How to squeeze electricity out of crystals - Ashwini Bharathula
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0:08 - 0:11This is a crystal of sugar.
-
0:11 - 0:14If you press on it, it will actually
generate its own electricity. -
0:14 - 0:18How can this simple crystal
act like a tiny power source? -
0:18 - 0:20Because sugar is piezoelectric.
-
0:20 - 0:23Piezoelectric materials
turn mechanical stress, -
0:23 - 0:24like pressure,
-
0:24 - 0:25sound waves,
-
0:25 - 0:26and other vibrations
-
0:26 - 0:29into electricity and vice versa.
-
0:29 - 0:31This odd phenomenon was first
discovered -
0:31 - 0:35by the physicist Pierre Curie
and his brother Jacques in 1880. -
0:35 - 0:38They discovered that if they compressed
thin slices of certain crystals, -
0:38 - 0:43positive and negative charges would appear
on opposite faces. -
0:43 - 0:45This difference in charge, or voltage,
-
0:45 - 0:49meant that the compressed crystal
could drive current through a circuit, -
0:49 - 0:50like a battery.
-
0:50 - 0:53And it worked the other way around, too.
-
0:53 - 0:57Running electricity through these crystals
made them change shape. -
0:57 - 0:58Both of these results,
-
0:58 - 1:01turning mechanical energy into electrical,
-
1:01 - 1:03and electrical energy into mechanical,
-
1:03 - 1:05were remarkable.
-
1:05 - 1:08But the discovery went uncelebrated
for several decades. -
1:08 - 1:11The first practical application
was in sonar instruments -
1:11 - 1:15used to detect German submarines
during World War I. -
1:15 - 1:17Piezoelectric quartz crystals
in the sonar's transmitter -
1:17 - 1:21vibrated when they were subjected
to alternating voltage. -
1:21 - 1:24That sent ultrasound waves
through the water. -
1:24 - 1:27Measuring how long it took these waves
to bounce back from an object -
1:27 - 1:30revealed how far away it was.
-
1:30 - 1:32For the opposite transformation,
-
1:32 - 1:34converting mechanical energy
to electrical, -
1:34 - 1:37consider the lights that turn on
when you clap. -
1:37 - 1:40Clapping your hands send sound vibrations
through the air -
1:40 - 1:43and causes the piezo element to bend
back and forth. -
1:43 - 1:47This creates a voltage that can drive
enough current to light up the LEDs, -
1:47 - 1:50though it's conventional sources
of electricity that keep them on. -
1:50 - 1:54So what makes a material piezoelectric?
-
1:54 - 1:56The answer depends on two factors:
-
1:56 - 1:58the materials atomic structure,
-
1:58 - 2:01and how electric charge
is distributed within it. -
2:01 - 2:03Many materials are crystalline,
-
2:03 - 2:04meaning they're made of atoms or ions
-
2:04 - 2:08arranged in an orderly
three-dimensional pattern. -
2:08 - 2:10That pattern has a building block
called a unit cell -
2:10 - 2:13that repeats over and over.
-
2:13 - 2:16In most non-piezoelectric
crystalline materials, -
2:16 - 2:19the atoms in their unit cells
are distributed symmetrically -
2:19 - 2:20around a central point.
-
2:20 - 2:24But some crystalline materials
don't possess a center of symmetry -
2:24 - 2:27making them candidates
for piezoelectricity. -
2:27 - 2:29Let's look at quartz,
-
2:29 - 2:32a piezoelectric material
made of silicon and oxygen. -
2:32 - 2:37The oxygens have a slight negative charge
and silicons have a slight positive, -
2:37 - 2:38creating a separation of charge,
-
2:38 - 2:41or a dipole along each bond.
-
2:41 - 2:44Normally, these dipoles
cancel each other out, -
2:44 - 2:47so there's no net separation of charge
in the unit cell. -
2:47 - 2:50But if a quartz crystal is squeezed
along a certain direction, -
2:50 - 2:51the atoms shift.
-
2:51 - 2:54Because of the resulting asymmetry
in charge distribution, -
2:54 - 2:57the dipoles no longer cancel
each other out. -
2:57 - 3:00The stretched cell ends up
with a net negative charge on one side -
3:00 - 3:03and a net positive on the other.
-
3:03 - 3:06This charge imbalance is repeated
all the way through the material, -
3:06 - 3:10and opposite charges collect
on opposite faces of the crystal. -
3:10 - 3:14This results in a voltage that can
drive electricity through a circuit. -
3:14 - 3:17Piezoelectric materials can
have different structures. -
3:17 - 3:22But what they all have in common is unit
cells which lack a center of symmetry. -
3:22 - 3:24And the stronger the compression
on piezoelectric materials, -
3:24 - 3:27the larger the voltage generated.
-
3:27 - 3:30Stretch the crystal, instead,
and the voltage will switch, -
3:30 - 3:32making current flow the other way.
-
3:32 - 3:36More materials are piezoelectric
than you might think. -
3:36 - 3:37DNA,
-
3:37 - 3:37bone,
-
3:37 - 3:38and silk
-
3:38 - 3:42all have this ability to turn
mechanical energy into electrical. -
3:42 - 3:46Scientists have created a variety
of synthetic piezoelectric materials -
3:46 - 3:49and found applications for them
in everything from medical imaging -
3:49 - 3:52to ink jet printers.
-
3:52 - 3:55Piezoelectricity is responsible for
the rhythmic oscillations -
3:55 - 3:58of the quartz crystals
that keep watches running on time, -
3:58 - 4:00the speakers of musical birthday cards,
-
4:00 - 4:03and the spark that ignites the gas
in some barbecue grill lighters -
4:03 - 4:05when you flick the switch.
-
4:05 - 4:08And piezoelectric devices may become
even more common -
4:08 - 4:13since electricity is in high demand
and mechanical energy is abundant. -
4:13 - 4:16There are already train stations
that use passengers' footsteps -
4:16 - 4:18to power the ticket gates and displays
-
4:18 - 4:22and a dance club where piezoelectricity
helps power the lights. -
4:22 - 4:25Could basketball players running back
and forth power the scoreboard? -
4:25 - 4:29Or might walking down the street
charge your electronic devices? -
4:29 - 4:31What's next for piezoelectricity?
- Title:
- How to squeeze electricity out of crystals - Ashwini Bharathula
- Description:
-
View full lesson: http://ed.ted.com/lessons/how-to-squeeze-electricity-out-of-crystals-ashwini-bharathula
It might sound like science fiction, but if you press on a crystal of sugar, it will actually generate its own electricity. This simple crystal can act like a tiny power source because sugar happens to be piezoelectric. Ashwini Bharathula explains how piezoelectric materials turn mechanical stress, like pressure, sound waves and other vibrations into electricity, and vice versa.
Lesson by Ashwini Bharathula, animation by Karrot Animation.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TED-Ed
- Duration:
- 04:56
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