Scientists don’t know how static electricity works

0:00 Static electricity is everywhere, from balloons to lightning storms to giving

0:04 yourself a shock by touching a door handle.

0:07 This common phenomenon has been studied since the time of the ancient Greeks.

0:11 So, you might be surprised to learn that scientists

0:13 don't know that much about how static electricity actually works.

0:18 I would say we know virtually nothing.

0:20 Um some people will get mad at me for saying that if they watch this video.

0:24 I think we know some things about particular situations,

0:27 uh but kind of the overall picture of what

0:29 happens most of the time for most materials, uh we don't know much.

0:34 The basic idea is that static electricity is an imbalance of charges

0:39 that occurs when charged ions or electrons move from one object to another.

0:43 This kind of charge transfer is

0:45 often called the triboelectric effect or tribocharging,

0:48 and it's caused by two surfaces rubbing against

0:51 each [music] other or even just coming into contact.

0:54 Whenever any two objects touch, they exchange charge.

0:58 [music] Uh it's literally unavoidable.

0:58 So, like, you know, right now sitting in my chair,

1:00 I'm moving around, I'm exchanging [music] charge with the chair.

1:03 So, we know it happens, but we don't really know why.

1:07 For example, if I give you [music]

1:09 rabbit's fur and a balloon, and I rub the balloon on the rabbit's fur,

1:13 I can't even tell you what is different [music] about

1:16 the rabbit's fur and the balloon that makes them exchange charge.

1:19 Could it be their electronic structure,

1:21 chemicals on the surface, their thermoelectric properties, flexoelectricity?

1:25 So, I literally can't even tell you what is the driving parameter.

1:29 And if we don't understand what's driving static charge,

1:32 then we don't really understand what

1:33 causes lightning in thunderclouds or volcanoes,

1:36 or what makes little bits of rock dust

1:38 come together around a star to form planets,

1:41 or how to avoid getting electric shocks off door handles.

1:44 Industries care a lot about static electricity,

1:47 mostly because they're trying to avoid sparks.

1:50 A classic example that's maybe a bit older now is a coal mine explosion,

1:53 but even in high-tech settings, for example,

1:55 in a clean room where they make computer chips,

1:58 uh these are environments where you have

2:00 to have everything extremely free of airborne particles,

2:04 because if that particle lands on your computer chip while you're making it,

2:08 uh it will mess everything up.

2:09 And the reason those particles attach is because of static electricity.

2:12 Scott's one of a relatively small group of researchers

2:15 trying to understand this at the fundamental level,

2:18 which is easier said than done.

2:20 And it's actually a really hard problem, because, you know,

2:22 when you when you rub two things together and they get really charged,

2:25 it seems like, "Oh, wow, they're really charged.

2:27 My hair's sticking to the balloon.

2:28 It's so big." But actually, you know,

2:30 maybe one in a hundred thousand surface atoms gained or received a charge.

2:35 And so, when you're trying to do any kind of technique

2:37 to see what that one in a hundred thousand was, [music]

2:40 uh it's a lot like looking for a needle in a haystack.

2:42 So, the team set out on a needle hunt,

2:45 and they started with an even more intriguing puzzle than balloons and hair.

2:49 Why does charge still move from one object

2:52 to another when they're both made of the same thing?

2:56 [music] Let's say you have a grain and you have another grain,

2:58 and they're both made of exactly the same material.

3:01 And if you contact them and you measure the charge,

3:03 maybe this one will be a bit positive and this one will be a bit negative.

3:07 And the question we wanted to answer is what's the difference.

3:10 Galia and Scott wanted to find

3:12 the difference between two apparently identical objects.

3:15 They were working with silicon dioxide,

3:17 the compound in quartz and a big component of sand.

3:20 And their approach to experimentation was

3:22 to simplify things as much as possible.

3:25 Take two objects, in this case a grain

3:28 and a flat plate made [music] of the same silicon dioxide,

3:31 hit them together, then see how the charge has changed.

3:35 And do this over and over again in different controlled conditions.

3:39 Setting up the experiment was tricky.

3:41 You cannot touch the grains in any way.

3:44 Because if you touch them, you're going to give them some charge,

3:46 and that's going to be more charge than they gained [music]

3:48 through the collision.

3:50 So, what we needed to do is isolate the grains.

3:53 So, we trapped them in an ultrasonic standing wave.

3:57 You turn off briefly the acoustic field.

3:59 You let the [music] grain fall on the plate,

4:01 bounce, and then you capture it again.

4:02 After each collision, they measured the charge.

4:05 Half the grains became more positive, half negative, seemingly at random.

4:11 But the same grain always went in the same direction,

4:14 either positive or negative.

4:15 Clearly, there must be some invisible difference

4:18 that determines [music] which way it goes.

4:20 Things that people have named as being

4:23 important in tribocharging include humidity, roughness, [music]

4:35 You can probably find a scientist that will tell

4:38 you that any of those is the most important one.

4:40 So, the team started testing theories,

4:43 trying to find the thing that makes the difference.

4:46 And finally, they landed on something that did seem to make a difference.

4:51 It was baking.

4:51 If you take the positively charging particle

4:55 and you bake it even at relatively low temperatures,

5:00 below 100 degrees, and you do the measurement again,

5:03 it's going to charge more negative.

5:05 [music] Baking the grains was the key.

5:06 And it turns out that the magic ingredient in this cooking process was carbon.

5:12 All around you are tiny little carbon compounds

5:15 that float through the air and land on everything.

5:18 It's often just a few molecules, but this stuff is absolutely everywhere.

5:22 And experiments showed that baking got rid of the carbon compounds,

5:27 cleaning the surface of the oxide grains.

5:30 This made each grain charge more negatively than it did

5:33 before until the carbon started slowly landing on the surface again,

5:37 and the effect wore off.

5:39 So, could bits of carbon explain

5:41 the invisible differences between otherwise identical samples?

5:45 What exactly goes on the surface is a little bit random.

5:48 It depends a little bit on whatever molecules were in the air at the time.

5:53 And so, this tells you that if you take two really clean

5:56 samples and you store them in a box for a few days,

5:59 they're going to end up slightly different.

6:02 And this slight difference explains why certain samples want

6:05 to charge positive and others want to charge negative.

6:07 Finally, there was a clear signal, a variable that actually impacted the static

6:12 charging of these little oxide grains.

6:15 But is this really the answer to the centuries-old puzzle of static electricity?

6:19 Are carbon compounds the one thing that always makes the difference?

6:23 So, we're very cautious just because this is such a messy topic.

6:27 Uh you know, that I personally have learned

6:29 my lesson and not to uh say that, you know,

6:33 the thing we saw here matters everywhere.

6:35 So, I would not be surprised at all if

6:38 these molecules from the environment matter for other materials.

6:42 Um but until we get a big body of experimental evidence,

6:45 I'm not going to stick my neck out.

6:47 It could be that surface carbon only matters

6:50 for the oxides which they tested in this particular experiment.

6:53 Different materials could have different key factors.

6:57 But knowing how it works in oxides is pretty useful.

7:00 They're actually one of the most common materials in nature.

7:03 So, something like 60% of the Earth's crust is made from silicon dioxide,

7:08 and the same is true for most other terrestrial planets.

7:11 So, recently, it was discovered that there's lightning on Mars,

7:14 and that was because of the oxide particles on Mars colliding,

7:18 getting charged up, and then somehow getting separated create,

7:20 you know, lightning and dust storms.

7:22 Whether the lightning is on another planet

7:24 or in the clouds pouring from an active volcano,

7:27 it all comes down to tiny static interactions between grains.

7:31 If you look at a system, for example, like a volcanic lightning,

7:35 you have these silicate particles that are thrown very violently in the air,

7:40 [music] and the temperature changes, the pressure, humidity,

7:43 there's all of these complicated things happening.

7:45 But if you don't understand just a little uh microscopic mechanism,

7:50 just what's happening when particle A and particle B collide,

7:55 then how can you explain the big picture, right?

7:58 And there are so many little open questions that maybe you could

8:02 answer by just understanding fundamentally what's

8:04 happening at the scale of the particles.

8:06 [music]