Diffraction on Loch Ness

We stayed at Foyers on the south side of Loch Ness. This photograph was taken looking north up the look from the mouth of the River Foyers. Note the shingle bank.

 

 

The exact end of the river is shown here - a gap between two shingle banks. Waves were being driven up the loch by the wind. They were straight wave fronts - what we call plane waves. When they went through the gap, they became semi-circular.

 

When plane waves go through a gap and become semi-circular, it's called diffraction. It is supposed to be most noticeable when the gap is the same size as the wavelength. Here the gap was much bigger than the wavelength. This could actually be an example of refraction. The water would be much shallower on the edges near the shingle so the water would move slower due to greater friction from the bottom. Those edges of the wave would go slower and get left behind, which could also account for the semi-circular shape.

Melvyn Bragg on the Braggs

I posted before about a Radio 4 programme by Melvyn Bragg about the Braggs last November. Since then I've met Professor Mike Glazer, who put him up to it. Professor Glazer has done a lot to remind the world of the contributions of William Henry Bragg and Laurence Bragg to X-ray crystallography. It is coming up to the centenary of the discovery and their joint Nobel Prize. Melvyn Bragg, an old boy of the school, has done another programme about them, having found out that he is indeed a distant relative. William Bragg was born 5 miles from here! Here's the link to the programme. http://www.bbc.co.uk/programmes/b0383vb0 The original programme from November is still available http://www.bbc.co.uk/programmes/b01p0s9s

Dolphins and stationary waves

We were up in the north of Scotland and went to see the dolphins at Chanonry Point on the Moray Firth. It is a narrow spit over a mile long that sticks out into the estuary. There's a road down it and a golf course, but it narrows to a shingle spit shown below.

 

 

On the rising tide, there is water coming in from the sea on your left. Meanwhile, river water is going out to sea from the right. The result is a stationary wave. It can't quite be the classic case that I teach about when the two waves are in opposite directions with same speed, frequency and amplitude, but you do get a wave feature that stays fixed in the same place on the sea. The waves go up and down but don't go left or right. You can see it as white water above the crowd in the picture above. It is clear on the water in the picture below.

 

The current carves a deep channel just off the end of the spit and the resident population of bottle-nosed dolphins gather to feed on salmon on the rising tide. Huge crowds gather to watch the spectacle. We saw at least a dozen dolphins, some of which are shown below. They performed as well, jumping clean out of the water. It is said to be the best place in Europe to see dolphins from the land. They can be as close as a few metres. You ought to go. If you don't like dolphins, go for the waves.

Durham Cathedral Bells

We visited Durham Cathedral today to visit the bells. Everyone we met was really nice. Here's a picture of the tenor bell, the heaviest. It's 28 cwt (that's short for hundredweight) - bell ringers like old units. 1 cwt is worth 50kg (2sf) in the UK, so the mass of the bell is 1400kg, which is 1.4 tonnes. That might be the mass of a small car. It took two people to ring the bell round so that it was upside down like this, but one person can them comfortably ring it because of the large size of the wooden wheel to which it is attached. The bigger radius means that the required moment to turn the bell can be achieved with a smaller force, since moment = force x distance.

 

 

Here are some clappers. The middle one was biggest. It was about 1m long with a 5cm diameter. Cross-sectional area is pi x r-squared so about 0.002 square metres. The volume is then 0.002 cubic metres, with the mass of the ball to be considered as well, of say radius 10cm, giving an additional volume of 0.0005 cubic metres. Total volume is 0.0025 cubic metres. I think bells contain a lot of copper, which has a density of 8920 kg per cubic metre, giving the clapper a mass of 22kg. This must be an underestimate! I'll go and think about it.

 

 

Here's the view down through the hole in the trapdoor to the ground floor. This is what 60 metres looks like!

 

Here are a couple of views from the top of the tower to show how high we were.

My TV aerial

 

After thinking about yesterday's post, I went up into the loft to look at my TV aerial. Sure enough, the elements are horizontal. Apparently that will be the direction of the electric field in the electromagnetic wave. Internet research tells me that this is a Yagi-Uda antenna, after the Japanese people who invented it nearly 90 years ago. The actual detector is the bit that the wire is attached to. The horizontal bits in front affect the field in such a way as to make the aerial directional. You can't see out of the loft but my gut feeling is that it might be pointing more towards Sandale transmitter than Caldbeck! The bit nearest to us on the picture is the reflector. The dipole detector is actually 2 separate sticks of metal with a small gap between then inside the junction box with the wire. The electromagnetic wave makes electrons in that wobble so the signal is detected and then amplified. In theory, the dipole aerial is half a wavelength long. I measured it as 20cm, which would suggest 750MHz waves using the wave equation c = f x lambda. Wikipedia quotes Caldbeck as up to about 500MHz. In theory, a longer dipole might be better. I studied this at university 27 years ago and reading the work on the Internet, it does sound familiar but I'll need to read up more to make sure I'm getting this right.

Caldbeck Transmitter

We went to Watchtree Nature Reserve on Sunday. I took this picture of the Caldbeck Transmitter through my binoculars.

 

I looked up the relevant details. Apparently it is 337 metres high, making it the third tallest structure in the UK. Wikipedia has details of the frequencies at which it broadcasts http://en.wikipedia.org/wiki/Caldbeck_transmitting_station Another site claims that the electromagnetic waves are horizontally polarised. I need to be more inquisitive about the local Physics features!

Transformers

We stayed at the Ski Station at the start of Glencoe, just round the corner from where they filmed for Skyfall. We found this transformer that steps down the voltage from 11000V for the Kingshouse Hotel. In the background is Buchaille Etive Mor, possibly the most photogenic mountain in Scotland.

 

 

This is the plate that tells you that it is 11kV.

 

 

Transformers are two coils wrapped around the same iron core. The primary coil acts as an electromagnet and magnetises the core. The mains electricity is alternating current, so the magnetic field in the core varies as well. This varying field induces a voltage (strictly an EMF) in the secondary coil. The size of this secondary output voltage depends on the ratio of turns between the two coils. In this case there will be a lot fewer turns on the secondary to reduce or step down the voltage to safer levels.

 

You can find out more about our electricity supply from this marvellous link: http://www.pylonofthemonth.org/

 

Pylon of the Month was one of the first things to "go viral" on the Internet around the Millennium. It has been revived by another Physics teacher. Respect!