Posts by David Haywood
Last ←Newer Page 1 2 3 4 5 Older→ First
-
Thanks for the translation, Linger -- close enough for government work! Apologies if my explanation confused rather than enlightened.
And don't feel all arts major-ish, Emma. Thermodynamics isn't difficult, I assure you. It's just very, very confusing...
Mind you, I won't be providing an explanation of entropy on the radio programme. I just know that some arts major is bound to get all philosophical and Thomas Pynchon on me, and start talking about entropy in literature...
I type these words with an overwhelming feeling of weariness. I've just had a tooth pulled in what my dentist described as "one of the most difficult extractions I've done for a few years". He said it would take only two minutes, but it took the best part of an hour.
Brownie points to the arts major (or anyone else) who can -- without Googling -- identify the author of this passage, which for some reason has sprung into my mind...
A warm flow of pain was gradually replacing the ice and wood of the anaesthetic in his thawing, still half-dead, abominably martyred mouth...
His tongue, a fat sleek seal, used to flop and slide so happily among the familiar rocks, checking the contours of a battered but still secure kingdom, plunging from cave to cove, climbing this jag, nuzzling that notch, finding a shred of sweet seaweed in the same old cleft; but now, not a landmark remained and all that existed was a great dark wound, a terra incognito of gums...
-
Andrew Hubbard wrote:
Surely the working fluid contains a certain amount of [thermal] energy at ambient temperature. Energy is added as the temperature of the fluid is raised, but extraction of 100% of the *added* energy occurs when the temperature of the fluid is reduced back to the original ambient temperature?
I think I see where your confusion lies, Andrew...
In my defence, you'll note that my explanation of the Carnot limit began with the words "To understand -- in simplistic terms -- why this Second Law of Thermodynamics means that an engine can never be 100 per cent efficient...". Thermodynamics is hard enough to explain in a lecture theatre to clever university students, let alone over the wireless to (ahem) Radio Live listeners. So I apologize if I have glossed over some of the details, and perhaps confused you somewhat...
It's rather difficult to explain the Carnot limit without recourse to what my Ph.D. supervisor used to call "The 'E' word" (entropy), which is a whole other can of worms. But -- what the hell -- I'll give it a go...
Let's start by considering a simple piston and cylinder containing a working gas such as air. The working gas has a pressure 'p' and a volume 'V' (obviously, the volume of the working gas is the same as the volume of the cylinder, at any point in time).
We can draw a graph of these two properties (pressure and volume) for an engine undergoing a cycle of thermodynamic processes, e.g.:
Now suppose you want to make an engine that runs on energy from a source of high temperature heat (at temperature T_h) such as a burner. And, furthermore, suppose you want your heat engine to work in the most efficient way possible.
The first thing you have to do is to get energy into your working fluid. Obviously, the best process to do this will be a constant temperature (isothermal) heat transfer, which will result in an expansion of the working gas at T_h (shown on the graph as A-->B). Any other expansion process that results in a reduction of the temperature of the working gas below T_h, will also result in a reduction of efficiency. [An analogy can be made here between temperature and pressure. Imagine a water turbine running from a dam -- ideally you want to keep the water level at the maximum so that you have the maximum pressure to turn the turbine. If you draw off water too fast then the water level (and therefore pressure) will drop and the power output of the turbine will be reduced.]
The isothermal expansion (A-->B) produces a work output -- yay! -- but at the end of the process (point B) you are faced with a dilemma. You want to get your piston back to it's original position, so that you can repeat the expansion/work output process again. But if you simply compress the working gas, then you have to do exactly the same amount of work to get back to the original state as you produced during the expansion process (and, of course, you end up rejecting your thermal energy back to the high temperature heat source). This would mean that you would get no net work output from your heat engine, which would be very unsatisfactory.
The answer, of course, is to perform another expansion process with the high temperature heat source disconnected from the system (shown on the graph as B-->C). The temperature (and pressure) of the working gas will fall, but because the system is not exchanging heat then you are not losing any of the 'potential' of the high temperature heat from the heat source.
This then allows you to compress the working gas at low temperature and pressure (shown on the graph as C-->D), which requires much less work to return the piston almost to its original position. Even though you have to put work back into the engine to do this, it is much less than the work output during the isothermal expansion process (A-->B) -- and so, overall, the engine produces a NET work output. This compression process must, of course, reject heat -- which it does to a low temperature heat sink at temperature T_C (usually the ambient environment). Again, an isothermal process is the best way to do this, in order to keep the temperature difference between the heat source and sink always at the maximum value.
Of course, the engine is not quite back to its original state, as the temperature of the working gas is still at T_C. To return the temperature to T_H, we disconnect the system from the low temperature heat sink, and perform another compression process (shown on the graph as D-->A). This requires a work input, which will be of exactly the same value as the work output in B-->C. But, of course -- to reiterate -- the engine will still have an overall net work output, since the isothermal work output A-->B is much greater than the isothermal work input C-->D.
Thus the engine has produced a work output, and returned to its original state, ready to do the whole thing all over again. This is how the Carnot cycle works.
THE IMPORTANT POINTS:
1. If you think about it, you will soon realize that there is no way to design a more efficient heat engine than this. You could replace B-->C and D-->A with a constant volume (isochoric) regenerative process, or with a constant pressure (isobaric) recuperative process, which will produce an equally efficient heat engine (these are the Stirling and Ericsson engines, by the way) -- but you simply can't design a more efficient system.
2. In order to return the engine cycle to its original starting point, you must reject heat from the cycle to the low temperature reservoir. There is simply no other way to run the engine in a repeating cycle. Because you are rejecting heat (i.e. rejecting thermal energy) then, clearly, you cannot have a 100 per cent efficient heat engine.
3. EXCEPT in the case when your low temperature reservoir is at the absolute zero of temperature (the point where all particle motion stops) at T_C=-273.15 degrees celsius. In this situation, there is no remaining heat to reject from the system at point C, and your heat engine becomes 100 per cent efficient. But, alas -- as I pointed out in the original programme -- you can't actually reach this temperature (see the Third Law of Thermodynamics).
I hope things have now become blindingly clear and your confusion has entirely evaporated. I think the misunderstanding arose from point 2 above -- to put it another way: we are not interested in the amount of energy that is rejected at ambient temperature (of course 100 per cent of the energy must be rejected as either heat or work), but rather the amount of work that is produced by the engine when the heat sink is at ambient temperature.
Although I have explained this at an intuitive level you can also prove it mathematically.
1. Starting from the definition of work as the integral of force with respect to displacement (F.dx), you can substitute in pressure and volume to show that work is the integral of pressure with respect to volume (p.dV). By taking the cyclic integral A-->B-->C-->D-->A you can develop develop an expression for the net work done by the Carnot engine.
2. For an isothermal process there is, by definition, no change in internal energy for the working gas -- and, therefore, heat transfer is equal to work transfer (Q=W). Thus the heat input to the engine is equal to the work produced during A-->B, which, in turn, is equal to the integral of P.dV from A-->B (see 1).
3. Thermal efficiency (n) is, of course, defined as the ratio of work output to heat input. Dividing your expression for net work output by your expression for heat input (and simplifying) will produce the following expression:
n = 1 - T_C/T_H
By inspection, you can see that the only solution for n=1 (i.e. 100 per cent efficiency) is when T_C=0 kelvin (where 0 kelvin = -273.15 degrees celsius) [note: I am disregarding the rather unhelpful case where T_H = infinity].
If you're interested in this topic, then it would be worth learning about entropy (and something called exergy) to gain a more profound understanding of how this all works. A good starting point might be that old workhorse "Fundamentals of Physics" by Halliday & Resnick. A really comprehensive book on this subject is 'Advanced Engineering Thermodynamics' by A. Bejan -- but this starts at a very high level (far beyond Halliday & Resnick).
Probably the best book for you would be 'The Ascent of Mount Thermo' by A.S. Tucker, which is head and shoulders better than any other thermo text I know in terms of explaining this subject. Unfortunately this is still 'forthcoming', but beta versions have been made available to the Mech. Eng. honours students at the University of Canterbury. You can probably buy a copy from the Mech. Eng. office (as I did).
P.S. On the subject of exergy, another way of answering this question is to say that your confusion arises because we are really talking about exergy rather than energy per se. I confess that I am guilty of misusing the word 'energy' all through this documentary -- in fact, it should really be called 'The Story of Exergy' rather than 'The Story of Energy'. But exergy really is a whole other story...
-
K J Aldous wrote
There is some debate about Blake's image of England's dark satanic mills. Some suggest it represented Blake's opinion of mechanical engineers, or the universities that housed them, or perhaps the churches where they, as hungry sheep, looked up and were not fed (Blake was a fan of Milton).
I had not heard this theory before, K J, but I have to admit that it sounds far more plausible than any explanation hatched up by clever-clever poetry experts. Your assessment of the contents of the Tate Modern is rather similar to my own as well.
The last time I visited the Tate, I was the recipient of a protracted explanation about how a stepladder leaning against a wall was symbolic of something terribly clever and artistically deep. When we came back half-an-hour later, a bloke in overalls had climbed up the ladder. Turns out that the 'installation' was rather more symbolic of a bloke about to replace a lightbulb.
Very much enjoyed your comments, K J -- nothing like a good laugh on a Sunday night.
P.S. How did you manage to get my gravatar attached to your comment? I am tempted to sue.
-
Full Brownie points to Mr Jackson, who is clearly much cleverer than Ozzie Freedom at Water4Gas (or any of his customers).
And, of course, the First Law of Thermodynamics dictates that the burning of hydrogen can't produce more energy than was required to make the hydrogen from water in the first place (water is simply "burnt" hydrogen, by the way, and therefore has no chemical potential energy).
The Second Law of thermodynamics also comes into play here as well. In addition to the efficiency losses in the electrolyser, the Water4Gas system's 'hydrogen fuel' approach pays a double penalty under the Second Law. Firstly, there are the entropy-related losses when the petrol is burnt to run the engine's alternator (the alternator makes the electricity in order to produce the hydrogen, and also has energy losses associated with it, of course). And, secondly, you have additional entropy-related losses when the hydrogen is subsequently burnt in the engine.
You could design a stupider system than this, I'm sure -- but you'd have to try really hard.
-
A Public Address Science reader (who wishes to remain anonymous) has asked my opinion on the technology shown here:
I can honestly say that this website (by a company called Water4Gas) is among the biggest loads of mumbo-jumbo, pseudo-science, and unabashed bullshit that I have ever seen in my life. I fall silent in admiration at the artistry of the unlikely-named proprietor, Ozzie Freedom, who is clearly either a world-class fraudster, a nutcase, or a complete idiot (or, possibly, all three).
Despite the claims that this "suppressed technology" device "burn[s] water", it is readily apparent from the photographs that it is simply a crude water injection system coupled to an (almost certainly) highly inefficient hydrogen electrolyser. Water injection systems are, of course, well-known technology (both the Spitfire and Messerschmitt BF109 used water injection in their engines, I seem to recall) which -- properly designed -- will intercool the fuel/air mixture and also acts as an anti-detonation agent. The electrolyser does nothing at all in this context -- in fact, it will actually reduce the efficiency of the vehicle (NOTE: Brownie points, and maybe a prize, to any PA Science reader who can use the Laws of Thermodynamics to explain why this is so).
Personally, I can't decide which is my favourite bit of the Water4Gas website. The news clip that the proprietor claims "shows a broad agreement that water is the solution to our energy problems" (it does no such thing) -- or this gem from the website's FAQ:
Q: What further modifications would I have to make to run my car completely on water?
A: All I can tell you is this: there are less than a handful of people still walking around, who know how to do that. Anyone who even gets near finds himself bought out, threatened or killed. Water4Gas technology is obviously not there because we're not hiding, yet still walking... I guess they'd leave us alone, and leave you alone too!
Or perhaps it's this cast iron guarantee for the Water4Gas technology:
... I personally guarantee that this knowledge will make you happy and proud about it, or we'll refund EVERY SINGLE PENNY of your money!
How do they come up with this shit?
-
Hi Roger,
Yep, there certainly is a Part 7. Public Address Radio has been in re-play mode over the summer break (Dec-Jan), but will resume with new episodes at 5pm on Saturday, February 2nd, on Radio Live.
Part 7 will play then, with Parts 8-12 to follow...
Cheers,
David Haywood -
Judi Lapsley Miller wrote:
It's quite amazing how recent the laws of thermodynamics are.
And, as another PA reader has pointed out, there's a few people (and their sucker customers) who still haven't caught up with thermodynamic developments in the last 160 years or so...
-
Part six of a twelve-part series looking at energy. This week: the changes wrought by the nineteenth century advances in the science of thermodynamics...
NOTE: References, further information, and a complete transcript of this week's episode are available here.
This episode of Public Address Science was originally broadcast on Radio Live, 17th November 2007, 5 pm - 6 pm.
Public Address Science dedicated RSS feed and story archives.
-
Emma Hart wrote:
A phrase which can still mean two entirely different things. How entertaining/traumatic is watching you iron?
It was only a cheap $9.95 iron from the Warehouse. It's amazing to think that for only $9.95 they can afford to sell you an iron AND a piece of hilarious double-entendre. How do they make a profit?
-
Matthew Poole wrote:
... the parliamentary term limit as set in the Constitution Act is entrenched by the Electoral Act with a 75% Parliamentary majority or simple electorate majority. Can't remember if that's single- or double-entrenchment.
Actually, I don't think the AND/OR parliamentary vs. referendum-type amendment is what is meant by single/double entrenchment (although that would seem a perfectly logical interpretation of the terms).
I seem to recall that double entrenchment refers to the entrenching legislation being itself entrenched, e.g. legislation that requires a parliamentary super-majority to change the electoral term, also requires a super-majority to change that legislation.
This has always seemed kind of stupidly obvious to me. On the other hand, I also own an iron with the words 'remove clothing before ironing' emblazoned on the side -- so I guess it's the same principle.