What's the most efficient machine?

11 September 2018

Question

There are no perpetual motion machines but what is the most efficient machine known?

Answer

Steve sent this question in to our brainy panel. Chemist Peter Wothers, Astrophysicist Matt Bothwell and Human Physiologist Sam Virtue all had different thoughts on the matter... 

Peter - What people were trying to do when they were trying to create perpetual motion machines would make something that would always be moving. And that is something that, of course, does happen in that, for instance, vibratrating bombs so a simple, going back to our oxygen molecule O2, this molecule is always vibrating so it is always moving. In fact, even if you cool this thing down, even to absolute zero, it is still going to moving which is quite remarkable.

Chris - Really. I thought absolute zero did stop. Is that true?

Peter - I think it has to be moving, yes.

Chris - Really. Is that the case. Wow! So nature has already invented a perpetual motion machine, is that what you’re saying?

Peter - But the problem is you can’t get it to do anything useful. So if that’s what you want out of your machine so this comes back to the efficiency thing, then there’s a problem.

Chris - Okay. So Peter has challenged Steve’s contention that there are no perpetual motion machines. What do you think Matt about the idea about efficiency? What does he actually mean by efficiency though Matt? What is that?

Matt - The efficiency of a machine is just the amount of useful work you can get out of the machine divided by how much energy you put in.

Chris - And so what sorts of levels of efficiency might we consider for various things around us in the world around us then?

Peter - The question actually is why is some of this energy lost, and so why can you not convert all of your energy into useful work? And then this comes down to the idea of disorder and entropy, that this is the key driving force and so everything is getting more and more disordered. Some of this energy, in a sense is actually creating a lot of disorder and this is what every machine has to do - you can’t convert all of your fire heat energy into lifting a weight, for instance, or something is always going to be lost.

Chris - Indeed, a coal fired power station is what, 50 percent efficient, isn’t it, between 30 and 50 percent? A car is about 30 percent efficient so, in other words, 70 percent of the fuel that you burn does not turn into movement of your car, which seems ridiculous, doesn’t it, when we put it those terms?

Peter - It sounds really ridiculous. I think some of the more efficient engines will then be working on fuel cells which is a very efficient way to get that energy out of - well the chemical energy there.

Chris - And this is where you’re exploiting the energy and bonds components, isn’t it, to actually make bonds in order to release some energy?

Peter- Yep. But even so, it is absolutely impossible to convert 100 percent of your energy into useful work energy. So you’re never going to get to 100 percent.

Chris - So it’s worth going to the gym because that’s kind of useful for where you come from, isn’t is Sam, with the obesity business? It’s a damn good job that the body isn’t 100 percent efficient because it would be a lot harder to get that weight off down the gym.

Sam - Also, biological systems are, essentially, machines as well, and they’re pretty good. I mean the efficiency of us, as humans, when we’re running will be over 30 percent and that’s fairly equivalent to some of the better motor cars, so yeah. And what humans really turn their energy into is heat, so that’s what we mostly lose it as. If we go to lift a weight or something down at the gym, maybe about 30 percent of the food we’d be consuming is going into lifting it and the rest is going into heat production.

Chris - It’s kind of good though because when you’re chilly and you shiver, you’re basically making your muscles do lots of pointless work and releasing lots of heat as a byproduct which  you then shove back into your body to warm up on a cold day?

Sam - Yeah, absolutely. And actually there’s another type of organ we work on which is called brown adipose tissue, and this is an organ where it’s express purpose is not to store energy, but to convert energy into heat. So small organisms like mice, rats, and in fact infant humans have a lot of brown fat. We know know some adult humans have brown fat. And so I wonder if I could cheat and argue that this is a very very highly efficient organ because all it does is convert chemical energy into heat, and that’s it’s purpose. So, in a sense, it’s very inefficiency by the classical definition of have much work do you get out of it makes it incredibly efficient at doing it.

Chris - Because, as you say, kiddies have a lot more as a proportion than adults - we lose it as we get older, don’t we? Is that because children have a very big surface area to volume ratio so their rate of heat loss can be much higher, so they have a bigger challenge staying warm so they compensate by having this brown fat?  Or is there some other reason why we lose it? Does it burn itself out as we get older?

Sam - No. It’s essentially what you’ve just suggested in the first point - it’s a device for making heat. Many adult humans, and a good proportion of them do have brown fat, but it’s certainly more prevalent and more active in infants. And so, being a bit of a geek, I have a three and a half year old and a one year old and I borrow the thermal camera from the lab and decided to try and photograph my son to see if his back lit up, which it did look quite convincing

Chris - It’s between the shoulder blades, isn’t it, you have a big patch of it?

Sam - Yes. And we have mainly in humans it’s found more actually in the neck, and there may be some between the shoulderblades as well in infant humans. But yeah, so it was probably not ethical.

Chris - So that’s your own inbuilt central heating system?

Sam - Into heat, yeah.

Chris - Matt?

Matt - I think my vote for a most efficient system, because I’m just piggybacking on what you said about biological systems, I think my vote for a most efficient system might be a person on a bike. You can transport 100 kilogrammes or more, hundreds and hundreds of miles on a relatively small input of energy.

Chris - So the wheel was an amazing thing?

Matt - It was, yeah.

Chris - I think it’s 5 thousand years old the wheel. So we probably made one of our most important invention in efficiency terms, you’re arguing, a long time ago.

Matt - Yeah, that’s right.

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