D35. Kelvin Water Drop Generator
ELECTROSTATICS - Electrostatic Potential
The electrostatic generator consists of water falling from two spigots through two metal cylinders and in two cans that are cross-connected to the cylinders. It is capable of generating about 10kV before discharging across a spark gap. The electrodes and sparks are very visibly TV-projected on the video screen.
How it works:
Suppose that the can, just below the spigot on the right, has a slight negative charge, with everything else in the system initially uncharged. A water drop detaching from the spigot in the presence of this slight negative charge on the can underneath it will acquire a slight positive charge. This occurs because the water contains free charges and positive free charges will be attracted (and negative free charges repelled) by the negatively charged can, leading to the charge arrangement in Fig. 3.
That positive droplet charges up the bucket into which it drops positively and because that bucket (lower right in FIG. 2) is connected by a wire to the can on the upper left: that can charges up positive. This will cause the droplet falling on the left to acquire a negative charge, charging up the bucket on the left negatively. Because this bucket is connected by a wire to the can on the upper right, this will then enhance the initial negative charge on the can on the upper right. This makes the droplets falling on the right even more positive, ad infinitum, until breakdown.
In FIG. 2 the electric field on the right of the drawing goes from the positive bucket below to the negative can above (on the left the field is reversed). Gravity is the force that is moving positive charge against the direction of the electric field, that is, from a point of low electrical potential to a point of higher electric potential. In this way, some of the gravitational potential of the water is transformed into electrical as the water drop falls! The initial charge on the can in the upper right can be either pos-itive or negative. In either case, the initial charge is amplified.
Location: Room 26-003