"Deep time" at Red Wharf Bay

In his introduction to the re-print of Clarence Ellis's book The Pebbles on the Beach, Robert Macfarlane says that geology gives him a sense of what he calls "deep time". Ellis was born on Anglesey and it was on Anglesey that I got the sense of "deep time". We found these alternating layers of limestone and sandstone. The former was laid down in tropical seas and then the land rose, leaving rivers to deposit the sand and swamps the black mud shale deposits. Human history goes back in the thousands of years but I get the impression that each layer must represent a time longer than that. I'd be interested in finding out how long a time each layer represents and how you would work it out.

Getting an IV characteristic for a red LED

 We used a potential divider to change the potential difference across a red LED. We measured the current through it for each voltage. To obtain negative voltages, we swapped the connections round on the power supply.

The graph looks like this:

There is no light for negative voltages but even with positive voltages, nothing happens until around 0.7V and even then we were getting current of 0.02A with 1.5V.

Contact resistance

We were doing an experiment to find the EMF and internal resistance of a cell by clipping in different sizes of resistor. The idea is that the resistance changes the current and the current heats the internal resistance, changing the output p.d. If you plot output p.d.against current, the y-intercept is the EMF and the internal resistance is - gradient. But with the 1 Ohm resistor I was asked if the direction of the resistor affected the current like it would with a diode.

The answer is that it doesn't, but that there is extra resistance added into the circuit by the way that the clips grip the wire. It is unavoidable and changes in size every time you reattach, hence the seemingly different reading. It is a small resistance but might be a reasonable fraction of 1 Ohm and thus affect the current readings for very small resistances.

A simple cell

We made simple cells by soaking filter paper in a strong brine solution and connecting up two metals of differing reactivity on either side of the paper. Here we had nickel and copper. I'm working on an explanation as to how the differing reactions mean that more electrons are pushed by one side than by the other.

Wind in the wire: Abergwyngregyn

It was a windy day when we walked under the wires and there was a steady roar that I worked out was the sound of the wind in the wires. I'll estimate 100Hz. Let's assume that it has set up the first harmonic of a stationary wave. The equation for calculating the frequency is:

Where L is the length of the wire (1000m as measured on the map), T is the tension of the wire and mu is the mass per metre. The wire is aluminium with steel core. Let's round up the density of aluminium to 3000 kg per cubic metre and estimate circular cross-section of 4cm diameter. Cross-sectional area = 0.001 square metres and mass per unit length of 3kg. That would give a tension of 120 billion Newtons. Breaking stress for the steel is 7.9 x 10^8 Newtons per square metre. This figure would exceed that so some of my estimates or assumptions must be wrong.

Diffraction at RSPB Conwy

This was going to be a piece about how the ripples on the water help us to see the invisible gusts of wind. Each gust sent a set of short wavelength ripples across the surface - an interesting real-time visualisation tool. But it didn't photograph well. However, I did notice that as well as the high frequency noise on the surface, there were also lower frequency waves crossing the pond.You can see them in the clearer water beneath the clump of reeds in the water in the middle of the picture. Why are they suddenly visible there? The answer seems to be that their wavelength is closer to the size of the clump so they are able to bend into the sheltered space behind the clump by diffraction. The much shorter wavelength noise isn't able to diffract and so goes straight past.

Variable resistor controlling a bulb

 Here we took a rheostat that has 3 terminals and only connected 2 to make a variable resistor. Above, the variable resistor is set to zero so that all of the potential difference is across the bulb. But below, the variable resistor is on its highest resistance setting. You can tell because the current reading in the left hand meter has gone down. There is still a potential difference across the bulb, shown by the other meter. The bulb is off so you would have though that the potential difference would be zero. That's the problem with a variable resistor as a way of controlling a bulb. No matter what you do, the potential difference of the bulb will never go down to zero. If you want that when, for example, doing IV characteristics, you have to connect up all 3 terminals to wire the rheostat up as a potentiometer.