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Posts Tagged heat

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. 

Structural failure: Jam yesterday Marcus Wilson Jan 23

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We’ve had a bumper crop of plums from our two plum trees. Way more than we can eat our way through in the short plum season. It appears that we aren’t the only ones – the last couple of weeks have seen bucket loads of free plums turn up in the tea-room here. (And yet they are several dollars a kilo at the supermarket.) So, yesterday, Karen had a go making plum sauce, to add to the plum jam and frozen plums we already possess. As far as I can work out, the difference between plum sauce and plum jam is merely that plum sauce is more saucey. The mechanism appears to be the same – everything gets cooked up in a large pot and the hot sauce gets added to sterilized preserving jars. The lids go on and should seal shut as the temperature inside the jar drops and the small amount of air inside loses pressure.

A lid that’s popped downwards is a good sign that there is a decent seal between the lid and the jar. If there weren’t, then air could get in and equalize the pressure between inside and outside, and up would pop the lid. Conversely, if there were, for some reason, some multiplying nasties in the contents, producing carbon-dioxide, the pressure would build up inside and the lid will buldge upwards. That’s a sure sign that what’s inside isn’t edible. The trick is to spot it before it explodes at the back of the pantry and splatters jam and glass everywhere.

That’s why there’s a warning on shop-bought jam – if the ‘button’ isn’t down, don’t eat the contents.

However, all this only works if the jar is up to scratch. I came home from work yesterday to discover that parts of the kitchen had been painted in plum sauce. This wasn’t the work of the baby – it was down to a jar that had failed. When the sauce went in, the lid went on, and, sure enough, the lid popped downwards. But it wasn’t the only thing that ‘popped.’  The jar did as well, leaving it with no bottom.

There are a couple of reasons I can think of why this might have happened. First, it could simply be down to rapid thermal expansion of the glass. When the hot sauce goes in, the inside of the glass jar gets hot much more quickly than the outside, and so there is stress built up as a result of different amounts of thermal expansion on the inside and outside. This is what happens pouring boiling water into a cold glass.

Or, perhaps the bottom of the jar acted as a popping lid. If there were some air trapped at the bottom, it would reduce its pressure as it cooled, and create a force on the bottom of the jar. If that force were great enough, it could break the glass. An implosion, rather than an explosion.

Now, neither should have happened because the jar concerned was a proper preserving jar, designed for the purpose of having hot stuff poured into it and being sealed. But, for some reason, it did. I was tempted to bring in the jar to work today and get one of our materials engineers to examine it to determine the mechanism of failure. It would have been interesting, but I thought they had better things to do with their time, so we may never know exactly what happened here. But the good news is that there are still several intact jars of sauce, so we will be well supplied for the coming months.

 

 

 

The power of steam Marcus Wilson Jan 18

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Over Christmas, we were staying with my wife’s sister and her family in Dunedin. Early one morning (sometime before I got up, anyway – that is my definition of early) a loud ‘bang’ came from the direction of the kitchen, followed by the sound of eight paws beating a hasty retreat. There are two young cats in that house, and my  first thought was ‘pesky felines – I wonder what they’ve done now’. And I went back to sleep.

As it turned out, the cats were entirely innocent. The culprit was the coffee pot (now ex-coffee pot). It was a stove-top machine – one where water goes in the bottom half, the coffee is placed in a holder and inserted on top of the water, and the top half of the pot (which will contain the finished product) is screwed on. Whether there was too much coffee in it, or it was packed in too tightly, or some other problem, we don’t know, but clearly the steam made in the bottom wasn’t finding its way to the top. Instead, as the water boiled, the pressure inside the bottom half just kept increasing, until there was a mechanical failure and the top half departed from the bottom half in a hurry.

The bang I heard was the top half hitting the underside of the range hood at high speed. The dent in the latter was sizable. There was also a trail of coffee up the wall above the stove, and onto the ceiling, where coffee grinds were pasted in place. And splatterings of coffee were in every cup, box, drawer, container and cupboard that had the misfortune to be in the kitchen at the time.

It was rather fortunate that no-one was near it at the time. Beware the ideal gas law: Pressure times volume is proportional to temperature. In this case, with a constant volume, high temperature implies high pressure, and there is only so much pressure a coffee-pot can take.

 

 

Undiscovery in physics Marcus Wilson Nov 28

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With the recent undiscovery of Sandy Island I’ve begun wondering what other things might be ripe for undiscovery. Wasps, for example. Wouldn’t it be great if we realized that there wasn’t actually any evidence for the existence of wasps after all. Their discovery had been just a mistake made by an entomologist back in the depths of history. We can all tell our children not to worry about them – they don’t exist. Our chickens would love to see the neighbour’s cat undiscovered (as would we – at least from our garden).  I’m sure a variety of places might feature strongly too. Hamilton is bound to be on the list of some people; but, I can assure you, the last time I looked it was still there.

I don’t think in physics there has been a great deal of undiscovery in the last few centuries. I struggle to think of any real undiscoveries.  Sure, there have been changes to our thinking. For example, relativity superseded Newtonian physics, but it would be wrong to say that Einstein undiscovered Newton’s Laws of motion. The latter are still a cornerstone of physics – but their applicability has been reduced to the realm where things aren’t travelling close to the speed of light. That would be more like discovering the coastline of Sandy Island is a bit different to what the maps have it. One might say that the Michelson-Morley experiment undiscovered the aether, but in reality the aether had never been discovered – it was just a well-accepted hypothesis. Likewise Joule’s experiments with heat put pay to the idea that heat was a fluid, but since no-one had claimed (supported by real evidence)  to have observed this fluid, it wasn’t really an undiscovery either.

Underlying modern science (by which I mean Galileo and beyond) is experimental evidence. No change in understanding of science, in any discipline, is going to happen without well collected and well analyzed data. This makes undiscovery of something (by which I mean overturning of some knowledge, theory or principle that has been believed based upon evidence, as opposed to mere hypothesis) unlikely. There have been a few instances of reports of new things that have been made prematurely, with unreliable evidence, such as cold fusion and faster-than-light neutrinos, and these have been embarrassing for the groups concerned and undiscovered very rapidly.  But undiscovery here has happened because they were never properly discovered in the first place.

That said, neither was Sandy Island properly discovered. My spell-checker’s underlining of the word undiscovery may be for good reason.

I’d love to hear readers’ thoughts on this one. Is there any piece of modern science that has been genuinely undiscovered?

 

 

 

 

Back to work Marcus Wilson Jul 25

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It’s been great having time off work with Karen and Benjamin, but, as they say, all good things come to an end. So today it’s back to work, with my first class in about an hour and a half. I spent my final afternoon of parental leave probing the underlying geology of our driveway, trying to work out why it was flooding in patches. (The fact that it had deluged for two days pretty-well non-stop was clearly half the reason, but why wasn’t the water clearing?) As it turned out, the gravel top surface was hiding sizeable potholes underneath, which were just filling with water, which wasn’t draining. When you’ve got time to do that, you know that you don’t need to be on parental leave anymore!

But that’s not the subject of today’s blog. It’s Microwave sterilizers. They are a handy device for rapid sterilization of bottles and other baby parephanalia. Fill with 200 ml of water, zap on full power (in our case 900 W) for 5 minutes, and it’s done.

A quick physics calculation will show what’s happening. A 900 watt microwave will put out a total of 270 thousand joules of energy in five minutes (that’s 900 joules per second times 300 seconds). A paltry 4.2 joules will take 1 ml of water up in temperature by 1 degree C  (that’s the specific heat capacity). So to take 200 ml of water from the tap (at say 15 C) and take it to 100 C (boiling) requires 85 times 200 times 4.2 equals 71 400 J of energy. That’s about a quarter of what is being supplied.

So the water will reach boiling point. But where does the extra energy go (there’s about 199 thousand joules of it)? This is put into turning the water at 100 C into steam at 100 C. It requires a considerable energy input to make this step. The latent heat of vaporization of water is about 2300 joules per gram (or ml of water). This means that the extra 199 thousand joules will turn about 80 or so ml into steam.  That means there should be plenty of water left at the end of the proceedings.

Of course, the steam will start condensing on the bottles and internal surface of the sterilizer – these have a specific heat capacity too and will take some energy to come up to a similar temperature. But the rough calculation above I hope shows that the 5 minutes for 900 W statement by the manufacturer is pretty sensible.

Oh, and it has now stopped raining.

 

 

Why you shouldn’t bother buying a thermometer Marcus Wilson Jun 18

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Well, looks like winter has finally arrived here. There’s not much worse weather-wise than having a clear night with the cloud rolling in just as the sun rises. The clear night lets the temperature drop, as the ground radiates away more energy than it receives from the atmosphere and surrounding objects, and then the clouds stop the sun from warming the place up again.

Last weekend, we were getting odds and ends for the house, and I looked at thermometers for the baby’s room. (And, needless to add, still no baby.)  I was surprised to see that it is still legal to sell mercury thermometers here. They aren’t easy things to deal with at home  if you happen to break one. But, safety aside, I wouldn’t buy a mercury thermometer anyway, since they can be so unreliable. I illustrated this to Karen by pulling off five from their hooks, and looking at the range of temperatures they showed – the lowest was reading 16 degrees while the highest was recording 18.5 degrees C.  I suppose I could have got one that was reading in the middle.

I had a thermometer once that I calibrated by putting it into an ice-water mix, which should sit at 0 degrees if the water is reasonably pure. This particular thermometer read 3 degrees. With a calibration, at least you know where you are with it, but it still bugged me every time I read it.

Also, on a similar note, at one of our Osborne Physics and Engineering lectures a few years ago, a colleague of mine demonstrated this with a couple of mercury thermometers in a bucket of water. He asked the audience, a group of year 12 and year 13 school children, why they were reading different temperatures. There were some amazing answers put forward, involving some whacky thermodynamical thinking as to why one side of the bucket was strangely warmer than the other side. As far as I recall, no-one suggested that the thermometers could simply be lousy.

Which brings me to the point of all this. Next semester I teach my Experimental Physics paper, and one of the most useful things students will learn is never to blindly trust what a measurement device tells them.  Always ask, ‘how do I know it’s telling the truth?’

 

 

 

Some thoughts while stuck in traffic Marcus Wilson May 31

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Monday morning was one of those strange days where for some non-obvious reason there was far more traffic than normal. Maybe there had been an accident somewhere, or there was some event on, but, for whatever reason, it took nearly half an hour to crawl through the Hillcrest roundabouts.

All that start-stop on the car is a massive waste of energy. You get the car going forward, which requires energy, then you stick the breaks on and lose it all. That kinetic (movement) energy turns to heat in your brake pads and does nothing except contribute to global warming. That’s for a conventional car, anyway.  The designers of electric and hybrid vehicles know about this, and that’s why these cars have regenerative braking. The idea is that forward motion of the car is transferred not to heat, but to electrical energy, which can then be used again to get the car moving.

Practically, the concept is quite simple; braking is done in the first instance by the electric motor, which, if driven mechanically, will act as a generator. Ensure that the electrical path back to the battery is in place (otherwise the car will just freewheel) and then motor will act as an electrical brake – the turning motion of the engine will generate electricity and take the kinetic energy from the car.

Easy. So how much energy is involved. Here’s a few quick estimates for a crawling car in a traffic jam. At 20 km/h (or 5.6 m/s), a 1500 kg car will have kinetic energy (calculated by half times mass times velocity squared) of about 25 000 joules. Everytime you brake from this to zero, your brake pads take this away (in a conventional vehicle). If you are doing this say twenty times in a long queue for an intersection, that’s getting on for about  500 000 joules of heat energy created.

What can this energy do? Boiling a litre of water (taking it from say 20 degrees to 100 degrees) would take 4200 J/kg/K  times 1 kg times 80 K = 340 000 joules of energy. Here, 4200 J/kg/K is the specific heat capacity of water – the amount of energy needed to raise a kilogram (litre) of water by 1 degree Celsius or kelvin is 4200 J. In other words you could make several cups of tea for the energy you’ve wasted in the brakes. In terms of the electricity ‘unit’  (One unit, or kWh = 3.6 million J,  your brake pads have consumed about 0.14 units of electricity. That comes to about 4 cents worth at domestic rates.

But one important thing to remember with kinetic energy is that it grows with the square of the speed. So a car doing 100 km/h will have twenty five times the kinetic energy of one doing 20 km/h. Breaking just once from 100 km/h down to zero will also use a similar amount of energy to what’s been calculated above. If you’re one of those people who like to brake at the last moment think about each brake pad boiling water for a cup of tea when you do this.

Is it cold enough yet? Marcus Wilson May 25

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This week has seen some icy mornings in Cambridge – a reminder that we are sliding into winter. Our heat pumps have been going, especially first thing in the morning to warm the place up a bit, and the cat has relocated his primary sleeping spot from a chair by a window to a rug closer to the unit.

We are able to see the outdoor unit of our downstairs heat pump from the dining room window. Tuesday morning, when I looked at it, there was something odd about it. It looked somehow different. The indoor unit then made some gurgling noises and went into defrost mode, and I realized what – it was iced-up. Instead of seeing the multitude of metallic fins on the back and side of the unit, I was seeing white ice. With the unit in defrost mode, this disappeared very quickly into a pool of water around the pump, the fins reappeared, and the heat pump started heating again.

This unfortunately is a problem with heat pumps. When it gets close to freezing outside, the fins of the outdoor unit drop below zero in temperature. That’s because the pump is pumping heat from the outside to the inside. The many highly thermally-conductive metal fins are there to provide good thermal contact area with the air, so that heat can be drawn from it as efficiently as possible. When they drop below zero, ice is going to form on them. How quickly this happens depends on a number of factors, but it is hard to prevent. (Aircraft have similar problems.)

Once the ice has formed, the efficiency of the machine is severely reduced. That’s because ice is a fair thermal insulator. It doesn’t allow heat to pass through it quickly, so the pump cannot suck heat from the air very well. Also, I noticed the ice forms between the fins, thus massively reducing the effective area of the pump in contact with the air. Overall, it is better for the machine to stop heating the house and instead heat the outdoor unit to remove the ice, then start again.

In the same way it’s better to periodically defrost your freezer – it will mean it will be able to keep the inside colder with less electricity cost. That might seem somewhat paradoxical – getting rid of the ice will help cool the interior – but the ice here is a consequence of the cold, not the cause of the cold – and it is insulating the inside from the cooling elements.

 

Virga Marcus Wilson May 08

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I learned a new word today: virga. It was used in a short article in one of the Sydney newspapers, discussing yesterday’s weather. Virga is simply precipitation that evaporates before it hits the ground, and we had some here yesterday. So it was raining, but we didn’t get wet.

Virga can rapidly cool the air around it, causing downdrafts in the atmosphere which then heat as the pressure increases nearer the ground. Personally, I didn’t notice anything particularly bizarre, but then again I was indoors for most of the day.

There’s a nice video of virga on youtube.

Mysterious power generation Marcus Wilson Apr 30

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One consequence of being a physicist is that you can’t go anywhere without seeing physics calculations that need doing. I’ve just been to our library hunting down books on the medical technique of transcranial magnetic stimulation (TMS), which was an interesting exercise in itself, since one textbook I found also has a chapter on homeopathy. Hmm. So how much do I trust its section on TMS then?

Anyway, our new, glorious library and student centre gets better every time I go in. Not only has a new cafe opened up inside, but there are now screens telling you just how eco-friendly the building is being right now, by displaying data on the building’s power consumption, solar power generation, water consumption and water capture, etc.

So, in the last month (which I assume means April) the building has used 182 792 kWh of power (that certainly makes my electricity bill look tiddly!) but has generated a cool 1 847 531 kWh from its solar panels. Now, I know April has been unusually sunny this year (shame the sunshine couldn’t have come in summer when it was supposed to) but there is something clearly wrong with this figure.

One metre squared of area, under full sun, gets about 1 kW of power on it. That means in about an hour it captures 1 kWh of energy. I don’t know how much of the building is covered in solar panel or other capture device, but I reckon the footprint of the building is about square with a side of 40 or 50 metres, so let’s say about 2000 m2 in roof area. So, if that were covered in solar panel, under full sun it would capture about 2000 kWh in one hour. In April there are 720 hours, so that gives us 1 440 000 kWh of energy.

But I’ve assumed that the panels are illuminated 24 hours a day! That’s clearly rubbish. So halve that, since half the time it’s night. In April, the sun isn’t anywhere near the zenith, so let’s halve that again. We are down to about 400 000 kWh. Then there are cloudy days (albeit not too many this month) which will take it down again, and still a large factor to apply because the power conversion from light to electricity or hot water isn’t 100% efficient. I reckon we might be down to a more reasonable estimate of 100 000 kWh.

So what does the 1 847 531 kWh represent?  It’s either a mistake, or I’m misinterpreting the display.

 

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