Kelvin Water Drop Generator (D35) [5A40.70]
Electrostatics - Electric Fields
The electrostatic generator consists of water falling from two spigots through two metal cylinders and into 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.
Set-Up:
Fill the jug with water and set it on top of the extended stand. Hook the jug up to the rubber tubing. Position the cans over the buckets, making sure they are positioned such that the above spigots will drop water through the cans and into the buckets. Hook the buckets to the opposite cans using the wires. Now, open the jug and allow water to flow. The flow of water can be adjusted by using the small, provided metal clamp. The water should just be breaking up into droplets as it enters the can. Make sure to avoid air bubbles by pushing them out of the tubing and leaving the cap on the jug loose.
Let the apparatus run; it may take some time for a difference in charge to build up. Using a rod and fur on one of the buckets can also be used to accelerate the process of a building charge imbalance. If you are unsure whether or not the apparatus is working, look for the splaying out of the water stream as it exits the can. This indicates a buildup of charge.
Remember, this experiment will work less well in humid weather. This is because the more humid the air is, the higher the breakdown threshold becomes.
How it works:
Suppose that the can on the right, just below the spigot, has a slightly negative charge, with everything else in the system initially uncharged. A water drop, detaching from the spigot in the presence of this slightly negatively charged can underneath, 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.
The positive droplet gives its positive charge to the bucket into which it drops and, because the bucket is connected by a wire to the can on the upper left, that can now receive a positive charge. 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.
The electric field on the right 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 the 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 potential as the water drop falls! The initial charge on the can in the upper right can be either positive or negative. In either case, the initial charge is amplified.
Electrostatics, 8.02, 8.022
