Why don't perpetual motion machines ever work? - Netta Schramm

Around 1159 A.D., a mathematician called Bhaskara the Learned sketched a design for a wheel containing curved reservoirs of mercury. He reasoned that as the wheels spun, the mercury would flow to the bottom of each reservoir, leaving one side of the wheel perpetually heavier than the other. The imbalance would keep the wheel turning forever. Bhaskara's drawing was one of the earliest designs for a perpetual motion machine, a device that can do work indefinitely without any external energy source. Imagine a windmill that produced the breeze it needed to keep rotating. Or a lightbulb whose glow provided its own electricity. These devices have captured many inventors' imaginations because they could transform our relationship with energy. For example, if you could build a perpetual motion machine that included humans as part of its perfectly efficient system, it could sustain life indefinitely. There's just one problem. They don't work. Ideas for perpetual motion machines all violate one or more fundamental laws of thermodynamics, the branch of physics that describes the relationship between different forms of energy. The first law of thermodynamics says that energy can't be created or destroyed. You can't get out more energy than you put in. That rules out a useful perpetual motion machine right away because a machine could only ever produce as much energy as it consumed. There wouldn't be any left over to power a car or charge a phone. But what if you just wanted the machine to keep itself moving? Inventors have proposed plenty of ideas. Several of these have been variations on Bhaskara's over-balanced wheel with rolling balls or weights on swinging arms. None of them work. The moving parts that make one side of the wheel heavier also shift its center of mass downward below the axle. With a low center of mass, the wheel just swings back and forth like a pendulum, then stops. What about a different approach? In the 17th century, Robert Boyle came up with an idea for a self-watering pot. He theorized that capillary action, the attraction between liquids and surfaces that pulls water through thin tubes, might keep the water cycling around the bowl. But if the capillary action is strong enough to overcome gravity and draw the water up, it would also prevent it from falling back into the bowl. Then there are versions with magnets, like this set of ramps. The ball is supposed to be pulled upwards by the magnet at the top, fall back down through the hole, and repeat the cycle. This one fails because like the self-watering pot, the magnet would simply hold the ball at the top. Even if it somehow did keep moving, the magnet's strength would degrade over time and eventually stop working. For each of these machines to keep moving, they'd have to create some extra energy to nudge the system past its stopping point, breaking the first law of thermodynamics. There are ones that seem to keep going, but in reality, they invariably turn out to be drawing energy from some external source. Even if engineers could somehow design a machine that didn't violate the first law of thermodynamics, it still wouldn't work in the real world because of the second law. The second law of thermodynamics tells us that energy tends to spread out through processes like friction. Any real machine would have moving parts or interactions with air or liquid molecules that would generate tiny amounts of friction and heat, even in a vacuum. That heat is energy escaping, and it would keep leeching out, reducing the energy available to move the system itself until the machine inevitably stopped. So far, these two laws of thermodynamics have stymied every idea for perpetual motion and the dreams of perfectly efficient energy generation they imply. Yet it's hard to conclusively say we'll never discover a perpetual motion machine because there's still so much we don't understand about the universe. Perhaps we'll find new exotic forms of matter that'll force us to revisit the laws of thermodynamics. Or maybe there's perpetual motion on tiny quantum scales. What we can be reasonably sure about is that we'll never stop looking. For now, the one thing that seems truly perpetual is our search.

Channel: TED-Ed

Views: 8,172,577

Rating: 4.9024792 out of 5

Keywords: TED, TED-Ed, TED Education, TED Ed, Netta Schramm, perpetual motion, perpetual motion machine, energy, thermodynamics, Bhaskara the Learned, Robert Boyle, magnet, wheel, renewable energy, first law of thermodynamics, second law of thermodynamics, light bulb, battery, capillary action

Id: A-QgGXbDyR0

Length: 5min 31sec (331 seconds)

Published: Mon Jun 05 2017

Reddit Comments

[u/wattm]

Because, Lisa, in this house we respect the laws of... THERMODYNAMICS!

[u/[deleted]]

Is this a a Ted Talk for kindergartners? What a retarded question. Why does your bike slow down when you aren't pedaling. Why did my parents marriage abruptly end.

[u/DasWyt]

Actual question: aren't solar panels, at least for us humans, essentially practical perpetual energy machines? That is, by the time the sun is done doing it's thing we won't be able to survive anyway. So can't we just use solar panels and be done with it?

[u/purplehayes1986]

Physics

[u/Manatitur]

Something I don't like about many educational videos is that at the end of the video they always have to bring up false hope.

What I mean is: He spent minutes trying to explain why perpetual motion is really not possible. Nothing indicates that it is or will be possible. Why does he have to come up with some exotic matter or quantum woo to somehow still keep some small door to the possibility of it existing open?

Same with videos about aliens or interstellar travel. After a long explanation why it's really unlikely that humanity will ever be a spacefaring, alien-meeting species, videos about this topic can never just be realistic and rule out the possibility of it happening. There must always be a disclaimer that somehow, if we really believe in it, everything we imagine is still possible! (exaggeration)

I find that really dishonest.

[u/Photoguppy]

Just out of curiosity (Don't shoot me) could a feedback loop such as what happens when you move a microphone close to an amplifier be considered perpetual motion?

I don't mean to say that it's a perfect perpetual motion machine but rather, would that scenario fit into the general scheme of a perpetual motion design?

[u/thehomesketch]

There ain't no motion like perpetual motion, cause perpetual motion don't stop.