Statics and hair standing on end

For this next demo, you'll need a balloon, a tap, and access to some hair! Dave Ansell took Adam Murphy through this electrifying experiment...

Dave - For this demo, what you're going to need is a balloon, either a long thin one or a short round one and a tap, which you can turn down. And some hair. Many people find their head hair works well. Cat hair is supposed to be brilliant. But I find that the best is my leg hair?

Adam - Okay. So I've got a purple princess balloon because that's what the shop had, I'm standing beside my tap and I needed a haircut two weeks before we went into isolation. So I definitely have enough hair to do this. What do I do with this stuff, Dave?

Dave - First blow up your balloon, I'll blow up mine.

Adam - Okay, I've got my balloon all blown up. Now what do I do with this?

Dave - What I want you to do is rub it on some hair. I'll pull up my trouser leg and rub it on my leg.

Adam - Okay. Just rubbing this on my head hair now. I am in dire need of a haircut. Okay, so I've got a head rubbed balloon. What do I do now, Dave?

Dave - So did you notice anything odd happening as you were rubbing it?

Adam - Yeah, my hair is standing on end now.

Dave - So what's happening is that the electrons, which are little tiny particles which whiz around atoms and are negatively charged, slightly prefer being in the balloon than being in your hair. So whenever a bit of hair touches onto a bit of a balloon, a few electrons get transferred from one to the other. Not many, a very, very small number get transferred. The reason why you're rubbing is because as you rub more different bits of hair get to touch different bits of balloon. So more electrons are transferred, and you get a bigger negative charge transferred to the balloon. Now what I want you to do is turn on your tap, so you've got a very thin but continuous stream of water coming out along it.

Adam - Okay. Turning on my tap now.

Dave - To make sure the balloon is really well charged, give it another quick rub on your hair. And then move the bit of the balloon which you've charged up close to that stream of water.

Adam - Okay, so when I bring the balloon near the water, it starts to pull the stream of water towards it. Like they were magnets coming together.

Dave - Totally. What happened with your hair earlier, is that your balloon was becoming negatively charged, and your hair was becoming positively charged, because it lost its negative charge and positive attracts negative. So your hair was attracted to the balloon. But with the stream of water, the stream of water isn't positively charged, but when you put the balloon near it, it will tend to repel any negative charge, and the electrons then be able to move more easily than the central nuclei of the atoms, the positive nuclei of the atoms.

所以负电荷,一些电子会推动away from the balloon leaving positive charge near it, which attracts it. So the stream of water is attracted to the balloon. And also I've just noticed that if you bring the balloon near a very, very slow stream of water, you hear a kind of cricket, that's a little kind of crinkling noise. This is because the stream of water is actually breaking up and little drop of water are hitting the balloon. And because these droplets of water are coming from the side nearest the balloon, they tend to be positively charged. This tends to kill the charge on your balloon very quickly, and so the effect gets weaker and weaker over time.

Adam - So is this how we could store electricity then, Dave?

Dave - This is probably the only way we can store electricity directly. You have an object, a large object, which will accept some charge and it will charge up. It does that, the voltage will increase and then you can release that energy quickly and do something useful with it. The obvious example is if you get really charged up by walking along, I don't know, a strange plastic floor, and then you touch something else or somebody else, you often get a little spark, a little shock, and that's the electrical energy, which has been stored on your body being released through your finger and causing a little bit of pain. This is a tiny amount of energy, so it's not going to be, at all significant compared to the national grid, for example, unless you design your object so they store a huge amount of charge, and the electronic component which is designed to do this is called a capacitor. And big banks of capacitors can store a significant amount of energy, but not really a useful amount of energy still. They're useful for things like smoothing electrical signals, but you couldn't drive a car on it for very long.

Adam - So we won't be hooking hot air balloons up to the grade anytime soon.

Dave - Yeah, no, we're not going to be using hot air balloons. No. I mean, even the really extreme example of this effect, which is lightning, although it's really impressive at the time it happens and you get sort of, millions of volts and millions of amps going along. It happens for such a small portion of time that actually, the amount of energy produced is not really very useful to us.