This is a transcript of an episode of Public Address Science which was originally broadcast on Radio Live, 17th November 2007, 5 pm - 6 pm.
You can listen to the original audio version of the programme by clicking on the 'Play the audio for this post' link at the top of this page or the 'Audio' button at the bottom of this page.
Background:
[Sound of singing at 'The Last Night of the Proms']
Voiceover:
Along with athlete's foot and haemorrhoids, one of the most annoying things you may ever experience is the horrible noise you're hearing right now -- also known as "The Last Night of the Proms". The sound of embarrassing British patriotism is akin to the howling of dogs as far as I'm concerned -- but what makes it even more deeply irritating is the special pride-of-place given to the rather good song that they're murdering as I speak: Jerusalem.
Rather mysteriously, the flag-waving English are clearly of the opinion that Jerusalem is a patriotic hymn -- although even a cursory glance over the lyrics reveals that this is not the case. In fact, it would perhaps be more accurate to describe Jerusalem as an environmental protest song: perhaps the first environmental protest song ever written.
The author of the original poem, William Blake, was railing against the industrialization of the English landscape and populace [1][2]. The dark satanic mills that these sheep-like Englishmen are singing so cheerfully about, aren't in any way metaphorical -- but rather the very real sulphur-belching cotton mills and paper mills that had resulted from the British exploitation of energy from coal.
Background:
[Sound of wind over high-tension power lines]
Voiceover:
In last week's episode, I talked about the way in which the chemical energy from coal was first employed to produce useful work in the context of pumping water from coal mines. In the subsequent decades, an improved version of this engine -- redesigned in 1765 by James Watt [3] -- began to be employed to provide useful work in other industrial situations. For factories, in particular, this was a Godsend. No longer were they dependent on sites with favourable wind and flowing water
to provide energy: factories could now be located for easy transportation of goods via canal or sea, or for proximity to existing populations of potential workers.
Background:
[Sound of spinning machine]
Voiceover:
And the increases in productivity were remarkable. To give but one example: hand-spun cotton in the early 1700s required more than 1,100 hours of labour to produce a single kilogram of yarn -- but, by 1825, it took only three hours of labour to produce the same amount using a machine (only 1/370th of the previous labour cost) [4].
Unfortunately, by modern standards, the conditions for most of the workers in these new factories can only be described as horrific. With a few notable exceptions the hours were long, the working environment was appalling, the pay was minuscule [5], and severe injuries (and even fatalities) were commonplace [6]. Children as young as seven were employed -- working as much as thirteen hours per day [7]. And the sulphurous smoke and pollution produced by these factories fully justified the description of "dark [and] satanic".
Background:
[Sound of steam engine]
Voiceover:
One of the contributory factors to these acrid fumes was the incredibly low efficiency of the early steam engines. Although Watt's engine was major improvement over its predecessors, its efficiency was still only 4.5 per cent [8]. Each joule of useful work that it produced required an input of 22 joules of chemical energy from coal. And this meant that vast amounts of coal had to be burnt in order to provide comparatively modest amounts of useful work.
So why was engine efficiency so low? Well, the short answer is that science hadn't caught up with the advances in engineering. The early engines weren't energy-efficient because -- in a scientific sense -- energy hadn't actually been discovered yet. In
fact, no-one had even realized that work and heat were both manifestations of the same thing. They were even measured in entirely different units: heat in something called British Thermal Units (or BTUs), and mechanical work in units of foot-pound-force.
Background:
[Sound of stirrer in bucket of water]
Voiceover:
It wasn't until the mid-1800s that the equivalence of heat and work was demonstrated by an English scientist called James Prescott Joule [9]. His apparatus is so simple that nowadays it could be duplicated in anyone's backyard shed. It simply consists of a known weight attached to a rope -- which, in turn, is attached (via a pulley system) to a stirrer in an insulated bucket of water. As the weight is dropped, the stirrer rotates, and stirs the water in the bucket.
What Joule was able to show with his experiment was that the work done by the falling weight in stirring the bucket of water didn't just vanish into the aether -- but was actually transformed into thermal energy, which stayed within the insulated bucket and raised the temperature of the water.
Background:
[Sound of steam engine]
Voiceover:
Joule had shown that [in the normal scheme of things] the "ability to do work" (or 'energy' as it's now called) is not actually something that can be created or destroyed -- merely transformed from one form to another. This means, for example, that the chemical energy from coal is not 'used up' when it is burnt in a steam engine, but merely transformed into other forms of energy: a small amount into useful work, but most of it into heat or thermal energy -- either going up the smokestack as hot exhaust gas, or rejected into the cooling water. A portion of the original coal energy is even transformed into
the acoustic energy that our ears detect as sound.
This was such an important scientific revelation that Joule was honoured by having the measurement unit of energy named after him, and his findings were immortalized as the First Law of Thermodynamics [10] (where 'thermo' refers to heat; and 'dynamic' refers to work). It was the final nail in the coffin for machines that claimed to be able to create energy from nowhere (otherwise known as perpetual motion machines of the first kind). And, for the first time, it gave the means for engineers to calculate just how lousy their engines actually were -- as they made the comparison between heat input to a steam engine and the useful work output.
Background:
[Sound of wind over high-tension power lines]
Voiceover:
But here's an interesting question: from Joule's discoveries about energy it would seem perfectly reasonable to conclude -- as some nineteenth century engineers did -- that the maximum achievable efficiency for a coal-powered steam engine should be close to 100 per cent. After all, if heat and work are equivalent forms of energy then (allowing for a few minor losses) nearly all of the heat obtained from burning coal should be able to be converted into useful work in a well-designed engine. Shouldn't it?
Well, no, actually. Joule's discovery shed light on earlier work by a French engineer, Sadi Carnot. Carnot had speculated that there was an upper limit to the efficiency of engines. This theory of his 'Carnot limit' was further developed by Joule, and one of Joule's collaborators, William Thomson (who later became Lord Kelvin) -- as well as the
German mathematician and physicist, Rudolf Clausius [11]. And eventually this became known as the Kelvin-Planck statement of the Second Law of Thermodynamics [12].
Background:
[Sound of running water]
Voiceover:
To understand -- in simplistic terms -- why this Second Law of Thermodynamics means that an engine can never be 100 per cent efficient, you have consider the fact that all heat engines require a working fluid. This is the substance inside the engine which is heated by burning the fuel, and which produces a work output -- usually by 'pushing' on a piston or a turbine. In a steam engine the working fluid is steam and water; in a car engine or a jet engine, the working fluid is air.
As an engine performs the process whereby heat is transformed into work, then the temperature of the working fluid is reduced. And in order for an engine to transform all of the input heat energy into useful work, then the temperature of the working fluid would have to be reduced to the point at which all molecular motion ceases: a temperature of exactly -273.15 degrees Celsius.
Background:
[Sound of wind over high-tension power lines]
Voiceover:
Now there are obvious practical objections to running part of an engine at this sort of temperature. But, even apart from this, the coldest known naturally-occurring temperature is found in interstellar space -- at 'only' -270 degrees Celsius [13] -- still three degrees short of the temperature that the working fluid in an engine would have to reach in order to fully convert heat into work. In fact, there are theoretical reasons to think that such a temperature can never be reached -- a theory, by the way, known as the Third Law of Thermodynamics.
Because we live on a nice warm planet, the minimum temperature that the working fluid in an engine can reach is, well... just above room temperature. In other words, about 300 degrees Celsius above the temperature needed to fully
convert heat into work. Given the high-temperature constraints on engine materials, this means that the maximum efficiency that can be reached for an engine -- even in theory -- is only around 70 per cent.
Background:
[Sound of a steam whistle]
Voiceover:
The discovery of the first and second Laws of Thermodynamics was something of a good news/bad news story for 19th century engineers. The bad news was that their engines were horribly inefficient; the good news was that the inefficiency wasn't quite as bad as it looked because, in practice, the maximum theoretical efficiency of a heat engine is a lot less than 100 per cent.
Background:
[Sound of a diesel engine]
Voiceover:
And, I suppose, the other good news was that the Laws of Thermodynamics gave engineers some very good information about how to improve the efficiency of their engines. In the late 1800s, this culminated in a machine designed by a German engineer called Rudolf Diesel.
Diesel was a university-educated engineer who set out to design an engine according to thermodynamic principles so that it would attain very high efficiency [14]. His early attempts to fuel his engine with coal-dust were failures, but he finally achieved success when he used a relatively new engine fuel called 'oil' [15]. Diesel's invention would eventually supplant the steam engine in most applications, including ships and the machine that revolutionized transportation in the 1800s -- the train.
But more on that next week...
- Read about the Laws of Thermodynamics in Wikipedia.