L 18 Thermodynamics [3] Heat as moving light Æ radiation
Heat transfer • Radiation is the heat transfer by • convection Å done this electromagnetic waves – thermal light waves - invisible to the eyes • conduction Å done this • thermal radiation is a small part of the • radiation electromagnetic spectrum – waves are • emitters of radiation characterized by their frequency or • seeing behind closed doors wavelength • Greenhouse effect • different colors in the visible correspond to Heat Capacity Heat Capacity different wavelengths from red to blue How to boil water
electromagnetic spectrum visible electromagnetic waves: LIGHT
thermal shorter wavelength Î more energy TV microwaves, cell phones radiation
visible light thermal radio radiation UV radiation x-rays waves visible produces sunburn
Thermal Radiation What produces thermal radiation? • The warmth you feel from the sun is the sun’s thermal • all objects whose temperature is above radiation absolute zero emit thermal radiation • It travels through the • The hotter the object, the more radiation it vacuum of space to reach emits, the amount of radiation is ~ T4 earth, no material is • We all continuously emit thermal radiation necessary (takes 8 minutes) • you can feel its effects even • We also absorb it from objects and people though you cannot see the around us radiation. • If we just emitted radiation we would • you can feel the thermal eventually cool to absolute zero! radiation from a fireplace
1 Emission and Absorption are Thermal radiation spectrum balanced • The intensity of radiation increases with temperature • the color shifts toward the blue at higher temperatures • The UV radiation from the sun is just beyond the violet
sources of thermal radiation Radiation emitted by hot objects • the incandescent light bulb ( the ones that have a tungsten filament, • The hotter they are, the more they emit filament) are sources of can get very hot and not melt • the efficiency with which an object emits both visible light and heat. thermal radiation is characterized be a • when electricity flows parameter called its emissive Æ e through a wire it gets hot. • it emits radiation even • e is a number between 0 and 1 though you can’t see it • a good emitter has an e close to 1 • as it gets hotter it glows • a poor emitter has an e close to 0 red then orange then white evacuated glass bulb
good emitters are good absorbers good/bad emitters-Leslie’s cube
• an object that is a good emitter is also a infrared radiation good absorber of thermal radiation copper cube filled with hot sensor • a poor emitter is also a poor absorber water • 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 this side is burn, you wrap it in aluminum foil painted black
2 Practical considerations seeing behind closed doors
• wear light clothing in we can “see” behind summer Æ light closed doors clothing absorbs less because of the heat sunlight signature left by • cover all body parts in warm objects on winter Æ warm body walls parts (like your head) emit radiation Infrared sensors can pick up temp- erature differences of 0.05 degrees C.
The Greenhouse effect Greenhouse effect Su n’s • the sun’s visible light can penetrate vis ibl e li through the atmosphere and reach infrared radiation ght is trapped the surface where it heats it • the visible light energy is converted to thermal light energy
O 2 C • the thermal radiation is reflected from CO2 in the atmosphere
• http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/
Greenhouse effect The ozone layer • is a naturally occurring • concentrations of CO2 trace element in the have been increasing atmosphere O3 • Æ rise in earth’s • It absorbs solar temperature ultraviolet radiation, especially the harmful • same effect occurs in UV-B rays your car during the • it is destroyed by Cfc’s day. chlorofluorocarbons • loss affects us and environment
3 How do I boil water? Heat Capacity or specific heat • The heat capacity is the • How much heat does it take to boil water? amount of heat that is • Simpler question Æ how much heat is required to raise the required to raise the temperature of water temperature of 1 g of a by so many degrees? substance by 1 degree C. • it is measured in Calories • The answer depends on how much water • for water it is 1 cal/g °C you have and how hot you want to get it hot plate • in general: • The answer would be different for a heat Q = m c (temp change) different material, say aluminum. specific heat
Some heat capacities
Substance Specific heat in cal/g °C
water 1
Ethyl alcohol 0.58
Steel 0.11
Aluminum 0.215
lead 0.03
4