Download presentation
Presentation is loading. Please wait.
Published byConrad Marshall Modified over 8 years ago
1
L 18 Thermodynamics [3] Heat transfer Heat Capacity convection
conduction radiation emitters of radiation seeing behind closed doors Greenhouse effect Heat Capacity How to boil water
2
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 radiation
3
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)
4
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
5
heat conduction HOT COLD Heat Flow Heat Flow rate depends on A / L
Cross sectional area A L HOT COLD Heat Flow Heat Flow rate depends on A / L
6
Thermal Conductivity Material Thermal conductivity Copper 400 Silver
420 Stainless steel 14 wood 0.15 glass 0.8 wool 0.04 Goose down 0.025 styrofoam 0.01 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)
7
Thermal Conductivities of Metals
Thermal Conductivity (W/mK) Silver 406 Copper 385 Aluminum 205 Brass 109 Iron 80 Steel 50
8
Grandma’s silver spoons
9
radiation Heat as moving light
Radiation is the heat transfer by electromagnetic waves – thermal light waves - invisible to the eyes thermal radiation 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
10
electromagnetic spectrum
microwaves, cell phones thermal radiation TV radio waves x-rays visible
11
visible electromagnetic waves: LIGHT
shorter wavelength more energy visible light thermal radiation UV radiation produces sunburn
12
Thermal Radiation The warmth you feel from the sun is the sun’s thermal radiation 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 radiation. you can feel the thermal radiation from a fireplace
13
What produces thermal radiation?
all objects whose temperature is above absolute zero emit thermal radiation The hotter the object, the more radiation it emits, the amount of radiation is ~ T4 We all continuously emit thermal radiation We also absorb it from objects and people around us If we just emitted radiation we would eventually cool to absolute zero!
14
Emission and Absorption are balanced
15
Thermal radiation spectrum
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 (11,000 F)
16
sources of thermal radiation
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 radiation even though you can’t see it as it gets hotter it glows red then orange then white tungsten filament, can get very hot and not melt evacuated glass bulb
17
Radiation emitted by hot objects
The hotter they are, the more they emit the efficiency with which an object emits thermal radiation 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
18
good emitters are good absorbers
an object that is a good emitter is also a good absorber of thermal radiation 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
19
good/bad emitters-Leslie’s cube
infrared radiation sensor copper cube filled with hot water this side is painted black
20
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 radiation
21
thermal radiation all objects that are at a temperature above absolute zero emit thermal radiation (waves) the higher the temp, the more they emit the color (wavelength) of the emitted waves goes from redorangeyellow blue as the temperature increases
22
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 temp- erature differences of 0.05 degrees C.
23
Which one is best? silvered evacuated un-silvered and un-evacuated
24
infrared radiation is trapped
The Greenhouse effect Sun’s visible light infrared radiation is trapped C O2 EARTH
25
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 radiation is reflected from CO2 in the atmosphere
26
Greenhouse effect concentrations of CO2 have been increasing
rise in earth’s temperature same effect occurs in your car during the day.
27
The ozone layer ozone, O3 is a naturally occurring trace element in the atmosphere It absorbs solar ultraviolet radiation, especially the harmful UV-B rays it is destroyed by Cfc’s (chlorofluorocarbons) loss affects us and environment
28
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.
29
Heat Capacity or specific heat
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 hot plate mass of sample specific heat
30
Specific heat in cal/g °C
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
Similar presentations
© 2024 SlidePlayer.com. Inc.
All rights reserved.