Julius Sumner Miller: Lesson 24 - Heat Energy Transfer by Radiation

00:14:12
https://www.youtube.com/watch?v=6WnYfUVE3kc

Resumo

TLDRDans cette intervention, Julia Suna Miller explique le transfert d'énergie thermique par rayonnement. Après avoir décrit les mécanismes de conduction et de convection, elle se concentre sur le rayonnement, un mode de transfert énergétique qui n'a pas besoin de matière pour se propager, comme illustré par la chaleur de la lampe ou celle du Soleil. Plusieurs démonstrations sont réalisées : un radiomètre est utilisé pour montrer l'effet du rayonnement, et des expériences avec des canettes de métal et des thermomètres montrent comment la couleur et le matériau influencent le transfert thermique. En outre, la conception d'un thermos est expliquée pour minimiser la perte par rayonnement. Le problème amusant de la crème dans le café est posé pour explorer davantage les principes du rayonnement thermique.

Conclusões

  • 📡 Le rayonnement thermique peut se propager dans le vide, comme entre le Soleil et la Terre.
  • 💡 Une ampoule peut chauffer instantanément la peau par rayonnement sans chauffer le verre.
  • 🕹️ Un radiomètre montre l'effet immédiat du rayonnement sur sa rotation.
  • 🏺 L'amiante, en raison de sa réflexion, peut refroidir une canette d'eau plus vite qu'un noir ou brillant.
  • ⚖️ Les thermomètres noirs et blancs démontrent les différentes vitesses de chauffage par rayonnement.
  • 🚿 Une fine couche d'amiante isole mieux que plusieurs couches, à l'instar d'une canette brillante.
  • 🍵 La réflexion sur l'ajout de crème dans le café chaud est influencée par le rayonnement.
  • 🔍 Les problèmes paradoxaux stimulent l'apprentissage plutôt que les réponses simples.
  • 📐 Un thermos minimise les pertes thermiques grâce à ses parois doublées d'argent.
  • 🔎 Explorer les spectres électromagnétiques permet de comprendre le rayonnement visible et invisible.

Linha do tempo

  • 00:00:00 - 00:05:00

    Dans cette première partie, la conférencière Julia Suna Miller discute des différents mécanismes de transfert de la chaleur, notamment par conduction, convection et rayonnement. Elle explique que le rayonnement, contrairement aux deux autres méthodes, fonctionne même en l'absence de matière, comme c'est le cas pour l'énergie thermique du Soleil traversant l'espace vide. Elle démontre cela à l'aide d'une lampe incandescente dont la chaleur est ressentie à distance par rayonnement, sans que le verre ne se réchauffe immédiatement. Elle utilise également un radiomètre pour illustrer comment différentes surfaces réagissent au rayonnement dont l'accélération des mouvements des ailes noires et brillantes du radiomètre attribuée à la différence d'absorption.

  • 00:05:00 - 00:14:12

    Dans cette seconde partie, elle présente une démonstration intrigante sur le refroidissement de trois boîtes similaires mais avec des surfaces différentes: une brillante, une peinte en noir et une recouverte d'une mince couche d'amiante. Elle révèle que contrairement à l'intuition qui suggère que les corps noirs perdent plus rapidement de la chaleur, c'est en fait la boîte recouverte d'amiante qui refroidit le plus vite, une propriété liée au rayonnement. Elle examine aussi l'impact de la couleur sur les thermomètres exposés au soleil et l'effet de l'épaisseur d'un matériau isolant sur l'absorption thermique, suggérant que les vêtements amples peuvent aider à réguler la température corporelle. Finalement, elle conclut avec le principe du thermos et la problématique de garder un café chaud plus longtemps en ajoutant de la crème.

Mapa mental

Mind Map

Perguntas frequentes

  • Quels sont les trois mécanismes de transfert d'énergie thermique discutés ?

    Les trois mécanismes sont la conduction, la convection et le rayonnement.

  • Comment le rayonnement thermique est-il illustré grâce à une lampe ?

    En allumant une lampe incandescente et en ressentant la chaleur directement sur la peau sans chauffer le verre.

  • Quels sont les matériaux comparés pour leur capacité à refroidir de l'eau chaude dans des canettes ?

    Une canette brillante, une peinte en noir et une recouverte d'amiante.

  • Quelle conclusion tire-t-on de l'expérience des canettes ?

    La canette recouverte d'amiante refroidit la plus rapidement grâce à la réflexion du rayonnement thermique.

  • Que démontre l'expérience des thermomètres avec des ampoules noires et blanches ?

    Le thermomètre noir chauffe plus rapidement sous le soleil mais atteint la même température à l'équilibre thermique.

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    [Music]
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    w
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    [Music]
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    how do you do ladies and gentlemen and
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    boys and girls and mothers and fathers
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    and people I'm Julia Suna Miller and
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    physics is my business and we come once
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    more to the subject of heat energy
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    transfer in an earlier program I talked
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    about heat energy transfer by conduction
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    and then I talked in another program
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    about heat energy conduction by the
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    mechanism of
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    convection and let me remind you that
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    for conduction we have say a metal rod
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    nothing goes no stuff moves very far
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    energy is what goes but in the case of
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    thermal energy transfer by convection we
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    had the actual motion of stuff like hot
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    air and hot liquids and stuff so in
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    these two we had
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    transfer where there was something now
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    we come to heat energy transfer by the
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    mechanism of
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    radiation and this is a puzzling sort of
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    thing because it works best as I like to
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    say where there ain't nothing for
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    example the heat energy of the of a star
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    called the Sun comes mostly through
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    empty space so radiation consider the
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    following quite a marvelous thing to
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    contemplate here I have an incandescent
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    lamp and I'm going to excite it
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    momentarily that is connect it and
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    energize it so it burns it lights now
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    watch what I'm going to do I'm going to
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    connect
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    it put my arm here and I felt it hot
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    right away but that is still cold that's
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    a marvelous thing so the radiation from
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    that lamp reached my arm over this
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    distance of a foot which isn't very far
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    because this radiation travels at the
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    velocity of light which is about 186,000
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    miles per second so it didn't take very
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    long to get to my arm still the glass
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    did not have enough time to be thermally
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    excited and to warm up as we say so my
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    arm was warmed by
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    radiation another
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    demonstration several programs of mine
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    and this one also have opened with this
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    little
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    radiometer and I am led to say to you
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    that although this is a commonplace
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    little device would you believe it it
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    has not yet been thoroughly understood
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    so I invite you to go to the physics
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    literature and read about it there have
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    been at least a thousand notes in the
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    journals written about this but I will
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    say a word about it I am covering it
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    from the radiation from the lamps in the
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    studio and when I remove my hands we
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    will find that its speed has slowed down
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    very very much then when I expose it to
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    the radiation its speed will pick up now
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    the veins in this little rascal are very
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    light and blackened on one side and
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    shiny on the other and if you look
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    circumspectly at its method of rotation
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    you will find that the black faces
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    always Retreat from you the Observer now
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    I'm going to take my hands away look how
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    slow it is and look look look look how
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    much faster faster faster it's going
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    very much faster and the black faces are
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    retreating now I have another one here
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    and this one isn't going
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    well I excited a little mechanically
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    that one isn't going at all and I leave
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    that as a puzzle for you why isn't this
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    one
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    turning why isn't it turning you see I
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    consider raising paradoxes and dilemas
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    and uncommon questions much more
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    virtuous than just talking answers next
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    demonstration which is a most
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    unreasonable thing very unreasonable
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    indeed reason is ravaged by this
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    demonstration we will look first at
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    three cans that I have I have one can
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    two cans three cans and to begin with
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    they are identical here they are here
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    they are three cans from fruit juice let
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    us say now one of them I left
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    shiny the other one I painted on the
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    outside black black and the third one I
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    covered with a very thin layer of
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    asbestos
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    asbestos
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    right shiny black
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    asbestos now what am I going to do a
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    wonderful
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    demonstration
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    I'm going to put some hot water in these
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    to the same level in all three cans
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    water water water and a cover if you
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    wish with a hole in the cover and put a
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    thermometer in each one a thermometer we
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    will imagine that I have it right here
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    here is a little stick which we
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    understand is a
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    thermometer now this is hot
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    water 140° f say which is about the
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    temperature which you take a bath now
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    question I let these stay here on the
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    tabletop and as time goes they cool off
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    in what order do they cool or more
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    exactly which one cools the fastest now
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    nearly everybody in the world says that
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    the black one cools the fastest because
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    black bodies are good radiators now part
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    of that is true but that answer is not
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    correct I'm going to tell you a strange
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    thing because I am a kind-hearted fellow
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    I'm going to tell you but having told
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    you the answer there is a greater
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    mandate upon you to find out why it is
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    so here it is the one that is covered
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    with the asbest cools the fastest now
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    who to Thun it that verb is the past PL
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    perfect subjunctive of the verb to think
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    who would believe that the one that's
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    covered with asbest would cool the
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    fastest and yet it does and I will give
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    you a hint the program this day today
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    this program Bears on the subject of
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    radiation and that therein lies the
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    secret
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    so chiny can black can asbestos covered
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    can thin layer very thin layer another
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    commentary here I have a fourth such tin
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    can and I have put seven layers of that
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    asbestos seven and this can is brought
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    down to the property of the shiny one
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    which is an amazing thing
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    absolutely No One Believes it next
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    demonstration the case of the four
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    thermometers notice the title I give it
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    this is not a case for Sherlock Holmes
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    but for juliia
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    S problem I have here two identical
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    thermometers to
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    absolutely identical this one however is
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    painted white at the bulb and this one
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    is Painted
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    Black now we will imagine we will
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    imagine we will imagine that I go out
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    into the sunshine into an open field
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    with these two
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    thermometers to begin with they read the
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    same temperature we let a little time
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    elapse certainly they show a rise an
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    elevation a growing temperature question
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    what exactly do we see now when I ask
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    this very few answer it correctly but
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    I'm going to tell you again because I'm
  • 00:08:39
    kindhearted this one the black one shows
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    an elevation much quicker faster sooner
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    than the white one but after a while
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    they come to thermal equilibrium with
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    themselves and the surroundings and read
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    the same
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    temperature so having told you about a
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    white thermomet and a black thermometer
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    I'm going to raise another question here
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    I have two identical
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    thermometers and they are wrapped at the
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    bulbs with cotton batting here you see
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    the cotton batting is very tightly fixed
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    here it is very puffy and loose and what
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    I want to say about these is this that
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    the same weight of cotton batting is
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    here as is here so the same amount of
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    stuff insulates both of them now I go
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    out into the sunshine with these two
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    thermometers and I ask you what do they
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    show
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    immediately and this is a wonderful
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    thing and I'm just wondering if I'm kind
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    enough in my heart to tell you because
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    I've told you so much
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    already uh I'll leave this for you to
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    explore but let me say that this is why
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    for example it is uh on occasion useful
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    to wear clothes that are not too tight
  • 00:09:59
    fitting and I have given you a hint
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    about these two
  • 00:10:04
    thermometers more on
  • 00:10:07
    radiation the so-called Thermos bottle a
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    Dua flask Dua D eew d e w a r d James D
  • 00:10:19
    an
  • 00:10:20
    Englishman how is this made well I'm
  • 00:10:23
    going to show you it is a glass
  • 00:10:26
    bottle with two walls an outer wall and
  • 00:10:30
    an inner wall closely sealed tightly and
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    the air between is taken out and if we
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    could see very sharply there I'll play
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    that to the camera there is a little tip
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    of glass where the vacuum pump has been
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    connected it's very difficult to see let
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    me see if I can get another one oh yeah
  • 00:10:49
    may be able to get this there it is
  • 00:10:51
    there's the tip of glass sealed off with
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    the air between mostly taken out now a
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    question I like to ask ask you see these
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    little pads of felt there's one there
  • 00:11:03
    and one and one and I ask where are they
  • 00:11:05
    they are not on the outside they are not
  • 00:11:08
    on the inside no they are between the
  • 00:11:11
    two walls of glass so as one might say
  • 00:11:13
    they are on the inside of the outside or
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    the outside of the
  • 00:11:18
    inside now to more properly finish such
  • 00:11:21
    a container we silver it also on the
  • 00:11:25
    inside as well as on the outside why to
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    minimize the radiation
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    losses indeed as you know we can put
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    some stuff in which is cold and it stays
  • 00:11:36
    cold or we can put some stuff in which
  • 00:11:38
    is hot which stays hot so we have talked
  • 00:11:41
    about
  • 00:11:42
    radiation and now I have a classical
  • 00:11:45
    problem which you can investigate in a
  • 00:11:48
    private way with much
  • 00:11:51
    fun problem problem I call it the
  • 00:11:55
    problem of the black coffee here here it
  • 00:11:59
    is breakfast
  • 00:12:00
    time breakfast time I pour me a cup of
  • 00:12:04
    black coffee here I am pouring a cup and
  • 00:12:07
    the here is the cup of black coffee now
  • 00:12:10
    I am a user of cream in my coffee so I
  • 00:12:14
    am on the verge of adding cream from the
  • 00:12:17
    pitcher and let us say it's cold cream
  • 00:12:19
    from the refrigerator I'm about to put
  • 00:12:22
    the cream in the coffee when the phone
  • 00:12:24
    rings
  • 00:12:26
    ding now I have to go answer the
  • 00:12:29
    telephone and I expect of course to be
  • 00:12:31
    delayed a little time
  • 00:12:34
    question I want to find my coffee as hot
  • 00:12:37
    as possible when I come
  • 00:12:40
    back should I then add the cream before
  • 00:12:44
    I go to answer the phone or should I
  • 00:12:47
    wait until I come back see they
  • 00:12:51
    beautiful thing and I wonder if I should
  • 00:12:53
    give you a hint the hint is this it
  • 00:12:57
    depends also on on this business of
  • 00:13:01
    radiation and I want the coffee to
  • 00:13:04
    radiate away its heat energy at a lesser
  • 00:13:07
    rate and therefore I should add the
  • 00:13:11
    cream uh at some time uh in this
  • 00:13:14
    discussion so you see ladies and
  • 00:13:17
    gentlemen this business of thermal
  • 00:13:20
    energy incredible and when one talks
  • 00:13:23
    about radiation he has to have in mind
  • 00:13:27
    the large scale view of our entire
  • 00:13:30
    electromagnetic spectrum which includes
  • 00:13:33
    the visible only a small part of the
  • 00:13:35
    whole region the infrared radio waves
  • 00:13:38
    electric waves ultraviolet x-rays gamma
  • 00:13:42
    rays and as I am led to say who knows
  • 00:13:45
    what is beyond and I thank you for
  • 00:13:57
    listening
  • 00:14:11
    I
Etiquetas
  • énergie thermique
  • rayonnement
  • convection
  • conduction
  • transfert thermique
  • radiomètre
  • thermos
  • démonstration
  • physique
  • expériences