Science Centre: Bell Jar

What happens in a vacuum? We remove the air from a bell jar to see if you can scream in space...
03 October 2013

Interview with 

Dave Ansell, Ginny Smith, The Naked Scientists


Clock face


What happens in a vacuum? Dave and Ginny remove the air from a bell jar to see if you can scream in space...

Ginny - Okay, next to the microwave, you've got another quite interesting contraption. What I can see is what looks like a sort of old fashioned alarm type bell inside a Perspex tube, why are we looking at this?

Dave - So basically, I have a sealed Perspex tube and inside of it there's a bell and if I press the button, to make the bell ring, you can hear the bell really quite well. You can also see the little fan in there, blowing air around. You can see there's some air moving inside. So, what I'm going to do now is a really, really classic experiment. What I've got this tube attached to is something called a vacuum pump. It's a bit like a really, really hardcore vacuum cleaner. A normal vacuum cleaner can take out about 10% of the air from something. This will do about 99% if you give it long enough. So, if you listen to the bell at the moment, it's quite noisy and I will slowly pump the air out.

Ginny - So, that's quite noisy as well.

Dave - Yeah, vacuum pumps aren't the quietest of devices. What you can hear when I'm talking to you is that when I'm talking, I make the air in front of my mouth vibrate. And so, the air moves backwards and forwards, and the air in front of that bashes into the air in front of that which bashes the air in front of that. And eventually, unless you're listening to me on a radio when the loud speaker in the radio makes the air vibrate. So the air bashes and bashes, and bashes until it gets to your ear, and that makes your ear vibrate, and you can hear me talking. So, what we're doing here is we're slowly taking the air out of the tube. So, the bell should hardly be able to make the air move to start with and then the air is so thin that it shouldn't be able to - any vibrations in that are going to have hardly any effect on the rest of the world and hopefully, it should be quite quiet.

Ginny - What I can see is that where the little fan was blowing some bits of tin foil around, they don't look like they're moving anymore. So, have you turned the fan off?

Dave - If you look carefully, the fan is still running. So, what I've done instead of doing that is I've taken the air away. So now, if we try ringing the bell, it's almost entirely quiet.

Ginny - Yeah, you can sort of hear that something is going on, but it's definitely not the kind of bell noise you could hear before.

Dave - And you can see the clapper moving, but it's very, very quiet. If I let the air back in again which take a while...

Ginny - If I press the bell while you're letting the air back in, we should be able to hear it starting to get louder. Can you all hear that?

Chris - So Dave, answer the age-old question, could someone hear you scream in space?

Dave - That depends, if you're actually out in space then there's no air there, so no. But if you're in a space station then yes, because there's air.

Chris - Dave and Ginny, thank you very much. Who's got some questions? We have one just here. This person, what's your name?

Laura - Laura.

Chris - Hello, Laura.

Laura - When you practice at home, does anything ever go seriously wrong?

Chris - Come on Dave, now it's time to be honest.

Dave - I've broken lots of things. I haven't killed myself yet which is always a good thing. I haven't badly injured myself. So, I think that's pretty good.

Chris - There's question over the back here.

Paul - Hi. My name is Paul. I come from Hardwick. When you're talking about vacuums, they keep telling is that space is vacuum but they keep talking about solar winds. How can there be wind in space in a vacuum?

Dave - You can never get a perfect vacuum and there's always a few particles left. If you go into deep, deep space, there might be only one or two in every cubic metre whereas in a normal air, there's hundreds and hundreds of billions, if not, more than that. And so, the vacuum in space is very, very good, better than anything you could get without really, really, expensive kit. You're talking millions of quid on Earth. But it doesn't mean there aren't still some particles there and the sun does throw off some little hydrogen nuclei and they get thrown out at incredibly high speed. And so, there's a little bit there.

Chris - I think the speed of it is about a million miles per hour that it goes extremely fast.

Dave - It's really scarily fast, but there's not very much of it so it doesn't do too much damage of moving things around very much.

Chris - It's also responsible for the aurora when it strikes the magnetosphere around the Earth.

Dave - It gets deflected by the Earth's magnetic field and then piles into the north and south poles, and gives lots of energy...

Chris - Makes Norway a lot of money out of its tourist trait, very nice. There's another question over here.

Fergal - This is Fergal again. I'm wondering how does the speed of sound depend on the air pressure, so, as you travel out into space and you were to do an experiment?

Dave - Interestingly, it hardly changes at all because the speed of sound is to do with the ratio between the density and the kind of springiness. And as you reduce the pressure, you reduce the density but you also reduce springiness, and that cancels out. What it does depend on is temperature. The lower the temperature, the lower the sound because you kind of reduce the springiness without reducing the density; at high temperature goes very fast, at low temperature it goes very slow. Which is the reason why if you fly up where the jet planes fly, it's about minus 30 degrees centigrade at which point the speed of sound is about 600 miles an hour not 700.

Chris - Does that affect when you would get a sonic boom then? So, higher up in the atmosphere, you'd go slower to break the sound barrier.

Dave - Yes, it is easy to break the sound barrier when you're higher up. You don't have to go nearly as fast.

Chris - Fantastic! Any other questions for Dave and Ginny. One in the front, another one.

Paul - It's me again, Paul. We just heard that Voyager left the solar system. How? What's making it move for all these years?

Dave - Okay, so essentially, Voyager had a big rocket on it and it was going really, really fast. It kind of did some cunning manoeuvres going around the different planets, especially Jupiter which gave it extra kick. Basically, it's just going so fast that it has enough energy to keep on going forever. It'll keep on going straight out the solar system and go. So, if you shoot something out of a gun fast enough, it won't come down.


Add a comment