Pipe flow, magnetic fields and surface chemistry... or what I did in my holidays...
Water flow has been used to generate electricity for many years, in the form of hydro-electric power. These work on the standard principle of electricity generation, you simply move a coil of wire inside a magnetic field. This causes electrons to flow through the coil, as the electrons interact with the magnetic field, and this is called Faraday Induction. On a microscopic scale it is governed by the Lorentz force, which tells us how electrons interact with electromagnetic fields (like the magnetic field) given by the equation;
F = q (E + v × B )
In this equation we have q, the charge of the electron, E the electric field (this is related to the current for most samples), v the velocity of the electron (this is the average speed at which an electron moves through a material), B is the magnetic field strength. In this equation the × is not an ordinary multiplication, it is called the cross product, and it means that when we use a cross product between B and v the resultant force (F) is in a direction different to both the magnetic field and the velocity, This can be seen in the figure below.
Here we have the geometry from the cross product , and the resulting 'Hall Effect'
The interesting result of this equation, is that we flow electrons or other charges (such as chemical ions like H+),they get deflected to one side. We do this when we apply a current through any material, such as a copper bar. When we put the electrons flowing through a finite box, this means all the negative charge collects on one side, the farthest it can go. As all materials are normally electrically neutral, this means the electrons have become delocalised (disconnected) from their atoms, so these atoms start to show a positive charge on the other side of the box (they are no longer screened by electrons). This is called the Hall Effect, and is very important in computer manufacturing, physics and materials, as it tells you some fundamental properties of the sample, if you do a more complete analysis.
Well at this point you might be saying that this is all very interesting, but what relevance does it have to water? But you can actually apply the same idea in water. Typical tap water actually contains many free ions, either from impurities such as calcium carbonate (which makes water hard), or even just H+ and OH- ions that have become un-bonded. As such when we flow water through a pipe, we expect to see a similar effect. What is exciting about this effect is it generates a voltage across the pipe. If we applied this to a standard piping in houses we may be able to generate a voltage big enough to power small applications.
Water molecules(red and white) with impurities (blue and yellow)
If it could be harnessed, then it could potentially lead to simple power generation methods. I attempted to measure this signal in my summer project. The results were in agreement with the Lorentz force, although unfortunately the voltages generated were insufficient for realistic applications. This was done all the way up to very powerful magnetic fields ~12T (typical MRI machines are 3T). I tried this for a whole host of different types of water, with different impurited, and they all obeyed the law derived from the Lorentz force.
V = d v B
Where V is the voltage across the pipe, d is the diameter. This is quite different to in normal metals... Intuitively we would expect some dependence on the number of ions. In normal metals voltages, and any electrical properties are effected by the number of electrons available to conduct, but this isn't true for magnetic fields in water. As this isn't of any real use, we were stumped. Fortunately we also found 'Electromagnetic Surface Effects', which don't even require a magnetic field. These are, in simpler terms, the equivalent of the potato clock, but in flowing water.
So how does this work? Well when you put 2 materials next to each other, there will be some interaction, due to chemical reactions between them. This manifests itself in the form of a difference in voltage between the surfaces. This is how the potato clock works. This seems rather strange at first thought, why should some reaction at the molecular level result in an electrical effect? However, we can make some sense out of this when we consider what is happening to the molecules. One type of ion will stick to a surface more than the other. This means that a layer of charge forms on the boundary, whether it be electrons or more complicated molecules. If we get a difference in charge, we get a difference in voltage. This is shown below
This is how a potential changes between two different surfaces. In this case this is on the atomic level. These effects can be tiny (like in this case), or in much larger
We get this surface effect between water, and a pipe surface. In this case one of the ions in the water will stick to the surface more than the other. This means that we get a thin layer of charged particles stuck to the surface of the pipe, so that the rest of the liquid becomes oppositely charged. The ions in the liquid are free to move around, unlike those stuck to the wall. This is called an electrical double layer, and is best explained by a picture below;
The solid surface has a thin layer of charge, called a compact layer. The rest of the liquid has ions free to move. When water flows the compact layer cannot move due to friction, in contrast to the rest of the liquid
Because the charges in liquid can move, when the water flows through the pipe its like a current is flowing through a wire. Here the ions carry the electric charge, unlike the electrons in a material. But we still get some of the effects we see in a conventional circuit. As the ions move along the pipe, charge builds up on one side, which causes another voltage (or potential difference). I also measured this effect over my summer project. The intriguing thing about this effect is that in principle it could power some small circuits in a house. This would be a completely free way to create small amounts of electrical energy in a everyday homes.
This voltage can even be very large in very pure samples. I recorded voltages of up to 7V over the summer holidays. The amount of current in these samples were not measured in my experiments, so the amount of actual power is unlikely to be large. But certain low power applications, like LED lighting, it might be possible to power them off these simple methods. I am attempting to verify this as a side project in my a current experiments. And if I succeed, then I will have truly re-invented the water clock!
