Science Centre: Rubber Band Fridge

03 October 2013

Interview with

Dave Ansell, Ginny Smith, The Naked Scientists

How can you make a tiny fridge using an elastic band? Ginny Smith and Dave Ansell Rubber bandsfind out...

Ginny - Okay, if everyone looks under their chair, you should find a rubber band. So, we're going to need your help for this one and we're all going to have a go at this experiment together. Our panel I think has a rubber band, each of them? Good. So Dave, what do you need us to do with these rubber bands.

Dave - So, these are perfectly normal rubber bands. Hopefully, you've all found them and what we've going to be doing is looking at the temperature of the rubber band. Now, about the most sensitive part of your body for measuring temperature is your lips. So, what I want you to do is touch the rubber band to your lips and get an idea of what sort of temperature it's really like.

Ginny - Now, we're mainly doing this to make you look silly.

Dave - Everyone with rubber band moustaches, it looks wonderful from up here I can say. And then what I want you to do, if you stretch the rubber band slowly, you'll find the point where it starts to get quite stiff. Can you feel that? Okay, so what I want you to do is let it unstretch. Now, feel the temperature on your lip. Take it off then quickly stretch it to the point at which it gets stiff and then feel it again. And then leave it out and let it unstretch and feel it again and if you feel any difference.

Ginny - So, if we go over to our panel, you guys are having a go at this? So, can you feel anything different?

Rod - It feels warmer when you stretch it.

Ginny - Brilliant! Is that what everyone else is finding?

Crowd - Yes.

Chris - There are some pretty weird looking people here.

Ginny - Yeah, I think...

Chris - Elastic band moustaches.

Ginny - It's a good look. So, all we've done is stretched a rubber band and it's got warmer. Why?

Dave - So, that kind of make sense. As you stretch it, you're putting a load of energy into it and that energy must be going somewhere and in the case of the rubber band, it's going into warming up the rubber band. So, the rubber band has got warmer as you stretch it. Rubber is actually quite a strange material. It doesn't behave like a normal material. If you stretch a piece of steel or any other alloy, you kind of stretch bonds in the molecules and you kind of store energy. But rubber is different. Rubber is made up of great big long molecules. It's a bit like this piece of rope. When you stretch it, you're not stretching the bonds. What you're doing is just straightening out the molecules. Now, did you notice what happened when you let it shrink again?

Ginny - So, everyone is saying it got colder when they released it and then felt it again.

Dave - That's right because when rubber unstretches, what basically happens is that you've pulled out your rubber molecule and made it nice and long and straight, and then you let heat, so all the molecules inside are bouncing around because they're hot. Everything is jiggling. As it jiggles, it kind of jiggles around and gets more zigzaggy and get more wriggly. And so, it gets shorter.

Ginny - So, what Dave has got here is he's got a piece of rope which is pretending to be the molecules inside the elastic band and when he wiggles them about. Of course, the rope sort of makes waves, goes all snaky and then the length of it get shorter because it's moving around more, and that's what's happening inside the rubber band.

Dave - That's right and as the molecules bash into it, they lose a little bit of energy and they get colder. So, as the rubber band pulls against your fingers in order to shrink, the molecules lose some energy. They're actually losing it to your fingers if you pull in and it gets colder and you can actually feel that on your lips. You have effectively made a very, very small, very inefficient fridge.

Ginny - So, when you're hot and probably not at the moment because it seems to be absolutely freezing outside, but if we ever have a summer again, you just need a few rubber bands and you can cool down with no problem.

Dave - I'd like to see the machine.

Chris - Thanks, Dave and Ginny. So, any questions about elastic band physics? One just here.

Freya - Doesn't the rubber band have the temperature of the room temperature or is it its own temperature?

Dave - So, if you leave a rubber band for a long time, it will get the room temperature. And actually, that's kind of why it works as a fridge. If you stretch it, it gets hotter and if you leave it for a while, it'll get back to the room temperature because the heat will leave it because it's hotter than the rest of the world. Then as you unstretch it, it kind of itself gets colder. And so, it cools down a bit and then again, if you leave it for a long time, it'll warm up. So, if you leave it for long enough, it will get to the room temperature.

Chris - One just here.

Grace - Hi. My name is Grace and I've wondered what makes a magnet magnetic.

Ginny - So, magnets have little poles inside them. They have little elements that have to line up to make them magnetic. So, if you've ever wanted to make a magnet, you can do it by using another magnet to rub along a piece of iron or something like that. What that does is it lines up all these little poles inside them and if they're all pointing in the same direction, you get a magnet. You have to be really careful with magnets because you can demagnetise them if you jumble all those poles up again. So if you drop a magnet hard enough, it'll jumble them all up and then it won't be a magnet anymore. And also, if you heat it up. So, like we were just talking about, when you heat things up, everything inside it gets really excited and jumbles around. So, if you put a magnet in a flame, you can demagnetise it as well.

Dave - If you get to really fundamental things as to why anything is magnetic, magnetism is kind of effects of electric current. If you get an electric current flowing, you get a magnet. So, if you put a wire around and round and round, you get an electromagnet and you can put current through an electromagnet. Actually, in a magnet, on average, you have more electrons going around the atoms in one direction than the other direction. So actually, even permanent magnets are little electromagnets and yes, they become magnetic.

Chris - There's a question at the back here.

Lily - I'm Lily and you said when you stretch the rubber band, you made an inefficient fridge. So, how does a fridge really work?

Dave -  Okay, so a fridge sort of does something similar by using gas. So, if you squash a gas - I don't know if you've ever pumped up a bicycle tire very, very quickly. The end of your bicycle pump gets quite warm. So, if you squash a gas, the gas gets hotter. So, what's happening at the back of your fridge is, there's a pump which squashes the gas then it gets hotter. It then has that long wiggly tube at the back which loses that heat. You then set the squashed gas which is at room temperature, goes inside the fridge and is let to expand. If you expand a gas, it cools down again. So, if you've ever used an aerosol can for long time, it starts getting really, really cold. So you let the gas expand inside of your the fridge, then it gets cooler, sucks energy and heat out of your food, then get cycled back around to the pump and get squashed then the heat is lost. It goes round and round, and round and the fridge gets colder and colder.

Chris - Fridge in three easy steps. Last question down the front. You've got a question for us? What's your name?

Jacob - Jacob.

Chris - What would you like to know, Jacob?

Jacob - I want to know if we snap a rubber band, would it still do the same things?

Ginny - Yes. So, if you snap a rubber band, all that's happening is that those molecules there have pulled apart, but if you used a different area of it, it would still work. Probably, it wouldn't work so well if you used it right at the end because there's not much to stretch there. But yeah, it should still work.

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