Forget conventional hydroelectric installations – if you want to have fun generating electricity in the lab then the completely static tin can and bucket generator is for you. They are all the rage at the moment – at least in our lab here, where we’ve sidetracked a student from his summer project into making one to wheel out on Open Day. (It is, actually, vaguely related to what the student is doing for his project, so it’s not a complete sidetrack.)
Anyway, there are lots of clips on YouTube of these devices. Walter Lewin does a good job (of course) of demonstrating one. Basically, you have two dripping water streams (from the same ultimate source – e.g. a bucket) Each stream falls through a metal ring and into a metal bucket. But – the bit that makes it work is that you electrically-connect the left hand ring to the righ-hand bucket, and vice versa. This leads to a nice build up of charge in the buckets and sparks can fly between them.
So, what’s happening?
Imagine that the left hand ring (A in Walter’s video) acquires a small positive charge. This means that the water dripping through it is likely to be slightly negatively charged (the presence of the charged ring results in a polarization of the water in its vicinity). This negatively charged water falls through the ring and into the bucket (D on the video). Now, since the bucket is coneected to the OTHER ring (B on the video) the other ring acquires a negative charge. This then encourages the drips from the right hand stream to be positively charged, and so the right hand bucket ends up positive. This re-inforces the charge on the left hand ring (the two being connected). So charge keeps building up in the buckets, until the voltage between them is enough to generate a spark.
Lots of fun. And rather damp. As the buckets charge up (the left hand negatively, say), this means that the water falling towards it has the same charge as the bucket to which it is heading. It is repelled. Practically, this means that it is pushed away from the bucket, and hits the floor instead. That’s the limiting factor – if the charge on the bucket is big enough, no more liquid can fall into it! To get more liquid in, you have to let it drop from a greater height – so that each drop builds up more kinetic energy to overcome the electrostatic potential barrier to get into the bucket.
That’s where the energy is coming from – gravity.