Engineering Question #2842

Morten Aasbak, a 20 year old male from Bergen, Norway asks on July 13, 2005,

I came across a "water powered clock", and it can supposedly generate power from water alone. My question is simply: How? Here is a reference to this clock at

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The answer

John Jones answered on August 3, 2005

There is no chemical process that, starting with pure water and nothing else, yields electrical power. Nevertheless, if you purchased one of these clocks and added water, it would indeed start to run, and probably go on running for quite a long time -- perhaps even a year or two.

The resolution of this apparent paradox is that something else must be in the clock besides the water. My guess would be that there's a small bar of some reactive metal -- magnesium would be one possibility, (but many others are possible, such as zinc). When you add the water, some atoms of magnesium give up their electrons to form magnesium ions, which dissolve in the water. The electrons released flow through the circuit that powers the clock. The other end of the circuit is a small bar of some less-reactive metal, such as copper, which is also immersed in the water. The electrons flow into this bar, then recombine with hydrogen ions in the water to form hydrogen gas, which is given off.

Contrary to what the advertisement says, this isn't a fuel cell -- it's just a battery, and it will only last until the magnesium bar (if that's what it is) is used up. This can be a long time, though, since a digital clock and LED display only use a tiny amount of power.

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