Posts Tagged heat

Why you should clean your heat pump filters (the physics) Marcus Wilson May 28

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A couple of days ago I cleaned the filters in our heat pumps. What prompted me to do this wasn't the cold weather, but the visible build up of dust on the casing of the indoor units. It looked horrible. On opening the unit up, it was clear that the filters were well overdue a clean. Eryughhh. But it doesn't take long to do them, and in just a few minutes they're back inside the pump and its throwing out warm, toasty air again. 

Aesthetics is just one reason to attend to the filter. The second is that dust clogs up moving parts, which means the fan and the louvre on the front. Getting rid of that dust has to be a good thing in terms of mechanical performance. 

But there's also a third reason – one driven by physics. Your heat pump will be more efficient. How does that work?

The basic idea of the heat pump is that it takes heat out of the outside air and shifts it inside. It does it with an expansion-compression cycle, rather like a fridge. Although the air outside might be 0 degrees, it still has heat in it, which can be extracted and shifted inside. The result is that, outside, the air leaving the outdoor unit is lower in temperature than the air entering (to the extent that there isn't a lot that will grow in front of an outdoor unit – event the most stubborn of weeds get frozen out of existence once winter starts), while, inside, the air leaving he indoor unit is of higher temperature than the air entering. Hence the indoor temperature rises. 

But pumping heat from something cold to something warm comes at a cost. It's not the natural way that heat will flow. The bigger the temperature gap between indoors and outdoors, the harder it is to pump that heat. That means more power usage in the form of electricity. Heat pumps work really well for small temperature differences (e.g. the outdoor air is 15 C and you want to heat the house to 18 C) but not so well for large differences (e.g. -5 C to 18 C). The unit may still work at -15 C, but it's less efficient – you'll be getting fewer kWh of heat for every kWh of electricty. 

What has that got to do with the filter? Well, a dust-clogged filter starts restricting the air-flow through the indoor unit. That means there is less volume of air passing the heating element every second, to take away the heat.  If the heating element is still putting out the same amount of heat as before,  it means that it must get hotter. It's rather like the fan on a car radiator. The fan doesn't stop the car engine producing heat, but by increasing the air flow it brings the temperature down.  So a clogged filter means that the heating element inside the indoor unit is going to run hotter, if it's putting out the same amount of heat.  That's bad, since it means the heat pump now has a larger temperature difference to pump heat over, and therefore is less efficient. 

(I'm sure the reality is complicated by the control systems that heat-pumps use – so rather than running hotter it may simply pump less heat – but you don't want that either if you want to heat your home.)

So, cleaning those heat pump filters is a good idea, for a good physics reason. 

A lot of huffing and puffing Marcus Wilson Apr 15

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While there is some great fiction out there, one really shouldn't try to learn much physics from it. One case in point, which I am forced to listen to over and over by the youngest member of our house, is the story of the Three Little Pigs. 

I'm not talking here about the relative merits of various building materials for construction of houses. Straw, wood and brick all have their place. I refer to the rather rapid boiling of the pot of water that the Third Little Pig puts on the fire when the wolf comes knocking at the door. 

In the version of the story that we have on CD, thw wolf, fresh from his succesful huffing- and puffing- of the straw and wood houses, arrives at the home of the Third Little Pig, where  the First and Second Little Pigs have taken refuge. A house made of brick. The door is locked in his face. No problem for the wolf – or so he thinks. A little more huff and puff and this one will be blown in to.  But this time he's mistaken. The house stands still. The angry wolf now resorts to plan B. He puts safety regulations aside and climbs onto the roof of the house, with the intention of gaining ingress via the chimney. 

Time for the third Little Pig to move quickly. He gets a fire going, puts a wolf-sized  pot of water on in, and gets it boiling – just in time, as the wolf drops down the chimney. This version of the story ends with the wolf fleeing in pain (rather than cooked) and the Three Little Pigs jubilant. 

Actually, the story doesn't end, because Benjamin now pushes the button on the CD player to play it again. And again…

Now, just how much power does the Third Little Pig have at his disposal to boil a wolf-sized pot of water in the time taken for a wolf to climb up onto the roof and head down the chimney. The wolf is in a foul mood, so he's not going to hang around. Let's say it's going to take him  minute for this task. A wolf-sized pot might be around 100 litres in size. If it's full of water at about room temperature, this 100 litres of water has to gain 75 degrees Celsius in just 60 seconds. 

One litre of water takes 4200 joules of energy to raise its temperature by 1 degree C. That's called the 'specific heat capacity'. To raise 100 litres by 75 degrees, we therefore need 4200 times 100 times 75 = 31 500 000 joules. This happens in sixty seconds – thats about half a million joules per second. 

What does that mean? One joule per second is one watt of power. So here we have about 500 kW of power – a kW (kilowatt) being a thousand watts.  

This is something pretty substantial. If you've watched the disc on your electricity meter spin around, you'll know that it's rotation rate is a measure of your power consumption. Usually 200 revolutions equals 1 kWh of energy. Do the maths and you'll find that 1 revolution per second (a seriously high domestic consumption) equates to 18 kW of power.  500 kW equates to about 30 revolutions per second. Dizzy stuff.  

If the Little Pigs were relying on electricity they'd be needing to upgrade their mains connection. But they are using wood. Consumer NZ tells me that efficient, domestic wood-pellet fires can produce about 10 kW of power. To hit the 500 kW range, the pigs obviously have a sizeable one indeed. 





A load of rubbish Marcus Wilson Feb 19

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I've been reading through a student's report of his summer work placement. He  had a project on improving the performance of a heat exchanger used for getting rid of heat from a cryogenic cooler. The basic concept is that materials are being cooled to about 40 kelvin (that's 40 degrees above absolute zero, minus 233 degrees celsius, by removing heat from them. That heat needs to go somewhere, and the job of the heat exchanger is to dump it.

I was struck by the similarities of the problem of 'dumping waste heat' with that of 'dumping rubbish'.  What do we do with household rubbish – stuff we don't want that is generated by our day-to-day activities. Well, various things happen to it. 1. Some of it gets put in the pretty blue (in Waipa) recyling box and gets put out on a Thursday morning.  2. Some of it gets put into the equally pretty yellow pre-paid rubbish sacks and also goes out on a Thursday morning, and ends up in landfill :-( 3. Some of it accumulates in the garage, until such time that 1. or 2. applies (or it gets taken to the recycling centre / tip, which is the same as 1. or 2.) 

We do the same with waste heat. It is generated by just about everything that we do. Car engines generate waste heat, car brakes generate heat through friction, electrical appliances generate it, running up and down stairs creates it – basically it's an inescapable consequence of the second law of thermodynamics. Heat gets made, and usually we want to get rid of it. So, what do we do with it?

1. We can, if we're clever, recycle it. A sensibly-designed industrial plant will tap into the waste heat it makes to do useful things. Smart tumble dryers will use the waste heat in their exhaust to pre-heat the dry air being sucked into the machine, saving electricity. Heat engines can be put into effect where there is a consistent difference in temperature between two objects. 

2. We can dump it. That's what happens to most of it. We let it end up in cooling water, or the atmosphere, where we conveniently forget about it. However, unlike landfill, it's not usually a problem in itself (warm rivers near power stations might be, however). The amount we generate over the earth is pretty tiny compared to the amount that the sun gives us. The real issue is the amount of carbon dioxide and other greenhouse gases that have been generated in the process (plus, economically, the generation cost of the energy that is being wasted). 

3. We can store it and do something with it later. This isn't so easy, but can be done. We can exploit gels that have high latent heats, meaning that as they undergo a phase change they take in heat, and then as they undergo the reverse change they will give it out again. We can heat up objects with high heat capacity, keep them well insulated, and then release the heat later (e.g. night storage heaters). 

So does 'dumping' heat mean that we're treating energy in a similar manner to rubbish? Chuck it away and pretend it's not an issue. With the landfill problem, the first thing to address is not recyling, but simply not to consume so much stuff in the first place. If we did the same with heat, we'd have lower energy bills and lower greenhouse gas emissions. The two, and their problems, are perhaps not so wildly different. 

Here's a final thought then. My work emails (if I don't delete them) get stored somewhere in the world on a server belonging to a well-known and rather enormous company. How much power does it take to keep one of my emails on file? I don't know the answer to that one. If everyone in the world deleted their email and the data they really didn't need anymore, what difference does it make to the world's energy consumption? Anyone know?


Heat and water and making nappies Marcus Wilson Dec 06

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In the lab, my summer student has been working on a small device to keep a small piece of equipment at a stable temperature. It uses a Peltier device – in essence it's a solid-state heat pump. Pass through current one way, and heat is drawn from the top surface to the bottom; pass current through the other, heat is drawn from the bottom to the top. Therefore, by putting the equipment on the top surface of the peltier device, we can control how much we heat or cool it by via how much electric current (and in which direction) we pass through. 

There are a few things we need to consider, however, to get this to work well. One is the thermal resistance between the peltier device and the object. We need there to be a good thermal contact between the two, otherwise the flow of heat is going to be hampered. It would be rather like putting insulation around radiators in your house. It will keep the radiators nice and warm but it won't do much to the temperature inside your house. We need to ensure that the glue we use to hold our equipment to the Peltier has high thermal conductivity.

But also we are interested in knowing how quickly the equipment changes its temperature in response to heat input. This is quantified by its heat capacity – how much energy (heat) is required to raise its temperature by a given amount. Something with low heat capacity will change its temperature quickly, something with a high heat capacity will change its temperature only slowly.  A large lump of something, like the water in the university swimming pool, has a large heat capacity, and therefore takes a long time to heat up once it's been filled (and consequently remains very cold until January). Do we want our equipment to have a high or low heat capacity? That's not  entirely obvious. Our aim is for something that remains at fairly stable temperature – that neither heats up nor cools down quickly. Otherwise controlling the temperature becomes very difficult. That would suggest a high heat capacity for it. But we don't want it too big or our Peltier Device would never be able to bring it up to the temperature that we'd like. There's a bit of a balance to be had here.

What struck me this week was the obvious parallel with nappies. Well, I guess it's obvious to any physicist who changes nappies on a regular basis. The perfect nappy needs to take urine away from the skin quickly, and also have a high capacity to hold it. The first task is equivalent to the thermal conductivity, but with water. The fluid needs to be able to flow quickly from the skin to the absorbing bit of the nappy. The second task is the equivalent of the heat capacity – we need the material to absorb lots of water while not getting very wet (equivalent to absorbing lots of heat but not raising its temperature very much). The cloth nappies we use have a two different material textures. The first bit, that is in contact with the skin, sucks water away very quickly. The second part holds onto the water very well. Working together, they keep baby dry for longer, which sounds like a rather corny tag line for a nappy brand. 

And, yes, I've taken a clean dry nappy to the bathroom with a measuring jug and slowly poured water in to see exactly how much one would hold. Could you expect a physicist to do anything else?



Thermodynamics of learning Marcus Wilson Nov 27


Last week I attended a conference on Emergent Learning and Threshold Concepts, here at the University of Waikato. It was a very interesting couple of days. As far as academic conferences go, it was unusual in that it was really cross-disciplinary. We had engineers mixing with physiotherapists, and management consultants with dancers. It certainly was interesting to hear about how other disciplines approach educating their students. A challenge faced by everyone presenting, me included, was to make the presentations accessible to someone with no expertise in the area whatsoever. It was a job that was surprisingly well done. 

I'm not going to mention here what I talked about (you can find it on the ELTC website if you are that interested). Rather, I'll talk about what my colleague Jonathan Scott presented. He's been looking at Threshold Concepts and Learning for a while now and had some observations to make which he cased in terms of thermodynamics. Jonathan had to keep it pretty maths-easy for those in the audience that weren't mathematically inclined (probably most of them) and I think he did a good job. Here's a potted summary of things.  

When we learn something 'thresholdy', things get more ordered in our brain. Pieces of information fit together better. We can see how concepts work, rather than just being pieces of knowledge. Things come into order. In thermodynamics, order is associated with a quantity called entropy. Specifically, something well ordered has low entropy; something with little order has high entropy. Ice has less entropy than water (since its molecules have an ordered structure), but water has less entropy than steam (since even in water there is some degree of ordering among the molecules).  We give entropy the symbol 'S'.  (Actually, I've never stopped to think why it's 'S' for entropy  - Does anyone know?) 

Another key quantity in thermodynamics is heat. Heat is a form of energy. Practically, however, it's not always the best quantity to work with. That's because if we do experiments at constant pressure, which is what the laboratory usually has, gases and liquids expand when they heat up. That means a more useful quantity to work with is enthalpy. It's like heat energy, but it takes into account the fact that things can expand and contract, so the amount of stuff in say a 1 litre volume changes. When ice melts into water, for example, there is a change in enthalpy of the system. We need to put energy into the ice to melt it, which means that the enthalpy of the water is higher than that of the ice. We often give enthalpy the symbol 'H'. 

We can combine the effects of a change in enthalpy and a change in entropy in something called the Gibbs' Free Energy.  We give it the symbol 'G'.  Specifically, it's the enthalpy minus the produce of temperature (T) and entropy – in maths terms G = H – TS.  Now, here's the neat bit. To make a system change its state (e.g. ice into water) the change in Gibbs' free energy needs to be negative. For ice turning to water, we note that the change in entropy is positive (more disorder). The change in enthalpy is also positive. To get the change in G to be negative, we need the temperature T to be large enough. At atmospheric pressure, if T > 0 degrees C, it will happen. If not, it won't.  

What has that got to do with learning. Well, here's Jonathan's analogy. To learn a threshold concept, we need to have a move to more order. But a large, negative change in entropy means -TS is strongly positive and so if this is to happen we need to make the change in H (energy) strongly negative. In other words we need to 'take the heat out' of the system. If the system is 'the student', then this equates to getting the student to do lots of work. (Remember the first law of thermodynamics: Heat and work are equivalent). If a system does lots of work (on something else), it loses heat. A good example is gas from a pressurized bottle doing work as it moves to atmospheric pressure and expands  - the nozzle of the bottle will get cold. The bigger the ordering that is required in one's thoughts, the bigger the amount of work that the student needs to do.  The process is assisted by a lowering of the temperature – a 'cool' environment (as opposed to a hot one with too much going on)  helps the student learn. 

Perhaps all this is taking a physics analogy a bit too far. If we think of the message as being "to get thoughts to order together is actually quite difficult" then it's got merit – that is really what the Threshold Concept environment is about.

Finally, it's been noted that Threshold Concepts, are indeed, a threshold concept. Therefore if you struggle to see what I'm commenting on, you need to do some more work ;-)




My car is cold Marcus Wilson Oct 16


The last couple of days have seen our Engineering Design Show. This is where our 2nd/3rd/4th year Engineering students get to talk about and show off the various projects they've been working on in the last year. It's very interesting to see the range of activities going on, and there are some 'competitive events' – this year the 3rd year mechanical engineering students had to design a seed-planter to automatically put pine seeds into seed trays – accurately, quickly and cheaply. 

For me, the stand out talk was by one of the fourth years – Matt Dromgool. He talked about a solution to the problem of heating in electric cars.

This problem is something that had never occured to me, though it is extremely obvious when one thinks about it. In a petrol/diesel car, providing heating to the interior is easy. Just blow in some of the waste heat from the engine. A small radiator and fan does the job and often has the added bonus of saving your engine from overheating when the main fan fails. The one obvious disadvantage of this method is that you don't get instant heat when you start the engine from cold. When it's minus ten or lower outside, instant heat is often what you want. Some cars have wire-mesh heating elements built into the windscreen for quick defrosting on cold days. 

However, an electric engine doesn't generate much waste heat. There's not enough to warm the interior of the car (and, more importantly, the windscreen) adequately. So how do manufacturers of electric cars solve this problem? The simple solution is to stick in a resistive heater – just like a bar element on an electric fire. But where does the energy come from? From the battery, of course. The trouble is, to heat the car, you need a lot of heat. Matt put up some data from one electric car manufacturer that showed the range on a full charge dropping from about 160 km to about 50 km when the heater was turned on fully. That's a pretty severe drop in performance. 

Ironically, a lot of electric cars carry air conditioning systems. That's because many are little more than petrol cars with the petrol engine pulled out and an electric one slotted in – not much else gets changed – and the air conditioner stays in place. So an obvious solution is to allow  the air conditioner to run in reverse – to move heat from outside the car to inside, not the other way around. This is what a domestic heat pump does – the same system can either heat your house or cool it depending on the direction in which the fluid flows. So Matt's project looked at converting a conventional car air conditioner (which just does the cooling bit – it has no need to supply heat in a petrol car) and modifying it to allow it to run on a reverse cycle as well, to do the heating bit. A nice, simple, cost-effective solution. Sure, it still runs on electrical energy, but it uses much less energy than stuffing a resistive heating element in the car. It also carries the advantage that one can now get instant heating when required on really cold days. 

I wonder when we'll start seeing them in electric cars. 





Temperature is not Heat Marcus Wilson Sep 03

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First things first. PhysicsStop is back on-line after an enjoyable two-week break in warm and sunny southern England.

Second things second. What advice can anyone give to the parents of a fourteen-month-old with jetlag who insists that 4 am is time to get up, have breakfast, and feed the chickens (or the "Choo Chuk" as he calls them)? In the end we got up, had breakfast and let the chickens and the neighbours get some sleep. Consequently I'm rather tired and feel like it's ten past two, not ten past eleven. 

So, here's a bit of quick physics from our travels. If you're fortunate enough to fly with an airline that still gives out hot towels to its economy class passengers, you'll know that the towels can be extremely hot – almost untouchable at first. But, once you've unrolled them and used them for whatever purpose you can think of, they cool down very rapidly. They don't stay hot for long, and you hand them back cold.

In physics terms, the towel, as it is presented to you, has a high temperature, but it doesn't have a great deal of heat. While we can use these words loosely and almost synonomously in everyday conversation about the weather, in physics they are very distinct quantities. Heat is a measure of the thermal energy in something. It's measured in joules, just like any other form of energy. The energy resides in the thermal vibrations of the water molecules. Heat is an extrinsic quantity. If one doubled the amount of the material (had a towel twice as big), one would have twice the amount of heat. 

Temperature is a lot harder to define in simple terms. (Try making sense of the Wikipedia entry on it). There's a nice physical definition (rate of increase of energy with respect to entropy) but that's not terribly intuitive. It's easier to think of temperature as an 'average' thing – broadly speaking temperature is proportional to the average kinetic energy per atom in the material. Each atom in something that's hot will have more kinetic (movement) energy than something that's cold. Any physicists reading this will realise I've given a horribly simplistic definition, but it is roughly correct. A key thing is that temperature is an intrinsic quantity – if you double the amount you have (a towel twice the size), the temperature (average energy per atom) stays the same.

Our towel starts at a high temperature. However, because it is thin, there isn't actually a lot of heat in it. That means that it quickly loses what heat it had once unrolled, and the temperature, which is what you perceive on your skin, drops.  Contrast this to a lump of rock that's been sitting in your oven at 70 degrees for five hours. Pick that out and see how long you can hold it for (actually, don't try it). It contains far more heat than a hot towel, so it takes much, much longer for it to be lost.

Hotspot and Silicone Tape Marcus Wilson Aug 09

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Well, today’s big story is just perfect for PhysicsStop. Cricket meets physics. What more could I ask for.

In case you’ve just arrived from Alpha Centauri,  there have been accusations flying that both English and Australian batsmen have been trying to defeat the ‘Hot Spot‘ detector by putting silicone tape on their bats. The allegations have been vigorously denied from both sides. 

Hot Spot is used as part of a decision review system in professional cricket. The idea is that it will provide evidence as to whether the ball has hit the bat or not when assessing possible dismissals. It uses thermal imaging (infra-red) technology to look for the heat left behind when the ball makes contact with a surface. As the cricket ball just skims the edge of the bat, friction between the two will generate a small amount of heat at the point of contact. The thermal imagers can detect this heat and therefore prove whether the ball hit the bat or not. At least, that is the intention.

So how might silicone tape (a fairly innocuous medical product) give the batsman an advantage? The allegation being made is that a batsman would put tape on the outside edge of the bat, which reduces or eliminates the ‘hot spot’ left by a ball grazing the edge. Presumably they’d leave off the tape from the inside edge, so as to make sure that a fine edge on to their pads gets detected to counter any appeal for leg-before-wicket. (I admit that anyone who doesn’t know cricket will not have a clue what I’m talking about at this point, but hopefully you can still follow the physics part.)

Presumably the thinking is that silicone tape reduces the frictional forces between bat and ball, and therefore reduces the heat generated during a collision between the two. Would it work? One would need to try it out to be sure. But a quick glance at some values for coefficients of friction (e.g. here) will show that there is a vast range of values depending on the two materials. Some combinations surfaces have much more potential for friction (and therefore heating) than others. So it’s plausible that a low friction tape might have the effect. (Though one would think there might be more effective methods – e.g. spraying the edge of the bat with a lubricant spray. The thinking might be that applying tape to a bat is, bizarrely as it might sound,  actually legal in cricket.)

There’s been some discussion on the blogs that it has to do with thermal conductivity, though I’m not convinced by this argument. To defeat Hot Spot in this manner, one would need a material that gets rid of the heat very quickly by spreading it to other areas, so a noticeable hot spot doesn’t persist. The problem is that the thermal diffusivities of everyday materials are too low for this to happen. Thermal diffusivity controls how quickly heat spreads out by conduction. Even the very highly diffusive materials, with thermal diffusivities of around 100 mm2/s or so, would have a spot of heat spread out by only 10 mm in a second (The square-root of the product of thermal diffusivity and time tells you roughly how far heat will spread in that time). The Hot Spot frame rate is much shorter than this so there’s not time for the heat to diffuse away.

But I can think of another mechanism by which the tape might fool Hot Spot. The amount of infra-red light emitted by a surface doesn’t just depend on its temperature. Some surfaces are better emitters than others. A perfect emitter is called a ‘black-body’ in physics. However, be warned – an object that emits infra-red really well doesn’t necessarily look black to the eye – and conversely don’t think that because something is white that it doesn’t emit infra-red well. Some materials have properties that are very dependent on wavelength. It is possible (I don’t know) that silicone tape has a lower emissivity than wood, and therefore the effect, as viewed by an infra-red camera, would be reduced. Possibly it’s a combination of reduced friction and reduced emissivity.

Then again, possibly this is just a media propaganda stunt to try to get some interest back into the last two Ashes tests. (Again, non-cricketers won’t have a clue about that sentence).

All this would make a great student project. I’m sure there’d be physics graduates queuing up to do a PhD in defeating cricket technology. 




Don’t cook the baby Marcus Wilson May 31


 Last week we had a new oven installed. Our old el-cheapo one that came with the house was never in Karen’s good books. Small, dirty, incapable of getting to a high temperature and generally giving the impression that it was about to die at any moment. Indeed, it did a couple of Christmases ago – it refused to do anything. It turned out that the clock/timer chip had given up and was sending a permanent signal to the oven that it should turn off. The repair man took about a minute to diagnose it and another minute to bypass the clock chip – which meant we couldn’t use automatic on/off settings or anything fancy but at least we could turn the oven on and off manually.

But the days of that oven are gone, and it’s been replaced by something much more sensible. (I don’t do product placement so I won’t tell you make it is here) The thing that most strikes me about the new one, other than it does what you tell it, is how well thermally insulated it is. It’s a cabinet-mounted oven, rather than part of a freestanding cooker. When we turned up the old one high, the oven door got very hot and the cupboards next door became rather warm too. With the new one, the cupboards next door hardly change in temperature, and the oven door itself, is only slightly warm. This has the unintended benefit of meaning the baby is rather safer in the kitchen – if he puts his hands against the oven door he will barely notice a temperature change. (Not that we encourage him to try – his latest kitchen trick is trying to climb into the dishwasher – something that even the cat though better of)

The thermal resistance of a material is defined in terms of the temperature differential between its front and back surfaces needed to drive a given amount of power (heat per unit time) through the material. Thermal resistance obviously depends on both how thick the material is (double the thickness, double the resistance) and also its area (double the area and you’ll get twice the heat flow for the same temperature differential). We can take the area out of the equation by moving to a U-value – this describes the power loss for a degree kelvin (or celsius) difference between a unit area of the two surfaces. A low U-value means the material is a good insulator.  Still, U-value depends on how thick the material is. Taking this out of the picture and we get thermal conductivity, which is an intensive measure of how good a material is. Whatever is surrounding our new oven and is in our oven door surely has a pretty low thermal conductivity. The thought has occured that there is a vacuum somewhere – that has low thermal conductivity indeed!

 P.S. While putting in links for some of these terms I notice that there is some ambiguity in what people refer to as thermal resistance. I was taught it as being the temperature differential required per unit power (so SI units kelvin/watt – analogous to electrical resistance), but it also seems to be used as the reciprocal of the U-value. Not the same thing. 



The amazing vacuum microwave Marcus Wilson Apr 03

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 Happy Easter everyone. Sorry for lack of blog activity – lots of marking has been building up that I’ve needed to get through. 

Yesterday we experienced the vacuum-packing ability of a clip-container in a microwave. In this case, it was being used to cook some vegetables for Benjamin’s dinner. The veges were placed in the microwave, the lid put on, and then zapped for a few seconds. The problem was then taking the lid off, since it had sealed tightly shut. 

I’ve had a comment on my blog about this before, from someone who’s experienced it. I think what’s happening is that, as the contents heat up the air inside expands. It is able to push it’s way out through the seal. The mass of air on the inside is then rather less than what it was to start with. Once the heating has stopped, however, the temperature reduces and the air contracts. However, this time the seal doesn’t let air back in – instead the lid is sealed and the air inside reduces pressure. Consequently we are left with lower pressure on the inside than the outside.

Just how big a pressure difference do we have? Suppose the air inside is heated to 100 C, as opposed to the 20 C that it is on the outside. At constant pressure, volume scales as absolute temperature, so we have a volume increase of about (100 + 273) / (20 + 273) =  1.27 times. That is, about 30% of the air is pushed out in the heating process. This air doesn’t get back in during the cooling. Therefore, once cool, the container has 30% less pressure inside (pressure being proportional to volume at constant temperature).

What does this equate to in everyday terms? Air pressure is about 100 kPa, meaning a force of 100 thousand newtons over an area of 1 metre squared. 30% of this would be 30 000 newtons over a metre squared. Since a kilogram weighs about 10 Newtons, that’s about the equivalent of 3000 kg spread over a metre squared. 

Now, the little container wasn’t a metre squared in area. It’s about 10 cm times 6 cm (approximately) , which is 60 cm2 or 0.006 or a metre squared. Multiply that by 3000 kg per metre squared, gives us 18 kilograms. That is to say, the force due to the air pressure is equivalent to sticking about 18 kg of mass on top. Little wonder it was tough opening. 

This calculation has a few assumptions in it, not least that the air had cooled back to room temperature (it hadn’t). The reality I think is that it would be rather less force. I managed in the end to get a flat knife under the seal and let some air in – that got the lid off. 

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