Apr 2, 2007

How to produce static electricity with water

Posted in Geek, Physics, Science at 1:00 pm by David Bradley -- 4 Comments; add your comment

Yesterday, we ran a video showing you a water powered battery that can generate a 15kV spark using nothing more than some simple hardware and a professor who looks a bit like Einstein. Some readers may have worried that it was a spoof given the date (April 1) but this is a genuine piece of science based on the principles of static electricity.

Water is a polar molecule - there is a small difference in electric charge from one end to the other - but pure (deionized) water is also a very good insulator. As the droplets of water fall through the bottomless metal cans, their polarity induces a charge in the cans (which are by the way heavily insulated from earth (or ground). A positive charge builds up on the cans as the water molecules falling into the buckets become negative. This results in a charge separation or a potential difference between the paint cans and the buckets of water (which are also heavily insulated from earth). Eventually the potential difference reaches a threshold at which point the insulating properties of the air between the two balls breaks down and a spark leaps across the gap. This spark, which has a temperature of several thousand degrees Celsius carries a voltage of between 10 and 15 thousand Volts, far more than you need to power even the biggest set of plugin speakers for your mp3 player.

Several questions remain. Where does the energy come from to create this enormous potential difference and could this form of electricity be tapped by building some kind of power station at the top of a waterfall and using two enormous cans and buckets? Well to answer the first question just look at the vertical arrangement of the equipment. The energy comes from gravity, from the potential energy of the water, which is above the paint cans. The second question is a little more complicated to answer. It would be possible to build a bigger generator, although insulating the components from earth would be tougher and the dissolved salts in river water would make it far less efficient than a generator using deionized water, but those are probably not the main issues. Think about it, to make electricity generation useful we need a current to flow. How might you “tap” off a current from this type of generator when its product is effectively small-scale lightning? A capacitor in the spark zone, you say? But then isn’t the air acting as a capacitor, still doesn’t solve the problem of tapping off a current. Find an efficient and safe way to tap the power of lightning and you could make a fortune and solve the world’s energy needs. But, please don’t try those kinds of experiment at home!

Instead of generating static electricity, however, it is possible to use gravity’s power to move water to produce a current, much more readily…think water wheel, dynamo-type generator…think hydroelectric dam.

By the way, this experimental setup was originally devised by Lord Kelvin in the nineteenth century and is known as Kelvin’s Thunderstorm, it featured in Bill Beaty’s amateur scientist column in 1995, you can find a more detailed explanation here.

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