Electricity meter

 

It struck me yesterday that I'd never bothered to find out how an electricity meter works. I'd guessed that there must be some electromagnet thing going on with the rotating disc. Perusal of the Internet suggests that the disc is part of an induction motor. The disc is non-magnetic but there are electric coils above and below it. These act as electromagnets. Because the supply is alternating, the strength of the magnetic flux through the coils, and thus through the disc, varies. The changing flux induces an emf in the disc which causes eddy currents to flow by Faraday's Law. By Lenz's Law the act as small electromagnets themselves which repel from the coil electromagnets, turning the disc. The bit I have not yet understood is the way that this is wired to make the speed of the disc proportional to the energy supplied. Oh, and I also need to look up 3-phase again. It's been nearly 3 decades since I had a working understanding.

A clock with a crutch

 

I was asked to fix this clock because it was stopping after a few minutes every time it was set going. The problem was that the pendulum was not swinging evenly from side to side. You could tell by listening to the tick-tock. The tick and the tock weren't even. I understand that this means it doesn't swing far enough on one side to pick up the escapement cog. That means that the mechanism can't give the pendulum a push using the energy from the spring wind-up mechanism. Hence the pendulum loses energy and stops. The crutch mechanism appears to be a second part on the back of the pendulum that can be bent out of line to even up the swing even if the clock is not quite level.

How high is that plane? (part 2)

Here's the little finger method applied to the aeroplane. I suspect that my finger might be slightly nearer than previously measured due to the constraints of taking the photograph but would block out no more than 1.2 degrees of my vision if it were 60cm away from the camera. The plane is a quarter the size (I measured a big photograph) so the plane subtends 0.3 degrees of my vision. which is 0.005 radians. If the plane is 60 metres long in real life it is 11 km above me, about 7 miles. This photograph was taken near Oxford. It may be different in Wigton where the planes are preparing to head out over the Atlantic.

My little finger measures angles

A couple of weeks ago I posted about estimating the height of a plane by trying to estimate the angle that it subtends (blocks out) in my vision. I was looking at an astronomy website earlier and I was shocked to note that the Moon and the Sun only block out 0.5 degrees each. The plane didn't look as big as the Sun so I must have overestimated the angle. The site suggested using my little finger to help measure angles. My little finger is 15mm across and held out like this is 700mm from my eye. Hence the angle subtended is 15/700 = 0.021 radians or 1.2 degrees. So the full Moon or the Sun would only occupy half as much of my vision as my little finger. I will test this out as soon as possible and re-estimate the angle that the plane blocks out.

Back to the pylons at Saltholme

 

We went back to Saltholme. Here's a picture of the pylons above the reedbed. Last week I was musing that the smaller pylons were closer together than the bigger pylons (I know this seems obvious but I do like to measure things.). So this week I was driven at 80km/h along the straight road beside the pylons. I timed 11 seconds between the smaller pylons. That would be 244m apart -so my map was correct and they are really 250m apart. And I verified that the bigger pylons were 16 seconds and thus about 350m apart.

Red sky in the morning

 

I was taught "Red sky at night, shepherd's delight; red sky in the morning, shepherd's warning". Well, it's a very red sky this morning. This means that a frontal system is coming in from the Atlantic and it will rain later. At dawn and at dusk, the Sun is low in the sky. The heavier scattering by dust particles allows only the longer wavelength redder rays through. We don't notice much until it reflects off the clouds. The Sun rises in the east but Atlantic fronts come from the west, hence the redder light readily reflects off them. Have a nice day and enjoy the rain later! :)

Root mean square

 

I lit a small bulb using two separate power supplies. The yellow battery pack provided a d.c. supply and the other source was low voltage a.c.

 

 

I connected both sources to the oscilloscope. The flat line is the d.c. and the wave is the a.c. They both provide the same energy because they lit the bulb by the same amount. So the flat line must show the average voltage of the wave. This type of average is called a root mean square. Since the true mean of such a wave would be zero because it is as much negative as it is positive, we have to square it before we average it to get rid of the negatives. Then the root it back to normal size.

 

 

In fact, the flat line is lower than it should be so the two sources can't quite have lit the bulb equally.

 

Forces on a swing

 

Here's an odd experiment. I strung two sets of masses together with a piece of string. It's 400 grams on the left and 600 grams on the right. The 400 grams is in mid air because of the greater gravitational attraction of the 600 grams. Then I set the 400 grams swinging. Suddenly the 600 grams started to lift and fall, lift and fall. This shows that the force exerted by the 400 grams when it swinging is bigger than the 4 newtons exerted when it is still. When an object moves in a circle, there needs to be a centripetal force acting towards the centre of turning. A centripetal force is not a force in its own right: it is a resultant force, a combination of other forces. In this case, the centripetal force = tension in string - 4N. The 4 newtons cannot change, so to get a centripetal force the tension has to increase. It increases so much that it can lift the 600 grams off the ground momentarily. So if you are designing a swing, the cables need to be enough to support it when it is moving.

A Lot of Pylons on Teeside

This is the view from Saltholme nature reserve. There is an amazing large substation and more pylons than you can count.

 

 

Here is the Seal Sands petrochemical works which must use a lot of power and on the left of the picture the box-like building is the Hartlepool nuclear power station.

 

I measured the distance between pylons behind my house on a map. They were 250m apart. Today I timed as the car drove parallel to a power line. It took 16 seconds at a speed of about 80 km per hour. That would make these pylons 300m apart. Perhaps bigger pylons have a different spacing. 

Wigton church clock and escapement

This is the clock mechanism is the tower at Wigton. One issue that had always concerned me was that a pendulum always loses energy as it swings and yet pendulum clocks just keep going. I know that you wind up a clock to give it energy - I have wound up this clock; it's hard going! But I wasn't clear how this happens. It's the toothed gear wheels that do it. The mechanism is called an escapement. I'll do a future post to explain how our clock is given energy by winding up weights. However, as far as I can see, the force from the springs pushes on the pendulum by means of the gear wheels of the escapement.

 

Here is the lever from the clock mechanism to the pendulum.

 

Then the swing of the pendulum regulates the speed at which the gear wheel can rotate, thus fixing the speed at which the clock ticks.

Hydro-electricity on the bird reserve

We visited Loch Ken in southern Scotland. It's 9 miles long and part of it is a bird reserve.

 

 

A display board on the reserve explained that the loch was created by damming in 1937.

 

The dam actually looked more like a weir. I seemed to be possible to raise and lower it. You'll have to look hard to spot the top of the structure in this picture. The display board said that the actual power station fed by the loch is at Tongland. This is quite a few miles away. Wikipedia says that it is part of the Galloway Hydro scheme from the 1930s. They did it then because the National Grid had just been invented. Before that, all electricity generation was local; every town had its own power station and there would not have been the population to justify a scheme like this in Galloway. The power station is quoted at 33MW and it says that the water falls 106 feet. It also claims that these power stations were some of the least costly ever built in terms of £ per kW. http://en.wikipedia.org/wiki/Galloway_Hydro_Electric_Scheme

Close-packed bubbles

 

I found these bubbles in a stream near Coniston. They reminded me of the way atoms are packed together in materials. I think this is the hexagonal close packed model. The large bubble is similar to the way that large carbon particles stick out between the layers of iron atoms when you make steel, stopping the layers sliding over each other and turning a brittle material into a ductile one.

The power of the sea at St Bees

 We went to St Bees. We had seen TV coverage of the tidal surge earlier in the week. The grass above the promenade was covered in stones and debris. All of the life belt holders had been knocked flat. I did some calculations of the force of the wind hitting me in a December blog post. The theory says that the rate of change of momentum of the water hitting the life belt holder is the same as the force of the water. The assumption made is that the water stops dead. The theory is on the earlier post. I estimate that the life belt holder is 75 cm square. I'll estimate that the sea is doing 10 mph, which is 4.4 metres per second. The volume of water stopped in one second = area x speed. Change in momentum = mass x speed = density x volume x speed = density x area x speed squared. I get 11kN which is the equivalent of 1 metric tonne hitting the red board. If I could remember the bending moment theory I learned at university, I could calculate the minimum force from the fact that the steel bent at the bottom and thus tell you the minimum speed of the sea water. Trouble is that even a bit of Internet research isn't bringing it back quickly enough.

 

We are fond of the café and it has taken a bit of a pounding too. The banks below it don't look too secure.

 

Stargazing Live

 

We went to the Stargazing Live event in Cockermouth this evening. Thanks to the Astronomical Society people who put it on. I'm jealous of the auto-steer devices on some of the telescopes. My wildlife telescope can see as well as some of the smaller astronomical ones but it's very difficult to find a small point of light in a dark sky using mine. However, I did take this picture of the Moon. China landed the first lunar probe for nearly 40 years last month. I've labelled the landing site of Chang'e 3 on the picture below. Sorry it didn't come out too well.

 

 

Spring in Coniston

I found this playground toy. When I sat on it and rocked backwards and forwards, the time period was about 3 seconds.

 

 

With no one on, the time period was less than a second.

 

I'd hoped that perhaps it could be modelled as an inverted pendulum but clearly not. There will be an elastic restoring force when it is displaced. I'd have to go back to my degree work to remember how to calculate that and thus figure out if the motion can approximate to simple harmonic motion.

Solar power again

We went to the RSPB nature reserve at Leighton Moss. Here's their solar panel display. I last posted about this in December 2009. Then they had generated 8749 kWh of energy. So in 4 years and one month they have generated an extra 5557 kWh. That's about 1490 days. Assuming an average of 12 hours daylight every day that means 17892 hours and thus an average of 0.3 kW being generated. 300W is not much but I forgot to see how big the roof panel area was. That said, at say 10p a unit, it is £500 of electricity.

 

 

 

By the time we left the café it said it was generating 0 kW. This is the light level that generates no solar electricity ...

Darker in the mornings

 

My first day at work in 2014. It always seems darker going back after Christmas. I always though that it was just psychological after getting up in the light for a couple of weeks. In fact, if you look at the sunrise times, the sunrise gets later and later for a couple of weeks after the solstice. The days do lengthen because sunset time drifts out even more. The reasoning is to do with the tilt of the Earth and the fact that the Earth spins on its axis whilst also moving round the Sun. I'm working on a sensible explanation - maybe by next year!

How high is a plane?

 

This plane stood out today because of the length of its vapour trail. I'd noticed one the other day that stayed constant at about 3 plane lengths but this trail covered half the sky. I estimated that the plane itself blocked out perhaps 2 degrees of my vision when overhead. I'll think about how I can measure this more accurately. Two degrees is about 0.02 radians, where a whole circle, 360 degrees, comes out at 2pi radians. I know that I can calculate the height of the plane using the arc length formula, where angle in radians = length of plane/height above me. I estimate the length at 70m. This means that the plane was flying at 70/0.02 = 3500m or just over 2 miles up.

Not plugged in

The blue power light on my cable stays on for a while even when it's not plugged in. The computer will work on DC so the transformer will have a bridge rectifier in it. As well as the four diodes to make the current go one way only, there will be a smoothing capacitor to iron out the ac fluctuations. The light will stay on until the capacitor has discharged.

A Benchmark in Coniston

 

I've blogged about benchmarks before and here's one in Coniston. I can't find it on the benchmark spotters website but I did find it on the Ordnance Survey website. They reckon that there are about half a million of these left in the UK. Apparently this one is 50.823 metres above sea level. That's very precise. Think about how big a wave on the sea can be. You'd think that the nearest metre would be fine. But here the height is given to millimetre precision. Now mean sea level must be a construct but even so, with the continents moving and sea level shifting, that level of precision is potentially controversial. That said, if you don't measure to that level of precision perhaps it will be harder to detect changes in sea level or earth movements.

Getting reflection back to front

 

Here's an ambulance in Old Town, Swindon. AMBULANCE is written back-to-front on the bonnet so that it can be read in car mirrors. The way that mirrors flip things back-to-front is called lateral inversion. So how come they don't put the writing upside down? That's because rays that are travelling horizontally into the mirror are reflected back horizontally. The top ray and the bottom ray can't cross over as they would for a less or for a concave mirror. In fact, lateral inversion is not caused by the left and right rays crossing over either. It's basically an optical illusion. If you look at your reflection in the mirror, it has to be a reflection of the front of you. The rays can't cross over left and right, so the ray on your left will come from what would be the right hand of your reflection.

Spring tide for the New Moon

 

This was high tide in Dumfriesshire this lunchtime. It is the New Moon. The Moon is lined up with the Sun on the same side of the Earth. They are both pulling on the water. The Sun is a lot heavier but a lot further away. This, coupled with high onshore winds, meant that the sea was almost onto the dunes. The relative pull of each heavenly body is proportional to its mass divided by the distance away squared. For the Moon, this is 7x10^22/(3.8x10^8)^2 = 4.8x10^5. For the Sun it is 2x10^30/(150x10^9)^2 = 8.8x10^7. The pull of the Sun is actually about 180x bigger than the Moon. However, it is not this that affects the tides. It is the difference of pull from one side of the Earth to another. That's going to need further calculation.

Ringing out the old, ringing in the new

 

HAPPY NEW YEAR!

 

Last night we rang in the New Year. We rang out the old before midnight. That was done with the bells half-muffled to give a more mournful sound. It's done by strapping a piece of leather to one side of the bell clapper, as shown in the picture. The leather is a more springy material that the metal. It deforms slightly when it hits the side of the bell. This makes the collision last slightly longer. The same change in momentum taking place over a longer time means that less force is involved. This doesn't get the metal vibrating as much as a direct hit with a metal clapper.

 

Just before midnight we took the muffles off so that we could ring in the new with a full peal of loud bells.