L 18 Thermodynamics [3] Heat transfer convection conduction emitters of seeing behind closed doors Greenhouse effect Heat Capacity How to boil water Heat flow HEAT the energy that flows from one system to another because of temperature differences. But how does it flow? Three ways: convection conduction Convection heat is carried from place to place by the bulk movement of either liquids or gases does not apply to 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 stuff only energy moves iron is a particularly poor conductor of heat heat conduction HOT L Heat Flow Cross sectional area A Heat Flow rate depends on A / L COLD 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 Steel (stain.) wood glass wool Goose down styrofoam Thermal conductivity 400 420 14 0.15 0.8 0.04 0.025 0.01 1
Thermal Conductivities of Metals Grandma s silver spoons Metal Silver Copper Aluminum Brass Iron Steel Thermal Conductivity (W/m K) 406 385 205 109 80 50 electromagnetic spectrum 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 TV radio waves microwaves, cell phones visible thermal x-rays visible electromagnetic waves: LIGHT shorter wavelength more energy thermal visible light UV produces sunburn 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 2
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 evacuated glass bulb tungsten filament, can get very hot and not melt Radiation emitted by hot objects The hotter they are, the more they emit the efficiency with which an object emits thermal is characterized be a parameter called its emissive e e is a number between 0 and 1 a good emitter has an e close to 1 a poor emitter has an e close to 0 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 If you do not want the edges of your pie to burn, you wrap it in aluminum foil 3
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 thermal all objects that are at a temperature above absolute zero emit thermal (waves) the higher the temp, the more they emit the color (wavelength) of the emitted waves goes from red orange yellow blue as the temperature increases 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 The Greenhouse effect infrared is trapped C O 2 Sun s visible light http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/ 4
Greenhouse effect 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 CO 2 in the atmosphere Greenhouse effect concentrations of CO 2 have been increasing rise in earth s temperature same effect occurs in your car during the day. The ozone layer 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 How do I boil water? How much heat does it take to boil water? Simpler question how much heat is required to raise the temperature of water by so many degrees? The answer depends on how much water you have and how hot you want to get it The answer would be different for a different material, say aluminum. Heat Capacity or specific heat hot plate The heat capacity is the amount of heat that is required to raise the temperature of 1 g of a substance by 1 degree C. it is measured in Calories for water it is 1 cal/g C heat Q = m c temp change Some heat capacities Substance Specific heat in cal/g C water 1 Ethyl alcohol 0.58 Steel 0.11 Aluminum 0.215 mass of sample specific heat lead 0.03 5