Latest / The Curious Kidcast | Fun Facts and Curious Questions / Why are bubbles always round? | Science for kids explained

Transcript

Why are bubbles always round? | Science for kids explained · The Curious Kidcast | Fun Facts and Curious Questions

• 0:00
  Welcome to the Curious Kid cast. I'm your host, Charlie, and
• 0:02
  today I need you to stop whatever you're doing because
• 0:04
  we're about to answer a question so simple, so obvious, so
• 0:07
  staring you in the face that the fact nobody ever told you the
• 0:10
  answer is, frankly, a scandal. Why are bubbles round?
• 0:13
  That is it. That is the question.
• 0:15
  And by the end of this episode, you are going to know the answer
• 0:18
  and you are going to feel incredibly smug about it at the
• 0:20
  dinner table tonight. Curious kid cast?
• 0:23
  Here we go. Questions popping minds will
• 0:28
  blow big ideas. They're taking flight.
• 0:33
  Let's explore. Feel the line.
• 0:44
  Now today's question comes from a listener called Priya from
• 0:48
  Birmingham, England. Priya was in the middle of a
• 0:51
  very important science experiment, which is what she
• 0:53
  told her mum she was doing but was actually just blowing
• 0:55
  bubbles in the living room using a straw and a cup of washing up
• 0:58
  liquid. Classic.
• 0:59
  Anyway mid bubble she stopped looked her mum dead in the eyes
• 1:02
  and said why are they always round though?
• 1:05
  Her mum said I don't know go and ask your dad.
• 1:08
  Her dad said I don't know go and look it up.
• 1:10
  So she looked it up, found the curious kid cast and sent in her
• 1:13
  question. Priya this episode is for you.
• 1:16
  You are a scientist, the living room is your laboratory, right?
• 1:19
  So First things first, what actually is a bubble?
• 1:23
  And no, a floaty soap thing is not the answer I am looking for,
• 1:26
  even though it is technically correct.
• 1:28
  A bubble is a super thin layer of soapy water wrapped around a
• 1:32
  pocket of trapped air. Think of it like a very tiny,
• 1:34
  very shiny, very fragile sleeping bag, and the air inside
• 1:38
  is having the time of its life in there.
• 1:40
  The film has two surfaces, one facing outward and one facing
• 1:44
  inward, with a thin layer of liquid in between.
• 1:47
  That liquid, despite looking like it is just chilling, is
• 1:50
  absolutely full of drama. Here's why.
• 1:54
  The tiny molecules inside that liquid, which are so small you
• 1:57
  could never ever see them, are all attracted to each other.
• 2:00
  They love each other, they want to be together.
• 2:03
  They would write each other little notes if they could.
• 2:05
  Because they are all pulling on each other, the surface of the
• 2:07
  liquid tries to shrink itself down and take up as little space
• 2:11
  as possible. Scientists call this surface
• 2:13
  tension, which is basically just a fancy term for water being
• 2:16
  extremely clingy. Now, surface tension is not just
• 2:20
  a bubble thing. It is also why a water droplet
• 2:23
  on a leaf forms a little Dome instead of spreading flat.
• 2:26
  It is why certain insects called water striders can walk across
• 2:29
  the surface of a pond without sinking.
• 2:32
  Imagine that, Just casually strolling across a lake, not
• 2:35
  swimming, not a boat, just walking.
• 2:38
  Water striders are out here living a life that none of us
• 2:40
  will ever have, and they do not even appreciate it.
• 2:44
  But back to bubbles. In a soap bubble, surface
• 2:47
  tension pulls equally across the entire film, in every direction,
• 2:51
  all at once. It does not pull harder on one
• 2:54
  side. It does not have a favorite
• 2:55
  spot. It is completely fair, like the
• 2:57
  world's tiniest, most dedicated referee.
• 3:00
  And when a force pulls equally inward in all directions, trying
• 3:03
  to make the surface as small as it possibly can, the shape you
• 3:06
  end up with is a sphere every time.
• 3:08
  No exceptions. The sphere is nature's answer to
• 3:11
  the question, What is the most efficient shape to wrap around
• 3:15
  this air? And here is the genuinely mind
• 3:18
  bending bit. For any given amount of air, a
• 3:21
  sphere uses less surface than any other shape, less than a
• 3:25
  cube, less than a pyramid, less than a shape that looks like a
• 3:28
  wobbly cheese which I may have invented just now.
• 3:31
  A sphere is the undisputed champion of efficiency.
• 3:34
  It does the maximum job with the minimum material.
• 3:37
  If shapes were people, the sphere would be that one kid at
• 3:40
  school who finishes the test 1st and then just sits there looking
• 3:44
  calm while everyone else is still frantic.
• 3:46
  Basically writing. Think of it this way.
• 3:48
  Imagine you have a fixed amount of soapy film, and your job is
• 3:51
  to trap as much air as possible inside it.
• 3:53
  If you made a sphere, you would trap more air than if you made a
• 3:56
  cube or any other shape with the same amount of film.
• 3:59
  The sphere winds every single time.
• 4:01
  So when a bubble forms and the surface tension starts pulling
• 4:04
  inward, the bubble naturally settles into the shape that uses
• 4:07
  the least surface area, and that shape is always a sphere.
• 4:10
  Always the bubble does not think about it, it does not do any
• 4:14
  calculations. It just becomes round because
• 4:16
  the physics does not give it any other sensible option.
• 4:19
  There is also something called energy involved here.
• 4:22
  A stretched surface stores energy a bit like a rubber band
• 4:25
  pulled tight. The more surface area you have,
• 4:28
  the more energy is stored, and nature, much like a tired kid on
• 4:32
  a Sunday afternoon, always wants to use as little energy as
• 4:35
  possible. A sphere uses the least energy
• 4:37
  of any shape, which means a bubble becomes a sphere.
• 4:40
  Because it is, scientifically speaking, the laziest possible
• 4:43
  option. And in this case, being lazy is
• 4:45
  actually the correct answer. Tell your teacher Charlie said
• 4:49
  that too. Now the air inside the bubble is
• 4:52
  also doing something important. It is pushing outward in all
• 4:55
  directions. The soapy film is pulling inward
• 4:58
  in all directions. These two forces have to balance
• 5:01
  each other out, and when they do perfectly and evenly all the way
• 5:05
  around, you get a smooth curved surface with no corners, no flat
• 5:08
  bits, and no sharp edges. If the bubble tried to be a
• 5:11
  cube, the corners would be under way more stress than the flat
• 5:14
  sides. The film would not be happy
• 5:16
  about it and surface tension would immediately start
• 5:19
  smoothing. Those 1 is out to the whole
• 5:20
  thing became round again. The sphere is the only shape
• 5:23
  where every single part of the surface is under exactly the
• 5:26
  same amount of stress. Perfectly even, perfectly
• 5:29
  balanced. Basically the most fair shape in
• 5:32
  the universe. You might be wondering what
• 5:34
  about shaped bubble ones? You know the ones Star shapes,
• 5:38
  heart shapes, dinosaur shapes, That one weird one shaped like a
• 5:42
  crown that came in a party bag. No matter what shape the wand
• 5:45
  is, the bubble always comes out round once it floats away.
• 5:48
  Why? Because the wand only controls
• 5:50
  the bubble while the film is still attached to it.
• 5:52
  The second the bubble breaks free and floats off on its own,
• 5:55
  it is no longer constrained by the wand.
• 5:57
  Surface tension immediately takes over, starts smoothing
• 6:00
  everything out, and within moments the bubble is a perfect
• 6:02
  sphere. The wand is basically useless
• 6:05
  once the bubble escapes, which if the bubble could talk I
• 6:08
  imagine it would say something like.
• 6:10
  Cheers for the introduction mate, but I will take it from
• 6:12
  here now. I said bubbles are always round,
• 6:15
  but I should be honest with you because that is not the whole
• 6:18
  truth. A single free floating bubble is
• 6:21
  almost always a sphere. But when bubbles are squished
• 6:24
  together, like in foam or in a sink full of dishes that someone
• 6:27
  definitely did not leave there for three days, they press
• 6:30
  against each other and form flat faces where they meet.
• 6:32
  When a bubble has neighbors pushing in from all sides, it
• 6:35
  cannot just go off and be a perfect sphere.
• 6:38
  So instead it compromises forms flat shared walls and the whole
• 6:41
  thing looks like a strange bubbly honeycomb.
• 6:44
  It's like trying to sit comfortably in the middle seat
• 6:46
  on a packed bus. You want to stretch out, You
• 6:49
  can't. You do your best.
• 6:52
  And why do we use soap at all? Why not just plain water?
• 6:56
  Well, plain water does have surface tension, but it is
• 6:59
  actually a bit too stiff and snappy on its own.
• 7:01
  The film breaks almost immediately before you can even
• 7:04
  get a bubble going. Try blowing bubbles with plain
• 7:07
  water and you will just end up frustrated and slightly damp.
• 7:10
  Soap gets in between the water molecules and makes the film
• 7:13
  more flexible and stretchy, a bit like adding elastic to it.
• 7:16
  It gives the bubble time to actually form and float around
• 7:19
  before popping. Without soap, bubbles would
• 7:22
  basically be a disappointment, and there is no room for that on
• 7:25
  this podcast. Here is a bonus fact that will
• 7:28
  make you look at rain completely differently.
• 7:30
  Raindrops and water droplets also tend to become round, and
• 7:34
  for almost exactly the same reason.
• 7:36
  Surface tension pulls the outside of the droplet inward
• 7:39
  and the whole thing settles into a sphere because that is the
• 7:42
  most efficient shape. So the next time you are staring
• 7:45
  out of a classroom window at the rain, watching the droplets
• 7:48
  trickle down the glass, you are watching the same physics that
• 7:50
  makes bubbles round. You are basically doing science
• 7:53
  homework. You are not daydreaming, you are
• 7:56
  studying. Bubbles also connect to
• 7:58
  something mathematicians care about called minimal surfaces,
• 8:02
  which are shapes that use the absolute least amount of area to
• 8:05
  do a particular job. Soap bubbles find these shapes
• 8:08
  automatically. No equations, no calculator, no
• 8:11
  30 minute lesson about algebra. The bubble just does the maths
• 8:15
  instantly and gets the right answer every time.
• 8:17
  Scientists have actually used soap films to help solve
• 8:20
  complicated engineering problems because the film naturally shows
• 8:23
  the most efficient solution. So when Priya was in that living
• 8:26
  room blowing bubbles through a straw, she was running genuine
• 8:28
  physics experiments. Her mum should be proud, right?
• 8:32
  Then quiz time. 3 questions based on everything we have just
• 8:35
  learned. You can shout your answers out
• 8:37
  loud if you want, I can't hear you, but it's more fun that way
• 8:41
  and the people around you will find it very entertaining.
• 8:45
  Question one. What is the name of the force
• 8:47
  that pulls a bubble surface inward and tries to make it as
• 8:50
  small as possible? Is it a gravity, B surface
• 8:55
  tension, or C the power of friendship?
• 9:04
  The answer is B surface tension. Surface tension is caused by
• 9:08
  water molecules attracting each other, pulling the film inward
• 9:11
  and causing the bubble to form the smallest possible shape.
• 9:15
  If you said CI, respect the optimism but no question 2.
• 9:20
  Why does a bubble come out round even if you blow it through a
• 9:23
  star shaped wand? Is it A because the bubble is
• 9:26
  embarrassed by star shapes, B because once the bubble floats,
• 9:30
  free surface tension pulls the film into the shape with the
• 9:33
  smallest area, which is a sphere, or C because the stars
• 9:37
  on the wand are actually just decoration?
• 9:43
  The answer is B. The one shape only affects the
• 9:47
  bubble while it is attached. Once it floats, free surface
• 9:51
  tension takes over immediately and turns it into a sphere.
• 9:55
  Well done if you got that one. Question 3.
• 9:58
  For any given volume of air, which shape uses the smallest
• 10:01
  amount of surface to wrap around it?
• 10:03
  Is it a a cube, BA pyramid, or CA sphere?
• 10:13
  It is CA sphere. The sphere is the most efficient
• 10:18
  shape in nature for enclosing a volume with the least possible
• 10:21
  surface area. That is why bubbles around, why
• 10:24
  raindrops around, and honestly why a lot of things in nature
• 10:27
  end up round. Nature is very committed to the
• 10:30
  sphere, and I think we should all respect that.
• 10:34
  So there we have it. Your bubble question turned out
• 10:36
  to be one of those brilliant ones that sounds simple on the
• 10:38
  surface, pun absolutely intended, but actually opens up
• 10:42
  a whole world of physics, geometry, and the way nature
• 10:45
  prefers to do things efficiently.
• 10:47
  Bubbles around because surface tension pulls inward equally in
• 10:50
  all directions. Air pressure pushes outward
• 10:53
  equally in all directions, and the shape where those forces are
• 10:56
  perfectly balanced using the least surface area and the least
• 10:59
  energy is always a sphere. Nature is, at its heart, a
• 11:04
  little bit lazy, and that laziness creates something
• 11:08
  genuinely beautiful. The next time you blow a bubble
• 11:11
  and watch it drift away, remember you are watching a
• 11:14
  tiny, shimmering physics experiment that has solved A
• 11:17
  mathematical problem without doing a single second of
• 11:20
  homework. Incredible.
• 11:22
  Absolutely incredible. If you have enjoyed today's
• 11:25
  episode, please do share it with a friend, a sibling, a parent, a
• 11:29
  grandparent, A neighbor or honestly anyone who has ever
• 11:32
  stared at a bubble and thought, hang on, why is that round?
• 11:35
  And if you have got a question you would like me to answer on a
• 11:37
  future episode because no question is too silly, too weird
• 11:40
  or too random on this show, head over to curiouskidcast.com.
• 11:45
  Drop your question there and maybe your name will be the one
• 11:47
  read out at the start of the next episode.
• 11:51
  Don't forget to subscribe so you never miss an episode.
• 11:53
  And if you are listening on a podcast app, leaving a review
• 11:57
  really does help other curious kids find the show.
• 12:01
  I've been Charlie. Keep asking those questions and
• 12:04
  I will catch you on the next episode of The Curious Kid Cast.