Fun to Imagine: Jiggling Atoms
Hot and cold, ice and steam, a bouncing ball that slowly stops: Richard Feynman explains all of them with one picture. Everything is made of atoms, and the atoms are always jiggling. Imagine it his way and a cup of coffee, a drop of water and a bicycle pump suddenly make sense.
What Is Heat, Really?
How fast its atoms jiggle. Richard Feynman says hot and cold are nothing but the speed of the jiggling: more jiggling is hotter, less is colder. The atoms in hot coffee bang against the cup, the cup's atoms start shaking and the shaking spreads into your hand. What you feel as warmth is jiggling being passed along.
Into the floor, as jiggling. Each bounce shakes some of the floor's atoms and some of the ball's own. The tidy motion of the whole ball falling turns into untidy jiggling of countless atoms. Nothing is lost; the floor ends up a tiny bit warmer, which Richard Feynman calls unbelievable but true.
Put Your Instincts to the Test
Picture the atoms jiggling, then pick an answer for each question and see if your instincts were right.
It gets warmer. Richard Feynman noticed this as a boy. The piston moving in hits the air molecules and sends them back faster than they came, and faster molecules mean hotter air. Squash a gas and it warms up; let it expand and it cools.
The molecules at the edge are pulled inwards. Water molecules attract each other and want as many neighbours as they can get. One inside the drop is surrounded; one at the surface has neighbours on one side only, so it's tugged back in. Richard Feynman pictures them as people at the edge of a crowd, pushing to get back in. The result is called surface tension.
They lock into a pattern but keep jiggling. With less jiggling the molecules can't break free of their neighbours, so they settle into a neat pattern, like oranges in a crate. Each one still jiggles in its own place. Even ice at the South Pole is jiggling; only at absolute zero, about −273 °C, would the jiggling be as small as it can ever be.
Understanding the Science
Tap each card to see how one picture, atoms that never stop jiggling, explains heat, ice, water, steam and the bicycle pump.
Key Concepts
Temperature Is Jiggling
Tap to learn moreA thermometer measures how fast, on average, the atoms in something are jiggling. In hot things they jiggle faster; in cold things, slower. The molecules in the air around you are moving at about 500 metres a second, roughly twice as fast as a passenger jet, but each one only travels a tiny distance before it hits another and changes direction. That's why a smell takes a while to cross a room.
Heat Moves by Contact
Tap to learn moreWhen a hot thing touches a cold thing, its fast-jiggling atoms knock into the slow ones and speed them up. The hot thing cools and the cold thing warms until they jiggle at the same rate. This is called conduction. Richard Feynman stresses that what spreads isn't a substance called heat; it's just jiggling, an irregular motion, being shared out.
Atoms Never Lose Their Bounce
Tap to learn moreA rubber ball loses a little bounce every time it hits the floor. Atoms don't. When two atoms collide, all their energy of motion bounces back; they're perfectly elastic. That's why they keep jiggling forever without being wound up. The energy of the whole universe is never used up, only moved around: the law of conservation of energy.
Tidy Motion Becomes Jiggling
Tap to learn moreA falling ball has tidy motion: all its atoms move the same way together. When it lands, that motion turns into untidy jiggling in the ball and the floor. Hammer a nail or bend a paperclip back and forth and it gets warm for the same reason. In the 1840s James Joule stirred water with paddles driven by falling weights and measured the tiny rise in temperature, showing exactly how much work makes how much heat.
Solids: Oranges in a Crate
Tap to learn moreCool water enough and its molecules don't jiggle hard enough to escape their neighbours. They lock into a regular pattern, a crystal, each one jiggling in its own place. Hold one end of a piece of ice and the whole piece comes with it, because every molecule is lined up with the next. Ice's pattern is quite open, which is why ice is less dense than water and floats.
Liquids: Rolling Over Each Other
Tap to learn moreWarm the ice and the molecules start to break out of their places. They still stick together, but they tumble and roll over one another, so the water flows and takes the shape of its container. The pull between neighbours is still there, which is why water forms drops and why some insects can stand on a pond. That pull at the surface is called surface tension.
Gases: Hands That Don't Grab
Tap to learn moreHeat water further and its molecules jiggle so hard that, as Richard Feynman puts it, their hands don't grab as they pass. They fly apart and become a gas: steam. The molecules in steam are far apart, so steam takes up about 1,700 times the space of the water it came from. That's how a kettle's spout can push out so much of it.
Squash It and It Warms
Tap to learn morePush a piston into a gas and every molecule that hits it bounces back faster, so the gas heats up. Pull the piston out and the molecules bounce off something giving way, so they lose speed and the gas cools. A diesel engine squeezes air so hard that it gets hot enough to light the fuel with no spark at all. Spray from an aerosol can feels cold partly because the gas expands as it leaves.
Try It: Jiggle Lab
Freeze, melt and boil the water by choosing what's under the pump, then heat the steam with the pump alone. Drag the pump handle, or use the button.
Temperature is how fast the molecules jiggle. A hot thing passes its jiggle to a cold thing it touches, which is all heat is. Squash a gas and it gets hotter (a bicycle pump warms up as you use it); let it expand and it cools. Real water molecules in the air at room temperature move at about 600 metres a second. This model is flat and crowded, so its meter shows jiggle rather than degrees.
Richard Feynman's Pictures
Richard Feynman explained physics with pictures you can run in your head. Tap each one to see what it shows.
The Cup of Coffee
Tap to learn moreA hot drink on a table: its atoms jiggle a great deal and bang against the cup. The cup's atoms start shaking, they shake the table's atoms and the air's, and slowly everything nearby warms while the coffee cools. Richard Feynman's point is that nothing mysterious flows out of the coffee. The jiggling just spreads, the way a crowd's excitement spreads.
The Ball That Stops Bouncing
Tap to learn moreEach time a ball hits the floor it leaves a little patch of floor jiggling, and its own atoms jiggle more too. Bounce by bounce, the orderly fall of the whole ball turns into disorderly jiggling, until the ball sits still. All the motion is still there, as a floor that's a tiny bit warmer than before.
The Hammer
Tap to learn moreAnyone who has hammered something for a long time knows it warms up. Richard Feynman says it heats up simply because you're jiggling it. Every blow sets the atoms of the metal shaking harder, and that extra shaking is exactly what we measure as a higher temperature.
The Crowd at the Edge
Tap to learn moreImagine the molecules in a drop of water as a team of people who all want as many partners as possible. Those in the middle are happy; those at the edge have partners on one side only, so they keep pushing to get back in. That pressure from the edge squeezes the drop into a tight ball instead of a flat puddle. It's surface tension, seen from the inside.
The Disappearing Drop
Tap to learn moreWhile the drop holds itself together, a few molecules at the surface jiggle hard enough to break away and fly off into the air. One by one they leave, so the drop slowly disappears. The ones that escape are the fastest, so the drop left behind is a little cooler. That's why sweat cools you down.
Oranges in a Crate
Tap to learn moreCool water and the jiggling slows until the molecules get stuck in place, packed together in a neat pattern like oranges in a crate. They aren't still: each one jiggles where it is, but none has enough motion to get loose and break the structure down. Richard Feynman says that's all a solid is.
Hands That Don't Grab
Tap to learn moreHeat water hard and the molecules still have a tendency to stick, but they're moving too fast. Richard Feynman pictures their hands failing to grab as they pass, so they fly apart again. Each one goes its own way as a free molecule, and that's a gas: the steam rising from a pan.
The Bicycle Pump
Tap to learn moreAs a boy, Richard Feynman noticed his bicycle pump got hot. Push the handle down and the molecules coming up against the piston bounce off faster than they arrived, so the air heats. Pull it out and the molecules hit something soft and yielding, losing speed, so the air cools. Squashed gases warm; expanding gases cool.
Apply Your Knowledge
Use the jiggling picture to explain things you see every day.
Match each everyday event to what the molecules are doing
Click an item to select it, then click the matching description to place it.
Real-World Challenge
Blowing on hot soup cools it down. Explain why to someone at home using only Richard Feynman's picture of jiggling molecules, without the words ‘heat energy’. Then predict whether blowing works better on a wide bowl or a narrow mug and say why.
Feel the Jiggling Yourself
Three safe ways to see Richard Feynman's ideas at work. Ask an adult before you start.
Pump up a bicycle tyre or a football with a hand pump: 20 or 30 quick strokes, then feel the barrel near the bottom, where the hose joins. What to look for: The bottom of the barrel gets warm. That's where the air is squashed hardest, so its molecules are bounced back fastest. Stay safe: Stop when the tyre feels firm and never pump past the pressure printed on the tyre.
Fill one clear glass with cold tap water and another with hot tap water. Let them stand still for a minute, then add one drop of food colouring to each, gently. What to look for: The colour spreads much faster in the hot water. Its molecules are jiggling faster, so they knock the colouring about more. Stay safe: Use hot water from the tap, never boiling water from a kettle.
Unfold a metal paperclip and bend one part back and forth quickly, ten times, at the same spot. Then touch the bend. What to look for: The bend feels warm. Your work has turned into jiggling of the metal's atoms, just like Richard Feynman's hammer. Stay safe: The clip can snap and the ends are sharp, so touch it gently and keep it away from eyes.
What Has Changed Since This Programme
Fun to Imagine was filmed for BBC television in 1983. Here's what science has found since then.
Updated: Yes. The scanning tunnelling microscope, invented in 1981 and honoured with the Nobel Prize in Physics in 1986, can map single atoms on a surface. In 2021 a team at Cornell University made the sharpest pictures of atoms yet. They found that the blur left over came mostly from the atoms' own jiggling.
Updated: To within a billionth of a degree of absolute zero. Laser cooling slows atoms down by bouncing light off them, and in 1995 it helped scientists make a Bose-Einstein condensate, a cloud of atoms so cold they behave as one; the work won the 2001 Nobel Prize in Physics. Since 2018 NASA's Cold Atom Laboratory has made these ultracold clouds on the International Space Station.
Test Your Understanding
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Reflection
Richard Feynman joked that there's no teacher going to ask you questions at the end, so we've broken his rule. Here's one more, with no wrong answer: next time you rub your hands together to warm them on a cold day, what will the molecules in your skin be doing, and where did their extra jiggling come from?
Episode Discussion
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