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

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Perpetual motion machines — devices that can do work indefinitely without any external energy source — have captured many inventors’ imaginations because they could totally transform our relationship with energy. There’s just one problem: they don’t work. Why not? Netta Schramm describes the pitfalls of perpetual motion machines.

Lesson by Netta Schramm, animation by TED-Ed.

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.