaporation cools. WET CLOTH Brr HEAT WET BODY & TOWEL WET TONGUE Wet towel cools head HEAT HEAT Wetness cools person Wet tongue cools dog Condensation Condensation is the reverse of evaporation, a change of phase from gas to liquid that takes place at the surface of a liquid. A random molecule from the gas strikes the surface and sticks instead of bouncing back into the gas. Condensation heats. LIQUID GAS

11 Hot and Humid A 90 degree day in a dry climate is more comfortable than a 90 degree day in a humid place. In a dry climate you re cooled by evaporation; in a wet climate, you re heated by condensation. Heat index is the apparent temperature a person feels for a given humidity. Fog & Clouds Warm air rises. As it rises, it expands. As it expands, it cools. As it cools, vapor molecules condense into water droplets. This forms a cloud (or fog if warm, moist air cools near the ground). Cool As vapor expands, it cools and tiny, visible, water droplets (liquid) condense. Warm Warm breath feels cool when it expands Water vapor (gas) is invisible

12 Boiling When the temperature of a liquid is high enough that evaporation occurs everywhere, not just the surface, then the liquid boils. The temperature required depends on the pressure; lower the pressure, the lower the boiling temperature (boiling point). Tiny bubbles grow due to evaporation at their surface Demo: Low Pressure Boiling Water boils at room temperature if the pressure is low. Cooking at high altitudes is difficult due to this effect; coffee brewed in the mountains always tastes lukewarm.

13 Melting Melting is the change of phase from solid to liquid. Melting is a cooling process; the solid must absorb heat to melt. Sublimation Sublimation is change of phase from solid to gas without passing through liquid phase. Solid carbon dioxide (dry ice) sublimates at a chilly -109 F. Put dry ice into warm water to create dense fog of tiny water droplets.

14 Freezing Freezing is the opposite of melting, that is, the change of phase from liquid to solid. Heat must be removed from a liquid in order to freeze it into a solid. Lava (liquid) freezes into rock (solid), heating the seawater. Seawater (liquid) boils into vapor (gas), cooling the lava. Energy & Changes of Phase

15 Heats of Fusion & Vaporization 80 cal 100 cal 540 cal 720 cal Heat of Heat Capacity Heat of Total Energy Fusion Vaporization Heating a gram of water Check Yourself Is boiling a cooling or a warming process? Boiling is a cooling process. So can you cool your hand by putting it in boiling water? NO! Ouch! So why is boiling a cooling process? Because when a liquid boils it cools by itself releasing its most energetic molecules, just as with cooling by evaporation.

16 Chapter 18 The Laws of Thermodynamics The First Law of Thermodynamics The heat energy added to a closed system equals the increase in internal energy of the system plus the work done by the system on its surroundings. Q in = (int. energy) + W by This is the law of conservation of energy, written in a form useful to systems involving heat transfer.

17 Heat versus Work Both heat and work represent the flow of energy from one system to another. However, heat, as disordered energy flow, has some special properties: Heat always flows spontaneously from a warmer object to a cooler one, although the opposite would not violate conservation of energy. It is easy to convert work to thermal energy via friction (rubbing hands together), but conversion of thermal energy to work requires an engine and can t be done completely. Heat carries entropy (disorder) with it, but work does not. A lead block weighing 1 kg is dropped from a height of 1m. It does not bounce. What is the change in thermal energy of the block and floor? a. 0 b. 4.9 J c. 9.8 J d. cannot be determined

18 For isolated system (block+earth), energy is conserved: W on = 0 E tot = constant As time proceeds, the energy changes form: h Start with v = 0 PE = mgh v mgh = ½ mv 2 v = (2gh) 1/2 v = 0 mgh = U thermal = 9.8J kinetic energy converted to thermal energy in block and floor Potential energy Kinetic energy Thermal energy Irreversible process Key Points of Lecture 18 Heat Transfer by Radiation Greenhouse Effect & Global Warming Phase Change Latent Heat First Law of Thermodynamics Before Monday, read Hewitt Chap. 17. Homework Assignment #13 is due before 11:00 PM on Tuesday, Oct. 12.