Joules and Keswick

James Joule did his research on energy nearly 200 years ago. I was reading about his experiment where he allowed a falling weight to turn a paddle wheel in a barrel of water. He measured the temperature of the water. It went up because the mechanical work gave extra kinetic energy to the molecules in the water. I was thinking about trying to recreate it in my lab. Suppose I allowed a 1kg mass to fall 1m. It would lose mgh = 1 x 10 x 1 = 10 Joules of gravitational potential energy. If all of this went to raise the temperature of the water I'd be using dQ = mcdT. Say I used a 1 litre tub of water. The mass of water would be 1kg. Specific heat capacity of water = 4200 J/kg/degree C. So change in temperature dT would be 10/4200. To get a convincingly large temperature rise I'd need a much, much larger mass to fall through a much bigger distance. I wonder what he used...

Kettle handle at Haughtongreen bothy

This bothy is just off the Pennine Way north of Hadrian's Wall. Whilst we were there, I noticed that the kettle handle is black. Closer inspection reveals that the kettle itself is aluminium so that it won't rust. It is silver because that colour emits infra-red heat radiation badly. You don't want the main kettle to be emitting too much heat radiation because that would be inefficient. But the handle needs to stay cool. The handle is made of steel - you can tell from the rust. The black is enamel paint. It could be that black emits heat radiation better making for a cooler handle. But the handle is shiny enamel which isn't good. It's probably just that someone randomly thought that black would look better.

Thermal decomposition on Hadrian's Wall

We came across these limekilns on the north side of Hadrian's Wall near Housesteads. There was clearly a low ridge of limestone amongst the igneous intrusions. The used to dig out lumps of limestone and drop it into the kiln, having got a raging fire going at a temperature of over 1000 degrees Celsius. This is the reaction that took place.

The heat energy makes the calcium carbonate molecule fall apart to produce calcium oxide and carbon dioxide. It must be an endothermic reaction because we had to heat it up to get it to work.

Calcium oxide is a white powder that is often called quick lime. If added to water, it makes calcium hydroxide, which is often called slaked lime. Here's the reaction:

This is a very exothermic - it can give out a dangerous amount of heat energy. The calcium hydroxide is an alkali because of the OH- ions in it. OH- is called a HYDROXIDE ion.

The reason for doing all of this is that farmers can put the slaked lime alkali on their fields if the soil there is too acidic for growing crops. The calcium hydroxide can neutralise the acidic soils. The moorland soils in this part of the world are naturally acidic.

Flat topped clouds over the Tyne Valley

This was taken from Housesteads, looking towards Newcastle. The cloud in the middle spreads flat across to the left. Not quite the typical incus cumulonimbus anvil shape but not bad. I had an idea that it was caused by convection hitting a temperature barrier and being forced sideways. A bit of research has revealed that in the layer of air closest to the Earth which is called the troposphere, the temperature of the air does fall with height. The measure of this is called the lapse rate, in degrees Celsius per km. Hence convection works because the sunlight warms the ground and the ground warms the surface layer of air. This expands, becomes less dense and floats upwards. I teach that it will then cool, shrink, become more dense and sink. But I'm thinking about how the bubble of warm air will lose the internal energy needed to lower its temperature. How efficient would conduction be? Is it through adiabatic expansion? But the next detail I learned is that temperature goes the other way in the stratosphere, the next layer up. It is coldest where it butts up against the troposphere and then warms up as you increase in altitude. This would put a stop to convection and is why clouds spread sideways. But is the cloud in the picture really tall enough to be at the top of the troposphere? That would be 10km up. Hard to judge. I should have measured the height of the aeroplanes with my little finger method.

Nuclear display boards

Mass spectrometer

I found this mass spectrometer in a museum in Manchester. It works by turning the atoms in a compound into ions and then bending them with a magnetic field. They all experience the same magnetic field but the heavier elements are not bent as much so they are detected further from the centre of the circle. One clever bit is the use of crossed electric and magnetic fields on the way in which only allow particles of a particular speed to go straight on through a slot. It means that the speed in the main detector is also a control variable. Unfortunately we can't assume that the ions of one element all have the same charge. Some will be +1, some +2 etc. But you can make allowances for that in the analysis. Read the notice in the photograph to see what they are used for.

After the eclipse

 I didn't get to use the eclipse glasses on Friday so we took them onto the fells above Grasmere today. It was still very bright even through the aluminised Mylar plastic film. It's not far off trying to see through kitchen foil! It's important that the filters don't let infra-red or ultra-violet through because these can also damage your eyes. I'm now thinking of ways to test my eclipse glasses at these wavelengths. My eyes seemed all right through my polarising sunglasses AND the eclipse glasses. I aimed to be as protected as possible...

A solar eclipse comes with a very high tide because the Sun and the Moon line up their gravitational pulls on the seas. There was a very low tide at Silloth on Friday night. And an eclipse comes just before the New Moon. So tonight there was a lovely crescent Moon with the planet Venus as a bright "star" just above it.

Parallax error

I found this wonderful old ammeter. Notice that there is a curved mirror strip below the number scale. In the top picture you can clearly see the reflection of the needle in the mirror. This means that I was not reading the meter from directly above the needle. That's called a parallax error and in this case it means that the ammeter looks like it has a positive reading with nothing connected. Note also the non-linear scale. It is clearly not designed to make readings below 5A. Each scale division above 5A is worth 0.5A but these scale divisions are more compressed above 25A.

Eclipse eclipsed

 

Here are some views of the partial eclipse from north Cumbria. I was pleased with the views we had. But many were surprised by how dark it didn't get. Even 5% of the Sun through cloud is still very bright. Compare to the photographs I took on Skiddaw in full moonlight last month. It is possible that the flocks of birds moving were heading for the roost, but not conclusive. I looked up the list of future eclipses http://en.wikipedia.org/wiki/List_of_solar_eclipses_in_the_21st_century and was interested to notice that there are two a year, roughly 6 months apart. I'd never noticed that and need to learn a bit more.

What gives a plane lift?

We flew to Romania once. Clearly not in this plane. Mrs B said she'd only fly if I explained how the plane worked. So this is what I told her. The wings are an aerofoil shape. The curvature means that the air has to travel a longer distance over the top of the wing than under. This means that the particles become more spread out above the wing than below so there is less pressure above than below so there must be a resultant force upwards. This is a version of the Bernoulli Effect. I have come to understand in recent years that this is seen as controversial. I had heard of a rival explanation and was amazed to hear that it is actually Newton's Third Law. This version says that the wing shape is designed to push air down so the air pushes back up on the wing. The lift is basically the equal and opposite reaction. Here's a summary of the debate http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/airfoil.html 

Arcing horns in Manchester

We went to the Museum of Science and Industry in Manchester. For me, the highlight was coming face to face with a high voltage pylon insulator. It came a day after my friend in Australia sent me a proper explanation of the parallel bars on pylons that I blogged about last week. They now have a name: arcing horns.

 In certain conditions, there can be a voltage surge (overvoltage). This can lead to the electric field strength exceeding the breakdown in the air: you get a spark through the air. This is called FLASHOVER. It can spark round the insulator cups. This isn't good because it can burn tracks into the insulator that make future flashover more likely.

 The arcing horns are designed to provide a safe route for flashover that misses the surface of the insulators. Flashover can be made more likely by things like volcanic dust in the air which would reduce the breakdown field strength on the air. There is an excellent picture of flashover on this website from New Zealand http://www.nhrc.canterbury.ac.nz/Research/Transmission.shtml and scroll down.

More on these insulators to follow...

Whistling kettle at Wray Castle

We took our new whistling kettle to the amazing mock-gothic Wray Castle near Ambleside. I hadn't realised that explaining this type of steam whistle had been such a problem. Here's some new research from Cambridge University http://www.cam.ac.uk/research/news/how-the-kettle-got-its-whistle Basically the whistle is made of two parallel plates of metal with a small hole through the whole thing. The first hole is  a lot narrower than the spout and forces the stream into a jet. This loses some of its compactness as it moves into the space between the plates. When it reaches the second hole, these form into small vortices - like the eddies on the river mentioned two weeks ago - and it is this that gives the whistle. But researchers noted a second mechanism that it like blowing over the top of  an empty bottle.

Scalar field in the Langdale view

This is the view from the top of Black Fell. I've added numbers to show what the temperature in degrees Celsius might have been at the point shown. Temperature usually falls as you get higher. Temperature is a scalar quantity. It just has a size. It doesn't have a direction. This spreading out of the numbers across the landscape is called a SCALAR FIELD. There is this scalar number defined for any coordinate we can choose in the view. We could go on to calculate the temperature gradient at any given point - how many degrees Celsius the temperature changes per metre moved. But the gradient would be a VECTOR because it would depend on the direction in which you moved. We'd need to redraw the picture with arrows on to show this. The size of the arrow would represent the size of the gradient and thus we'd have made a VECTOR FIELD.

Another lovely film

My friend in Australia sent me this link http://physicsworld.com/cws/article/news/2015/mar/12/how-to-make-droplets-chase-each-other-and-self-assemble-into-devices. Guess what I'm going to be trying to sort out next week!

BBC2 Horizon on gravitational waves

There's an excellent documentary on the iplayer this week about the search for gravitational waves and how their discovery would provide evidence for the inflation model in Big Bang cosmology. It's a brilliant insight into how data is obtained and analysed. http://www.bbc.co.uk/iplayer/episode/b055knk1/horizon-20142015-9-aftershock-the-hunt-for-gravitational-waves

Fridge thermometer

If you look at this thermometer you'll notice that it has an unusual scale. We get used to thermometers going up to 100 degrees Celsius but why would you if it is being used in a fridge? It's a good example of adjusting the range to fit the data. It means that we can keep the scale divisions worth 1 degree Celsius. It is a rotary thermometer. I'm guessing that it contains a coiled bimetallic strip which uncoils as it warms and expands, thus moving the needle round the scale.

If a tree falls in a forest

This tree in the River Wear made me think of the famous old question: "If a tree falls in a forest and there is no one there, does it make a sound?". My take on it is that it must make the air molecules vibrate but that in order to call it a sound, there must be an ear to detect the vibrations and a brain to interpret it. To me, it seems similar to ideas in quantum physics. Detection is everything. We don't know what is going on between observations. We end up summing the probabilities of different possibilities.

Overhead cables and electric fields

I have been wondering for a while what the horizontal pieces with the curved ends are that sit beneath the stack of insulators. I think I might have worked it out. They seem to be connected to the wire suspension point so they will have the same potential (voltage) as the wire. If they were not there, there would be a single suspension point at that potential beneath the insulators. Single points concentrate the electric field lines into a smaller space. Electric field lines are like contours. The closer they are the stronger the field. Air can only take a certain field strength before it breaks down and conducts by sparks. By including this bar, the field lines are spread out making it less likely to spark. That's my theory. It looks different with horizontal suspension:

I'm not sure why there is only half a bar this time.

Scoat Tarn and Acid Rain

Scoat Tarn is tucked up in the hills above Wasdale. If you look carefully, it is somewhere in the middle of the picture. It has been used in studies of the amount of acid rain in the environment (look at the data in the latest report http://awmn.defra.gov.uk/resources/annualreports/index.php). So I decided to repeat an experiment I've done before into the effect of acid rain on the growth of cress seeds.
I got 5 half Petri dishes and some cotton wool. I also got some 0.010 Molar Citric Acid (that's effectively really, really weak lemon juice). I then watered the Citric Acid down further so that I was using 5 concentrations of weak acid: 0M (pure water, really) 0.002M, 0.004M, 0.008M, 0.010M (somehow I forgot to set up 0.006M!)
I soaked a piece of cotton wool with each solution and put it into a Petri dish. I then put 20 cress seeds on top of the cotton wool and left them in the same place so that they got the same light and temperature.

 

The entropy of a Lego Durham Cathedral

This is a wondeful fund raising idea at Durham Cathedral. You pay a pound a brick. When we were last there 18 months ago it was just the base layer. I'd been thinking about using Lego for a lesson on entropy. The idea was based on the Brian Cox Wonders of the Universe episode where he goes to the desert in Namibia and builds a sand castle. He says that there are fewer ways of arranging the sand grains into a castle shape than if you were to arrange them into a random sand pile. I thought it might be more obvious in Lego and that with a small amount of Lego, we could even count the number of ways. Anyway - the cathedral. It is obvious that you can't just put the bricks together randomly and end up with something that looks like a cathedral. Yes, ther are many identical bricks. They could be numbered and swapped round and it would still look like the cathedral. So there are in truth a large number of ways of putting this set of bricks together and having it still look the same, But you can't swap a roof brick for a wall brick, for example. Now what if we only wanted a random pile of unjoined bricks, The order wouldn't matter, You could then swap roof bricks for wall bricks and get the same effect. There would be many more ways of arranging the bricks to get the same overall effect. Entropy is a measure of disorder. Entropy S = k.LnW where k is the Boltzmann constant and W is the number of ways the bricks can be arranged. The Lego Durham Cathedral has a lower entropy than a random pile of the same bricks. Order has a lower entropy than disorder.

Eddy currents in Durham

I've been teaching about eddy currents induced in metal by changing magnetic fields, Trouble is, not many students seem to know what an eddy is. I found some eddies on the River Wear near Durham. They are shortlived mini-whirlpools in the water. In the same way, an eddy current is a circular current loop that's really going nowhere. But it does act as a mini-electromagnet which can cause problems.

Radioactive dice

OK, so they are not really dice, just wooden cubes with one side painted a darker colour. And they are not actually radioactive. But we use them to show what happens during radioactive decay. We say that the dice represent the NUCLEI of unstable radioactive atoms. We gather up 30 dice and drop them onto the table all at once. Any that come dark side up are said to have decayed. There are 4 in this picture. They are taken out and put on one side, The other 26 are scooped up and dropped again. The process can be repeated until only one of the dice is left in. You can plot dice left in against throw number. With only 30 dice there is a lot of scatter but you can put a best fit line through. The number of throws for the number of dice left to halve is thus the half life. More advanced theory notes that the probability of any one cube being dark side up and thus decaying is 1/6. Eaach time we'd expect (1/6 x number dropped) to be taken out. The rate at which the number drops each throw dN/dt = - 1/6 x number left in. This can be integrated to give number left in N=N0exp(-1/6xt). For any radioactive isotope, there is a different probability of a nucleus decaying. Instead of 1/6, this is called the decay constant lambda. Half life = Ln2/lambda.