Julius Sumner Miller: Lesson 23 - Heat Energy Transfer by Convection

00:14:27
https://www.youtube.com/watch?v=Kex97vjXvJY

概要

TLDRDans cette vidéo, Juliia Suna Miller nous fait découvrir la science du transfert de chaleur par convection. Après avoir exploré la conduction, elle se concentre ici sur la convection, où le transfert de chaleur se fait par le mouvement des fluides. Elle nous montre des expériences fascinantes, comme l'utilisation de la fumée pour illustrer le flux d'air, ou le mouvement de colorant dans l'eau chauffée pour montrer comment la chaleur fait circuler les liquides et les gaz. Elle explique comment l'air sec est plus dense que l'air humide, ce qui influence le comportement de la fumée. Des exemples comme la hauteur de la fumée sortant d'une cheminée selon le degré d'humidité de l'air soulignent ce phénomène. D'autres démonstrations incluent le rôle de la conduction dans les métaux où des boules fixées à des rayons métalliques tombent à des moments différents selon le type de métal et la conductivité thermique, ou comment la chaleur d'une lampe affecte le cheminement de la fumée. L'importance de la convection à grande échelle est aussi abordée, en se référant aux événements météorologiques tels que les tornades. Tout au long, elle enrichit sa présentation de vérités surprenantes, comme le fait que l'air humide est moins dense que l'air sec.

収穫

  • 🔥 La convection est le transfert de chaleur par le mouvement des fluides.
  • 💨 L'air humide est plus léger que l'air sec, influençant le mouvement de la fumée.
  • 🌡️ La conduction est le transfert de chaleur à travers les solides.
  • 💧 La densité affecte la convection dans les liquides et les gaz.
  • 🌪️ Les phénomènes météorologiques comme les ouragans découlent de la convection.
  • 🕯️ Une flamme s'éteint sans ventilation adéquate à cause de la convection.
  • 🔬 Les expériences démontrent la conduction entre différents métaux.
  • 📏 La taille et le matériau des objets modifient la conduction thermique.
  • 📏 L'humidité de l'air influence la direction du mouvement de la fumée.
  • 🔍 Des démonstrations simples peuvent illustrer ces principes physiques.

タイムライン

  • 00:00:00 - 00:05:00

    Juliia Suna Miller, experte en physique, aborde le sujet du transfert d'énergie thermique par convection. Elle rappelle d'abord la conduction, puis utilise une expérience avec une bougie et de la fumée pour illustrer la convection. Elle explique que la fumée se comporte différemment par temps sec et humide en raison de la densité de l'air, ce qui influence la flottabilité. Elle poursuit avec une démonstration à l'aide d'un cadre en verre rempli d'eau et de colorant, montrant comment la chaleur provoque un mouvement ascendant du colorant plus dense, illustrant ainsi la convection dans les fluides.

  • 00:05:00 - 00:14:27

    Elle décrit comment la convection joue un rôle crucial dans les phénomènes naturels de grande échelle comme les tempêtes et les tornades. En introduisant une expérience avec une bougie et un tube en verre, elle démontre comment l'air chaud est poussé vers le haut par l'air plus froid, contestant l'idée que l'air chaud 'monte' de lui-même. Miller élabore sur la conduction avec des tiges métalliques, montrant comment la forme et le matériau influencent la conduction thermique. Elle conclut avec une expérience sur la dilatation thermique, soulignant l'importance de la conduction thermique dans la dissipation de la chaleur, par exemple en utilisant une pièce en argent pour prendre la chaleur d'une cigarette sans endommager un mouchoir.

マインドマップ

Mind Map

よくある質問

  • Quel est le sujet principal de la vidéo ?

    Le transfert de chaleur par convection.

  • Comment la convection diffère-t-elle de la conduction ?

    La convection implique le transfert de chaleur par le mouvement des fluides, tandis que la conduction se produit dans les solides sans mouvement de matière.

  • Pourquoi l'air humide est-il plus léger que l'air sec ?

    L'air humide contient de la vapeur d'eau, qui est moins dense que l'air sec, ce qui le rend plus léger.

  • Qu'arrive-t-il à une flamme de bougie dans un tube fermé ?

    La flamme s'éteint faute d'air frais, à moins qu'une partition ne permette la circulation de l'air.

  • Qu'est-ce qui cause les courants de convection dans l'air ?

    Les variations de température qui modifient la densité de l'air, établissant ainsi un mouvement de l'air.

  • Quel est un exemple naturel de convection ?

    Les tornades et les ouragans sont des exemples de convection à grande échelle dans la nature.

  • Comment la densité de l'air affecte-t-elle le mouvement de la fumée ?

    La fumée monte lorsque l'air est sec et descend lorsque l'air est humide, car l'air sec est plus dense.

  • Quelle expérience utilise un cadre en verre avec de l'eau pour démontrer la convection ?

    L'ajout de colorant alimentaire et l'application de chaleur pour observer le mouvement du colorant à travers l'eau.

  • Comment l'expérience avec les rayons en métal démontre-t-elle la conduction ?

    En montrant comment des sphères métalliques fixées à des rayons tombent à différentes vitesses en fonction de la conductivité du matériau du rayon.

  • Qu'est-ce que démontre l'expérience du mouchoir et de la pièce de monnaie en argent ?

    Elle démontre la conductivité thermique du métal en empêchant le mouchoir de brûler.

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  • 00:00:02
    [Music]
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    how do you do ladies and gentlemen I am
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    juliia Suna Miller and physics is my
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    business and my special business today
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    is this subject the transfer of heat
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    energy by
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    convection convection on an earlier
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    program I talked about conduction and
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    here I have taken a piece of clothesline
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    and set it a fire so to say in a burner
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    and then shut out the flame so it is
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    giving me a lot of smoke abundant smoke
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    and I've got a real Smoke Stack there as
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    you see now let's look at this mechanism
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    here in this chamber is a candle a
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    lighted candle and smoke is coming out
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    of here at a vast rate
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    fantastic I think you can see it now I'm
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    going to put some covers on a cover on
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    that chimney and the smoke is trying to
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    come out here but not so well and if I
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    let this stay for a while I'm sure the
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    candle would go out most assuredly it
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    would go out if I I if I covered both
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    now watch I'm going to take both
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    off do you see not quite as well as I'd
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    like it but I'm going to do it again
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    watch it now watch
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    it watch it now I'm going to take this
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    one off this cover
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    watch I assert that the smoke is falling
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    down very little if it is coming up very
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    little of it this is a little different
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    oh there it is let me do it again I'm
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    going to fill this stack now watch it
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    watch it now I'm going to take that
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    cover
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    off watch it yeah yeah there it is there
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    it is so you saw smoke go down fall down
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    this a wonderful thing I'm going to pose
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    a little problem for you here is a house
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    with a smoke stack I say that on some
  • 00:02:17
    days I see the smoke come out and go up
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    up up up up then there are other days
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    the smoke comes out just out of the
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    chimney and falls down down down to the
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    Earth question on what kind of day does
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    it do the one and on what kind of day
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    does it do the other well I'm going to
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    help you a little bit because it's a
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    strange business it goes up up up on a
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    dry day on a dry day and it falls down
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    on a damp
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    day what does this tell us it tells us
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    that damp air is not as heavy as dry air
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    and therefore does not provide the
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    buoyant effect necessary to lift the air
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    up uh the smoke up and this is a
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    fantastic thing to say that that wet air
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    is lighter than dry air it has water
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    vapor in it wouldn't you think that it
  • 00:03:08
    would be heavier but it is not now while
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    that's smoking here is another I have a
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    framework a closed rectangle of glass
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    with some water in it I am going to put
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    some food
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    coloring in this corner of the
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    framework while watch it now some food
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    coloring there it is and you will see it
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    drift down diffuse down gravitate down
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    in a wonderful way look at it look at it
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    look at it coming down it's coming down
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    it's coming down now I'm going to apply
  • 00:03:49
    some heat to this tube watch it now you
  • 00:03:52
    see the coloring which is heavier than
  • 00:03:55
    water drifting down and diffusing now
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    I'm going to apply some heat energy here
  • 00:04:01
    and I say color go back color go back
  • 00:04:05
    that's what I command it to do oh it's
  • 00:04:08
    coming down it's coming down but I say
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    it should go back watch it now watch it
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    aha there it's going up it's going up
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    and it's going to come down here so I'm
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    going to play the burner the flame on
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    this side and I say coloring do not come
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    down do not I say do not well it's
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    coming down somebody says oh you cannot
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    fight nature Professor no I am not
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    fighting nature I am collaborating with
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    her and now it is migrating back there
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    it is and I could drive it the other way
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    so we are talking about
  • 00:04:46
    convection
  • 00:04:48
    convection the
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    transfer of heat energy by actual
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    transfer of stuff remember in an earlier
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    program I talked about
  • 00:05:03
    conduction which takes place in solid
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    things like metal rods and wooden rods
  • 00:05:07
    and the like now I am talking about
  • 00:05:10
    convection which takes place in fluids
  • 00:05:14
    because something must flow and fluids
  • 00:05:17
    is the generic name for gases and
  • 00:05:19
    liquids so we have this strange business
  • 00:05:23
    of convection another illustration of it
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    an imaginary experiment imagine that I
  • 00:05:29
    sitting here smoking a cigarette which I
  • 00:05:32
    don't do because I don't smoke so this
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    is the real part of the experiment and
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    here I have a reading lamp throwing some
  • 00:05:40
    light on a physics book and the lamp is
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    lighted question where does the smoke
  • 00:05:46
    from my cigarette go up here no
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    strangely enough it does not go up here
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    it goes up under the lamp and out from
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    under the shade why because the heat of
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    the lamp gives rise to a reduction in
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    the density of the air and a convection
  • 00:06:03
    current is set up now regarding this
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    convection I would remind you a massive
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    piece of business because when we have
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    convection in large scale on large scale
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    in nature what do we have Gales and
  • 00:06:17
    hurricanes and tornadoes fantastic
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    energy in the moving air and yet
  • 00:06:23
    consider the moving air I tried to grab
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    it as I am led to say there ain't
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    nothing there
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    but oh it is amazing there are three *
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    10 listen to this 3 * 10 to the 19
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    molecules of gas in a cubic centimeter
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    of air
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    incredible an illustration of how big
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    this number is supposing you and I and a
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    million other people went to the
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    Oceanside and picked up that many grains
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    of sand and we pile them all up in a
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    pile how big a pile do you think we'd
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    have we'd have a pile that is one
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    mile wide one mile one mile long and one
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    mile deep a cubic mile of sand and as I
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    am given to say further so you get an
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    idea how big this number is you know I'm
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    talking about there ain't nothing in air
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    three times 10 the 19 grains of sand
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    supposing you could count 10 per second
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    1 3 5 10 1 10 how long do you think it
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    would take to count that many grains of
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    sand answer 100,000 million million
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    years and I say ain't that something and
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    there is nothing here to grab incredible
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    more on convection a beautiful
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    demonstration I'm going to light a
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    candle here I'm going to light a
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    candle and here I'm going to and it rest
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    in a in a in a little saucer I'm going
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    to put this glass tube on top and now
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    nowh can get in at the bottom and the
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    candle flame will soon expire it will
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    asfixiate for want of air to breathe
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    watch it it's getting light it's getting
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    dimmer dimmer dimmer now I'm going to
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    put in a partition watch
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    it well I'm having a
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    little there it is there it is it has
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    been
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    revived it has been revived let me take
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    it out and I assure you the candle flame
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    will go out without this partition
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    watch it watch it it's on the verge of
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    going it's on the verge of going out on
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    the verge of going out yeah yeah and
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    I'll catch it now and give it a new
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    lease on life there it is and what must
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    we say well the physics is clear I put a
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    petition in this chamber in this chimney
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    so that cold air could fall down and the
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    hot air come up now don't ever say that
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    hot air rises because that ain't so
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    there ain't no Hindu levitation in this
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    business what happens is that the less
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    dense air is pushed up by the colder air
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    convection conviction oh a beautiful
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    thing to witness have you not seen
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    seagulls especially seagulls soaring at
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    certain places in the sky I once did
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    this in class and I'm led to tell you I
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    said to the class how many see that I'm
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    a bird on the wing a bird on the wing
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    and one fellow not so poetic minded said
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    Professor you look quite like a vulture
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    I thought that was true the fellow had
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    an idea didn't he right all
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    right so much for convection actual
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    transfer of heated stuff as compared
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    with conduction which is transfer of
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    heat energy alone now on an earlier
  • 00:09:47
    program I spoke about conduction and got
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    a little uh uh cut off because of time I
  • 00:09:53
    want to go back to one of those
  • 00:09:55
    experiments here is a metal hub from
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    which m Min Ates a number of spokes of
  • 00:10:01
    different materials brass copper iron
  • 00:10:04
    aluminum zinc nickel and so on and here
  • 00:10:07
    is a wonderful experiment to do here's
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    what I do I take a little steel
  • 00:10:13
    ball a little steel ball and with a
  • 00:10:16
    little bit of soft wax fix a ball to the
  • 00:10:19
    end of each Rod a ball to the end of
  • 00:10:22
    each Rod now what do I do I apply some
  • 00:10:26
    heat energy from a burner to the hub
  • 00:10:30
    the different rods conduct the heat
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    energy at different rates and obviously
  • 00:10:34
    the little balls fall off in an inverse
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    order to the
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    conductivity but an interesting question
  • 00:10:41
    arises I have here two identical kinds
  • 00:10:44
    of rods an iron Rod an iron Rod they are
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    absolutely alike in their composition
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    but they differ as follows and this is a
  • 00:10:53
    wonderful exercise they are identically
  • 00:10:56
    long identically long but one is
  • 00:11:00
    skinnier than the other as a matter of
  • 00:11:03
    fact one has that diameter and the other
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    that diameter this is a diameter D and
  • 00:11:08
    that's a diameter 2D and the problem is
  • 00:11:12
    as
  • 00:11:13
    follows supposing I put a little ball on
  • 00:11:17
    the end of each and what do I have I
  • 00:11:20
    heat The Hub I heat The Hub and in what
  • 00:11:25
    order do they fall off well I'm going to
  • 00:11:27
    tell you let's look at them because it's
  • 00:11:29
    very difficult problem this has twice
  • 00:11:32
    the diameter therefore it has four times
  • 00:11:34
    the cross-sectional area four times
  • 00:11:37
    therefore it conducts heat away four
  • 00:11:39
    times as fast but because it is twice
  • 00:11:42
    the diameter it has twice the
  • 00:11:45
    circumference and therefore it radiates
  • 00:11:47
    from the surface at twice the rate so
  • 00:11:50
    this one does not gain over this one
  • 00:11:52
    four times but rather only two times so
  • 00:11:55
    what do I do if I put a little ball on
  • 00:11:57
    the end of this one
  • 00:11:59
    no correction in the middle of this one
  • 00:12:02
    and on the end of that one they will
  • 00:12:04
    fall off at the same time and I hope
  • 00:12:06
    I've said that correctly but I got a
  • 00:12:08
    little filed up indeed if I said it
  • 00:12:11
    wrong then you will write me a letter
  • 00:12:12
    saying Professor what you said was not
  • 00:12:15
    right what you said was not
  • 00:12:17
    right notice I've got a little
  • 00:12:20
    perspiration in my eye that is as uh
  • 00:12:23
    sort of saline and it is not pleasant
  • 00:12:27
    more on this business of conduction a
  • 00:12:30
    wonderful demonstration which you can do
  • 00:12:33
    a silver dollar nice and clean you put
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    it tightly under a handkerchief so
  • 00:12:41
    tightly under a handkerchief there it is
  • 00:12:44
    tightly now I wish I had a cigarette we
  • 00:12:47
    will imagine that I have a lighted
  • 00:12:50
    cigarette this will pce of close
  • 00:12:52
    on lighted cigarette I put the lighted
  • 00:12:55
    end of the cigarette tightly down on the
  • 00:12:59
    the the handkerchief on the coin
  • 00:13:02
    question what happens nothing why
  • 00:13:06
    because the clean silver underneath
  • 00:13:09
    takes the heat energy away as it did in
  • 00:13:11
    a certain experiment I did with a wood
  • 00:13:13
    and metal rod and the handkerchief is
  • 00:13:15
    untouched this is a beautiful little
  • 00:13:18
    experiment to demonstrate to your kin
  • 00:13:22
    Enchanting
  • 00:13:26
    Enchanting next demonstration
  • 00:13:29
    this one you can do remember I said what
  • 00:13:32
    happens first when you put a thermometer
  • 00:13:33
    into hot water it shows a diminution in
  • 00:13:36
    the column take a flask like this fill
  • 00:13:38
    it with water put some food coloring in
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    it fit it with a one hole stopper in a
  • 00:13:42
    glass tube and the level is right here
  • 00:13:45
    more easily seen than the action of a
  • 00:13:47
    thermometer submerge this in a vessel of
  • 00:13:49
    hot water and what do you see down it
  • 00:13:52
    goes for the reason that the glass
  • 00:13:54
    vessel is heated first and expands this
  • 00:13:57
    concludes my recitation and I thank
  • 00:13:59
    thank you for listening
  • 00:14:07
    [Music]
タグ
  • convection
  • conduction
  • chaleur
  • densité
  • expériences
  • transfert d'énergie
  • fumée
  • humidité
  • ventilation
  • physique