L 18 Thermodynamics [3]

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1 L 18 Thermodynamics [3] Review Heat transfer processes convection conduction Greenhouse effect Climate change Ozone layer Review Internal energy sum of all the energy of all atoms in a system The temperature of a system is a measure of its internal energy Heat the energy that flows from one system to another because of temperature differences. There are 3 heat flow processes: convection conduction heat 1 st and 2 nd Laws of Thermodynamics System A at temp T A cold System B at temp T B >T A hot First Law of Thermodynamics: Energy is conserved. For two isolated systems A and B that exchange heat energy, the heat lost by one system = the heat gained by the other system. Second Law of Thermodynamics: In a spontaneous process involving two systems in contact, heat flows from the hot system to the cold system. In the top case heat flows from B to A But what about refrigerators? A refrigerator is a device that moves heat from cold to hot (inside to outside) Does this violate the second law? NO The process is NOT SPONTANEOUS. Work must be done (electrical) by a pump to move the heat from cold to hot This applies to air conditioners also heat Convection heat is carried from place to place by the bulk movement of either liquids or gases Convection does NOT occur in solids when water is boiled, hot liquid rises and mixes with cooler liquid, thus the heat is transferred Hot air rises: want heat into lower level of house (winter) cooled air into upper levels (summer) Conduction heat is transferred directly through a material, with no bulk movement of material only energy moves iron is a particularly poor conductor of heat

2 heat conduction HOT L Heat Flow Cross sectional area A COLD Heat Flow rate depends on A / L and a property unique to the material, called thermal conductivity The effectiveness of a material in conducting heat is characterized by a parameter called the thermal conductivity there are good thermal conductors (metals) and poor ones (insulators) Thermal Conductivity Material Copper Silver Stainless steel wood glass wool Goose down Styrofoam Thermal conductivity Thermal Conductivities of Metals Metal Silver Copper Aluminum Brass Iron Steel Thermal Conductivity (W/m K) Radiation is the heat transfer by electromagnetic waves thermal light waves - invisible to the eyes thermal is a small part of the electromagnetic spectrum waves are characterized by their frequency or wavelength different colors in the visible correspond to different wavelengths from red to blue electromagnetic spectrum visible electromagnetic waves: LIGHT TV microwaves, cell phones thermal shorter wavelength more energy radio waves visible x-rays thermal visible light UV produces sunburn

3 Thermal Radiation The warmth you feel from the sun is the sun s thermal It travels through the vacuum of space to reach earth, no material is necessary (takes 8 minutes) you can feel its effects even though you cannot see the. you can feel the thermal from a fireplace What produces thermal? all objects whose temperature is above absolute zero emit thermal The hotter the object, the more it emits, the amount of is ~ T 4 We all continuously emit thermal We also absorb it from objects and people around us If we just emitted we would eventually cool to absolute zero! Emission and Absorption are balanced Thermal spectrum The intensity of increases with temperature the color shifts toward the blue at higher temperatures The UV from the sun is just beyond the violet (11,000 F) sources of thermal the incandescent light bulb ( the ones that have a filament) are sources of both visible light and heat. when electricity flows through a wire it gets hot. it emits even though you can t see it as it gets hotter it glows red then orange then white tungsten filament, has a very high melting point, 3400 C evacuated glass bulb good emitters are good absorbers an object that is a good emitter is also a good absorber of thermal a poor emitter is also a poor absorber generally dark, dull objects are the best emitters/absorbers shinny objects are poor emitters/absorbers, they are good reflectors of If you do not want the edges of your pie to burn, you wrap it in aluminum foil. The aluminum foil reflects the heat rather than absorbing it.

4 good/bad emitters-leslie s cube copper cube filled with hot water this side is painted black infrared sensor Practical considerations wear light clothing in summer light clothing absorbs less sunlight cover all body parts in winter warm body parts (like your head) emit seeing behind closed doors we can see behind closed doors because of the heat signature left by warm objects on walls Infrared sensors can pick up temperature differences of 0.05 degrees C. Which one is best? A.silvered B. silvered and un-evacuated C. evacuated D.un-silvered and un-evacuated Why is it colder at the poles than at the equator? The Greenhouse effect SUN infrared is trapped Sun s visible light More of the Sun s energy per unit area falls on the equatorial regions compared to the polar regions the earth reflects about 30% of incident solar energy without the atmosphere the earth would be 30C cooler! Seasons due to change in tilt of the earth C O 2, H 2 O Without the greenhouse effect, the average temperature would be 20 F

5 Effect of greenhouse gases: H 2 O, CO 2, CH 4,... the sun s visible light can penetrate through the atmosphere to the earth s surface where it heats it the visible light energy is converted to thermal light energy the thermal is reflected from the greenhouse gases in the atmosphere CO 2 concentrations are about 0.04% Water vapor accounts for up to 66% Greenhouse effect and climate change concentrations of CO 2 have been increasing rise in earth s temperature similar effect occurs in your car during the day. Temperature change Climate change issues Are climate changes part of a natural cycle? What is the effect of human activity (anthropogenic) on the buildup of greenhouse gases? (NRC 2001) Because of the large and still uncertain level of natural variability inherent in the climate record and the uncertainties in the time histories of the various forcing agents (and particularly aerosols), a causal linkage between the buildup of greenhouse gases in the atmosphere and the observed climate changes during the 20th century cannot be unequivocally established. The IPCC (International Panel on Climate Change) 2/2/07: global warming is very likely caused by man recent revelations have cast some doubts on methods used to acquire and analyze data on climate change What are the economic repercussions of taking or not taking measures to curb greenhouse emissions The ozone layer: a related, but different issue ozone, O 3 is a naturally occurring trace element in the atmosphere It absorbs solar ultraviolet, especially the harmful UV-B rays it is destroyed by Cfc s (chlorofluorocarbons) loss affects us and environment