Science Interviews


Tue, 15th Mar 2016

The height of helium

Professor Hugh Hunt, The University of Cambridge

Listen Now    Download as mp3 from the show Cambridge Science Festival: Battle of the Brains

Professor Hugh Hunt wants to stop the arctic melting, and as strange as it mightBalloons seem - the giant floating helium balloons he brought wih him have got something to do with it, but what?

Hugh - Well they are balloons - helium filled balloons and itís all about buoyancy.  You might remember Archimedes and he got into a bath and he famously announced ďeureka.Ē  He understood buoyancyÖ

Smith - He was the first naked scientist - Hugh.

Hugh - Yes - one of the first naked scientists.  So I have this apple here and when I put it into the water it floats and thatís because the density of the apple is less than the density of the water.  Now with a helium balloon, the density of the helium is less than the density of air which is why the balloon floats.  What Iíd like to do is to measure how much this helium balloon lifts, so Iíve got some scales here - turn the scales on - and this appleÖ Whoíd like to come and measure the apple - would you like to come up?

Berrow - Whatís your name?

Emily - Emily Warren.

Hugh - Emily.  So Emily, how much is that apple weighing there?  Iíve put the apple on there, thatís how many grams?

Emily - 135.

Hugh - Right - 135 grams. Then when I put the apple into the bag here which is being lifted up by the helium balloon itísÖ

Emily - 132 grams.

Hugh - So how much has it got lighter?

Emily - 3 grams.

Hugh - 3 grams. So this little balloon here is lifting only 3 grams.  Thank you very much.  Now here Iíve got a bigger balloon, and you can see that this big balloon, well it has already got one apple in there, I can put a second apple in thereÖ

Smith - When you say big - itís 3ft wide.

Hugh - Well yes, it is quite big - about 80cm wide, and whatís interesting is, this balloon is about four times bigger than the small balloon.  But because the volume goes up as the cube of the size, itís four times bigger, but 4 cubed is 64 - 64 times bigger in volume. And 64 x 3 - itís about 200 grams.  Now 3 grams - 200 grams.  One apple was only 120 grams.  2 apples - canít quite hold two apples but you can see how that works. Now itís interesting with volume.  We can demonstrate how things go up with the cube.  Iíd like to have a volunteer - whoíd like to volunteer?  Whatís your name.

Johnny - Johnny

Hugh - Come here Johnny.  Iíd like to do a little experiment because I want to prove that volume - your weight goes up with your size cubed.  Now your height is about  150cm and my height is about 182 cms and if we do 150 divided by 182 and cube that, then that should be the ratio of our weights. 150 divided by 182 and we cube that and that tells us the ratio of our weights is about .55, and Iím 80kg so you should weight about 45kg - letís see if this works.  Right Johnny are you going to stand on there.  I reckon you should be about 45kg.

Berrow - So on the scales now of course.

Hugh - Okay 34kg but you should try this.  If you work out your weight it goes as a cube of your height.  So someone whoís half your height will be ⅛ of your weight. Now this is really important...

Thank you Johnny, you can sit down now.

This is really important because one of things Iím interested in doing is trying to refreeze the arctic.  Because we know about global warming but the arctic especially is getting very, warm and one of doing this is to spray tiny particles up into the stratosphere.  Thatís what volcanic eruptions do when they put tiny particles in the stratosphere and that helps to cool the planet.  Well, we could do this artificially and we might need a very big balloon to hold up a very big hose to spray these particles up.

Smith - And what particles are they that you want to put up?

Hugh - Well, we know that volcanoes would put up sulphur dioxide but maybe thereís other particles like titanium dioxide or silicon dioxide.  Tiny particles about half a micron in diameter - thatís under a 1000th of a millimeter which is about the wavelength of light, which is why these particles are good at scattering light.

Smith - What - so you want to put them up there and they will reflect light...?

Hugh - ...And they will reflect light back into space.  We only need to reflect a tiny bit of light and that will help to cool the planet.  Getting the particles up to a height of maybe 14, 16, 18 kilometers - we could do it with aircraft but that in itself is quite damaging to the environment.  So instead, if we have a big balloon, then we can use the big balloon to lift up a hose and one of the things about the hose is that trying to do that safelyÖ If I have a hose - well thereís a cable hereÖ

Berrow - So it looks like youíre attaching the chain from a bath plug to a 3ft wide pink balloon at the moment.  Thatís fairly accurate I would say?

Hugh - Yes, so Iíve now got this balloon on the end of a chain and then what happens is, when you start to have the balloon going high up into the sky, the wobbling of the chain may well cause the chain to break.  This chain will be a pipe to pump stuff into the atmosphere and weíve really got to get the engineering right to make sure that all doesnít go terribly wrong.

Berrow - So something like wind would have a huge effect on this because, obviously, itís going straight up into the sky?

Hugh - Yes, wind is a real problem.  The high winds in the jetstream and big storms. And actually, thinking about how we might cool the north pole is a very pressing problem for us to deal with right now.

Berrow - Does it matter that weíre thinking of putting particles at that level when people talk about the ozone layer and the fact that itís going away?  Does it matter that weíre starting to try and tamper with things like that?

Hugh - Well it really matters, and thereís huge questions about whether we should be meddling with the atmosphere, meddling with the climate in this way, but we are already pumping 35 billion of CO2 into the atmosphere.  Unless we stop doing that in a hurry and it doesnít look like we will, we may not have any choice.

Berrow - So this is possibly the answer?

Hugh - Well very reluctantly it may well be the answer but hopefully in the next five or ten years we will have figured out how to reduce our CO2 emissions.


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