Why Does Warm Air Rise and Cold Air Sink?

00:08:11
https://www.youtube.com/watch?v=U4GoJ2PXJj8

الملخص

TLDRIn this video, the speaker explores the concept of convection, particularly the phenomenon where hot air rises and cold air sinks. Employing a balloon experiment, the speaker demonstrates that cold air is denser and heavier than warm air, leading to buoyancy effects. The video further discusses Archimedes' principle, detailing how pressure differences in fluids create an upward buoyant force. The speaker emphasizes that warm air, being less dense, generates a greater buoyant force, which causes it to rise, consequently driving weather systems and convection currents. The video ends with the assertion that the principles discussed are vital for understanding weather patterns.

الوجبات الجاهزة

  • 🌡️ Hot air rises while cold air sinks, a fundamental aspect of convection.
  • 🎈 Demonstration with a balloon shows cold air is denser than warm air.
  • ⚖️ Buoyancy is the upward force that opposes gravity acting on an object in air or fluid.
  • 📜 Archimedes' principle states that an object submerged in a fluid is buoyed up by a force equal to the weight of the displaced fluid.
  • 🧪 The density of warm air is less than that of cold air, resulting in warm air rising.
  • 📏 Pressure differences in fluids (e.g., air) create buoyant forces, essential for convection.
  • 🌍 Understanding how air density affects movement is key to explaining weather patterns.

الجدول الزمني

  • 00:00:00 - 00:08:11

    The video explains the concept of convection in weather, challenging the common notion that hot air rises because it is lighter than cold air. An experiment using a balloon filled with warm air demonstrates that cold air actually has more weight due to higher density. The key factors are the concepts of mass, weight, gravity, and buoyancy. The buoyant force on an object is affected by the pressure differences in a fluid, which causes cold air to have a smaller buoyant force compared to warm air. Archimedes' principle is introduced as the basis for buoyancy, confirming that objects submerged in a fluid are buoyed up by a force equal to the weight of the fluid displaced. The video relates the buoyancy of warm air in the atmosphere to convection, asserting that warm air rises and cold air sinks, acting as a key driver in weather systems.

الخريطة الذهنية

فيديو أسئلة وأجوبة

  • What is convection?

    Convection is the process where warmer air rises and cooler air sinks, playing a significant role in weather systems.

  • Why does hot air rise?

    Hot air rises because it is less dense than cold air, meaning it takes up more space and has a lower weight for the same volume.

  • What is buoyancy?

    Buoyancy is the upward force exerted by a fluid that opposes the weight of an object immersed in it.

  • What is Archimedes' principle?

    Archimedes' principle states that an object submerged in a fluid experiences an upward force equal to the weight of the fluid it displaces.

  • How does pressure affect buoyancy?

    The pressure at the bottom of an object submerged in a fluid is higher than at the top, resulting in a net upward force.

  • How does air density affect weather?

    Differences in air density created by temperature differences are fundamental to convection, which drives weather patterns.

  • Can you explain the balloon experiment?

    The balloon experiment demonstrates that cold air is denser than warm air, and that a warmer balloon weighs less due to greater buoyant force.

  • Does the shape of an object affect buoyancy?

    Yes, the shape affects how pressure differences are distributed, but the overall buoyant force still adheres to the principles of Archimedes.

  • How can we calculate buoyant force?

    The buoyant force can be calculated using the formula: Buoyant force = density of fluid × gravity × volume of displaced fluid.

  • What happens to warm air over time?

    Warm air rises over time due to its lower density compared to surrounding colder air, leading to convection.

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التمرير التلقائي:
  • 00:00:00
    you've probably learned that hot air
  • 00:00:02
    rises cold air sinks this is the
  • 00:00:04
    mechanism behind convection and
  • 00:00:06
    convection plays a major role in weather
  • 00:00:08
    systems in this video I'll explain how
  • 00:00:11
    this comes about very often the rising
  • 00:00:16
    of hot air is explained by the fact that
  • 00:00:17
    hot air is lighter than cold air and
  • 00:00:19
    light things just happen to go up just
  • 00:00:21
    as a hot air balloon but that doesn't
  • 00:00:24
    really explain anything
  • 00:00:26
    so let's first test the idea that hot
  • 00:00:30
    air is lighter than cold air I've got
  • 00:00:32
    this balloon here and if i inflate it it
  • 00:00:41
    will be filled with my breath which is
  • 00:00:43
    about 37 degrees Celsius so this is warm
  • 00:00:47
    air okay let's put it on this scale okay
  • 00:00:59
    the mass is one point ninety seven grams
  • 00:01:02
    now let's put the balloon in the fridge
  • 00:01:06
    for about five minutes and then wait
  • 00:01:09
    again if I take it out of the fridge it
  • 00:01:11
    will be about five degrees Celsius
  • 00:01:23
    let's take the balloon out of the fridge
  • 00:01:25
    it's about five degrees Celsius let's
  • 00:01:27
    weigh the balloon again now the mass has
  • 00:01:33
    increased to two point 19 grams which is
  • 00:01:36
    an increase of 0.2 22 grams so indeed
  • 00:01:39
    cold air weighs more than warm air in
  • 00:01:44
    physics we actually distinguish between
  • 00:01:45
    mass and weight the scale doesn't
  • 00:01:48
    measure the mass although it gives you a
  • 00:01:50
    mass but it measures to weight it
  • 00:01:52
    measures the weight with which the
  • 00:01:54
    balloon or the object on the scale is
  • 00:01:56
    pulled down on the scale and in this
  • 00:01:59
    case it's actually measuring the net
  • 00:02:01
    force with which it's pushing down on
  • 00:02:03
    the scale and as we'll see the gravity
  • 00:02:09
    is not the only force acting on the
  • 00:02:10
    balloon it's also buoyancy due to the
  • 00:02:13
    air surrounding the balloon the weight
  • 00:02:16
    of the balloon has become less - to what
  • 00:02:18
    is called buoyancy when I put the
  • 00:02:20
    balloon on the weighing scale gravity
  • 00:02:22
    pulls it down
  • 00:02:22
    this causes the balloon to push down on
  • 00:02:24
    the scale this is what we call weight in
  • 00:02:27
    one of my other videos I demonstrated
  • 00:02:29
    that the pressure of the air decreases
  • 00:02:31
    with altitude as a result the air
  • 00:02:34
    presses harder on the bottom of the
  • 00:02:36
    balloon and does at the top so there is
  • 00:02:38
    a net upward force which we call
  • 00:02:39
    buoyancy now as it happens cold air has
  • 00:02:43
    a higher density than warm air so the
  • 00:02:45
    same amount of cold air takes up less
  • 00:02:47
    space than warm air therefore the cold
  • 00:02:50
    balloon is slightly smaller reaching
  • 00:02:52
    less high this causes the buoyant force
  • 00:02:54
    on the cold balloon to be smaller than
  • 00:02:56
    on the warm loom so it weighs more we
  • 00:02:59
    can actually calculate this difference
  • 00:03:00
    in buoyant force with some rough
  • 00:03:02
    estimates the change in pressure equals
  • 00:03:05
    the density of the air times the
  • 00:03:07
    acceleration due to gravity times the
  • 00:03:09
    change in height for small change in
  • 00:03:12
    height the density of the air can be
  • 00:03:13
    taken to be constant the balloon has a
  • 00:03:15
    diameter of about 20 centimeters if we
  • 00:03:18
    fill everything in the difference in
  • 00:03:20
    pressure turns out to be 2.54 Pascale's
  • 00:03:23
    this results in a force of F equals P
  • 00:03:26
    times a with a surface area of about 3.1
  • 00:03:29
    times 10 to the power of minus 2 meters
  • 00:03:31
    squared so for the warm balloon the
  • 00:03:33
    buoyant force is 7.87 times centered
  • 00:03:36
    - to Newtons the coal balloon is about
  • 00:03:39
    five millimeters smaller so the change
  • 00:03:41
    in height and the surface area are a bit
  • 00:03:43
    smaller - the buoyant force for the coal
  • 00:03:46
    balloon then is 8.22 times 10 to power
  • 00:03:49
    of minus two newtons we can calculate
  • 00:03:51
    the difference in buoyant force and
  • 00:03:52
    convert that to an equivalent mass so
  • 00:03:55
    the cold balloon should be about 0.4
  • 00:03:58
    grams lighter than the warmer loon this
  • 00:04:00
    is an upper limit because we did not
  • 00:04:01
    take into account the round shape of the
  • 00:04:04
    balloon some parts of the balloon
  • 00:04:05
    experience a smaller upward force so the
  • 00:04:08
    real difference in mods will be smaller
  • 00:04:10
    just as we've measured now this
  • 00:04:13
    principle of buoyancy is actually
  • 00:04:15
    nothing more than Archimedes principle
  • 00:04:17
    which states that any object totally or
  • 00:04:20
    partially immersed in a fluid is buoyed
  • 00:04:22
    up by force equally to the weight of the
  • 00:04:25
    displaced fluid by the object displaced
  • 00:04:28
    by the object and it can demonstrate
  • 00:04:30
    that with this weight of one kilogram
  • 00:04:33
    and I've got the scale here with which
  • 00:04:36
    measures up to 50 grams accurately so I
  • 00:04:39
    first need to turn it on so it sets
  • 00:04:41
    itself to zero and if I weight the Maus
  • 00:04:46
    then it's indeed 1 kilogram and then if
  • 00:04:49
    I submerge it on the water here then you
  • 00:04:54
    can actually see the mass drop it's now
  • 00:04:56
    point 85 kilograms so indeed there's an
  • 00:05:04
    extra force acting upward which is the
  • 00:05:06
    difference in pressure in the fluid so
  • 00:05:09
    the pressure at the bottom of the weight
  • 00:05:11
    is higher than the pressure at the top
  • 00:05:13
    of the weight and the difference in
  • 00:05:16
    pressure causes a net upward force now
  • 00:05:21
    because this this weight is kind of
  • 00:05:24
    cylinder like we can actually calculate
  • 00:05:26
    the difference in force and I'll show
  • 00:05:28
    you
  • 00:05:28
    we will imagine that a cylinder shaped
  • 00:05:31
    object is submerged under water only the
  • 00:05:34
    pressure of the water at the bottom of
  • 00:05:35
    the cylinder will cause an upward force
  • 00:05:37
    because the pressure at the sides of the
  • 00:05:39
    cylinder act horizontally
  • 00:05:42
    now this pressure equals Rho times G
  • 00:05:44
    times H the buoyant force then equals
  • 00:05:47
    pressure P times serves area a we will
  • 00:05:51
    fill in the formula for pressure the
  • 00:05:53
    buoyant force equals Rho times G times a
  • 00:05:55
    times H the height H times the surface
  • 00:05:59
    area a is just the volume of our
  • 00:06:01
    cylinder and equal to the amount of
  • 00:06:03
    displaced water remember that Rho is the
  • 00:06:06
    density of water so the buoyant force is
  • 00:06:08
    equal to the mass of the water times the
  • 00:06:11
    acceleration due to gravity which is
  • 00:06:14
    just the weight of the displaced water
  • 00:06:16
    this is Archimedes principle
  • 00:06:18
    now you can repeat the calculation for
  • 00:06:20
    the situation where the top of the
  • 00:06:22
    object is a bit below the water surface
  • 00:06:23
    you will get the same result and it also
  • 00:06:27
    works for more difficult shaped objects
  • 00:06:29
    but then the derivation is more
  • 00:06:31
    difficult from this calculation you can
  • 00:06:34
    also figure out when objects will float
  • 00:06:36
    if the density is less than that of
  • 00:06:38
    water because then the downward force
  • 00:06:41
    the weight of the object is lower than
  • 00:06:43
    the weight of the displaced water the
  • 00:06:45
    buoyant force and the same holds true
  • 00:06:48
    for air if the density of warm air is
  • 00:06:51
    lower than the density of the
  • 00:06:53
    surrounding air this warm air will start
  • 00:06:55
    to float will go up so we've seen that
  • 00:06:59
    Archimedes principle can be derived from
  • 00:07:01
    this difference in pressure in a fluid
  • 00:07:02
    or a liquid now you might wonder whether
  • 00:07:05
    it matters in what way you submerge this
  • 00:07:08
    object take this test tube for example
  • 00:07:11
    it's long and thin if you put it in
  • 00:07:13
    vertically and a difference in pressure
  • 00:07:15
    between the top and the bottom will be
  • 00:07:16
    quite large but the surface area of down
  • 00:07:19
    below where the pressure is acting on is
  • 00:07:21
    quite small if you put it in
  • 00:07:23
    horizontally then the difference in
  • 00:07:26
    pressure between the top and the bottom
  • 00:07:27
    is smaller but the surface area has
  • 00:07:29
    increased which compensates and the
  • 00:07:31
    upward buoyant force is just the same so
  • 00:07:35
    warm air indeed is lighter than cold air
  • 00:07:37
    for the same amount of air the downward
  • 00:07:40
    force the gravity on this air is the
  • 00:07:42
    same but the warm air takes up more
  • 00:07:44
    space and so the difference in pressure
  • 00:07:47
    between the top and the bottom of this
  • 00:07:49
    parcel of warm air will be larger and
  • 00:07:52
    the upward buoyant force will be larger
  • 00:07:54
    so
  • 00:07:55
    hot air or warm air rises and cold air
  • 00:07:58
    sinks this is the mechanism behind
  • 00:08:00
    convection and convection is like the
  • 00:08:02
    motor behind weather I'll explain that
  • 00:08:05
    in a different video I hope you like
  • 00:08:07
    this one thank you for watching
الوسوم
  • Convection
  • Buoyancy
  • Archimedes Principle
  • Hot Air
  • Cold Air
  • Density
  • Pressure
  • Weather Systems
  • Fluid Dynamics
  • Buoyant Force