The free end of the top brush will be curled up inside the empty soda can when we are done, and thus electrically connect the soda can to the top brush. We need a small glass tube to act as both a low-friction top pulley, and as a "triboelectric" complement to the rubber band, to generate static electricity by rubbing. Glass is one of the best materials to rub against rubber to create electricity. We get the tube by taking apart a small electrical fuse. The metal ends of the fuse come off easily if heated with a soldering iron or a match.
The solder inside them drips out when they come off, so be careful. The glass, the metal cap, and the molten solder are all quite hot, and will blister the skin if you touch them before they cool. Save the metal caps -- we will use them in a future project! The resulting glass tube has nice straight, even edges, which are "fire polished" for you, so there is no sharp glass, and no uneven edges to catch on the PVC and break the glass.
The next step is a little tricky. The small nail is placed through one of the two holes in the PVC union coupler, and the small glass tube is placed on the nail. Then the rubber band is placed on the glass tube, and the nail is then placed in the second hole.
The rubber band is on the glass tube, which is free to rotate around the nail. Now we glue the Styrofoam collar in place on the PVC pipe. I like to use a hot glue gun for this, since the glue can be laid on thickly to stabilize the collar, and it sets quickly and does not dissolve the Styrofoam. At this point we are ready for the empty soda can. Aluminum pop-top cans are good for high voltage because they have nice rounded edges, which minimizes "corona discharge".
With a sharp knife, carefully cut out the top of the soda can. Leave the nice crimped edge, and cut close to the side of the can so as to leave very little in the way of sharp edges.
You can smooth the cut edge by "stirring" the can with a metal tool like a screwdriver, pressing outward as you stir, to flatten the sharp edge. Tuck the free end of the top brush wire into the can, and invert the can over the top of the device, until it rests snugly on the Styrofoam collar.
The last step is to attach the batteries. I like to solder a battery clip to the motor terminals, and then clip this onto either a nine-volt battery, or a battery holder for two AA size batteries.
The nine-volt battery works, but it runs the motor too fast, making a lot of noise, and risking breakage of the glass tube. It does, however, make a slightly higher voltage, until the device breaks.
To use the Van de Graaff generator, simply clip the battery to the battery clip. If the brushes are very close to the ends of the rubber band, but not touching, you should be able to feel a spark from the soda can if you bring your finger close enough. It helps to hold onto the free end of the bottom brush with the other hand while doing this. To use our generator to power the Franklin's Bells we built in the previous section of the book, clip the bottom brush wire to one "bell", and attach a wire to the top of the generator, connecting it to the other "bell".
The pop-top clapper of the Franklin's Bells should start jumping between the soda cans. It may need a little push to get started. You may have at one time rubbed a balloon on your hair, and then made the balloon stick to the wall.
If you have never done this, try it! The Van de Graaff generator uses this trick and two others to generate the high voltage needed to make a spark. When the balloon made contact with your hair, the molecules of the rubber touched the molecules of the hair. When they touched, the molecules of the rubber attract electrons from the molecules of the hair. The you take the balloon away from your hair, some of those electrons stay with the balloon, giving it a negative charge.
The extra electrons on the balloon repel the electrons in the wall, pushing them back from the surface. The surface of the wall is left with a positive charge, since there are fewer electrons than when it was neutral. The positive wall attracts the negative balloon with enough force to keep it stuck to the wall.
If you collected a bunch of different materials and touched them to one another, you could find out which ones were left negatively charged, and which were left positively charged. You could then take these pairs of objects, and put them in order in a list, from the most positive to the most negative. Such a list is called a Triboelectric Series. The prefix Tribo- means "to rub". Most positive items at this end lose electrons. Most negative items at this end steal electrons.
Our Van de Graaff generator uses a glass tube and a rubber band. The rubber band steals electrons from the glass tube, leaving the glass positively charged, and the rubber band negatively charged. When a metal object is brought near a charged object, something quite interesting happens. The charged object causes the electrons in the metal to move.
If the object is charged negatively, it pushes the electrons away. If it is charged positively, it pulls the electrons towards it. Electrons are all negatively charged. A long narrow conveyor belt of insulating material like Silk, rayon or rubber wound around the pulleys P1 and P2.
Lower brush. Lower pulley P2 - A piece of nylon covered with silicon tape. B1 - Sharply pointed spray comb. B2 - Sharply pointed collecting comb. Due to corona discharge action of sharp points, a positively charged electric wind is set up, which sprays a positive charge on the belt as soon the motor is turned on, the lower pulley P2 begins turning the positively charged belt upwards, and the lower pulley P1 establishes a negative charge. Since the pulley P2 is capturing electrons from the belt which is passing over this pulley P2.
Here, we can see that the charge on the pulley P2 is more concentrated than the belt because a strong electric field is generated at the lower pulley. As the belt reaches the sphere, a negative charge builds upon the collecting comb B2 and a positive charge on the farther side of the comb B2.
This positive charge shifts to the outer surface of S. The discharging action of sharp points of the comb B2, a negatively charged electric wind is set up. Which in turn would neutralize the coated positive charge on the belt, and the belt would turn down again. The belt will collect the positive charge from comb B1, and then would be collected by the comb B2. This process continues, the charge accumulates on the sphere S and the excess charge shows up on the outer surface of the sphere.
Capacitance of electrical sphere. Where V is the Potential Difference. Hence, the Potential Difference V increases with an increase in charge Q. An ordinary lamp socket furnishes the only power needed. Van de Graaff applied for a second patent in December , which was assigned to MIT in exchange for a share of net income.
The patent was later granted. If the charged conductor is brought in to internal contact with a hollow conductor, all of its charge transfers to the surface of the hollow conductor and scatters uniformly over it no matter how high the potential of the latter may be.
It is placed on two pillars at a certain height from the ground. One is p1 and other is p2. An endless belt b made up of insulating material such as rubber, silk or a similar flexible dielectric material, is moving over two pulleys P1 and P2. The pulley P2 is present at the centre of the spherical conductor S and the pulley P1 is present vertically below P2 near the ground. A motor M is used whose main function is to create a spin in the belt.
Two sharp headed combs, spray comb B1 and collecting comb B2 are used. A discharge tube D is used in which the acceleration of ions is done. The point from where the ions originate is present at the head end of the discharge tube. But the other end of the tube is earthed.
The whole apparatus is placed in a steel compartment. Compartment is filled with Nitrogen or Methane. The pressure inside the chamber is maintained very high. Electric wind having a positive charge will be produced. Production of wind occurs due to the discharging of charge from the sharp edges. Belt moves continuously and will reach the sphere on moving.
When the belt will reach the sphere an induced negative charge will be produced on the sharp edges of the comb B2. At once a induced positive charge will be produced on the other side of the comb B2.
After this shifting of the charge,it will be transferred to sphere S. Similarly, due to the discharging action of the B2, wind will be produced but this will be negatively charged. The negatively charged wind helps to make the positively charged belt neutral. After this the belt will be totally discharged. Again after rotation, the belt will come down. It will take the positive charge from the comb B1.
Then again this charge will be taken by the comb B2. The above process repeats again and again. Due to the repetition the charge will start collecting on the sphere S. When the ionization of air starts then side by side leakage of charge will also take place. As we have discussed earlier that the generator is packed into a steel compartment filled with gas such as methane or nitrogen. So, leakage is minimized by this steel chamber. The larger the sphere and the farther it is from ground, the higher will be its final potential.
Primarily designed as a particle accelerator, the Van De Graaff generators are used in laboratories for demonstration purposes only.
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