Van de Graaff generator: from theory to practice
About Generator
The Van de Graaff generator is a physics experiments that visually is one of the most interesting that can be performed in the classroom, and the amount of concepts and phenomena that can be demonstrated with is simply absurd.
Basically, the Van de Graaff generator uses frictional electrification to function, as shown below:
Figure 1: Operation of a Van de Graaff generator.
A rubber belt, when moving, electrifies by friction with the brushes. The excess electrical charge caused by electrification is accumulated in the upper metallic sphere, creating electric fields up to 40 kV / m, enough to depending on the situation, momentarily breaking the isolation of the air and create small lightning.
Some very interesting feats can be performed with the generator. For example, if you touch the generator and turn it on while standing on the ground, nothing will happen because you will serve as a driver for the accumulated charges in the ball going to the ground. But when you step into an insulating material, is a wooden base or a plastic stool, charges no longer flow to the ground, but will accumulate in you, making an effect similar to the shown in the picture below.
Figure 2: Van de Graaff generator in action.
Although the effect of the tips of hair repelling is visually interesting, the potential of the Van de Graaff generator goes much further. Using other accessories accompanying the kit, other experiments can be performed, such as: types of electrification, electric field lines, electric potential, electric potential difference, charged points, lightning rods, capacitors and others.
Figure 3: The figure demonstrates the first electric field lines between two potentials of different formats. The second figure shows the electrical discharge between two metal balls that make up the kit.
Presentation Tips
An interesting way to start a presentation in class, with Van de Graaff generator, is questioning students with everyday questions, to check their prior knowledge of the content. For example:
“You tried to touch the two poles of a conventional battery (1.5V) at the same time, closing a circuit with his fingers? What happened? And with a 9V battery? I’m a little afraid to ask, and before continue reading already warning: DO NOT DO IT !!! But what happens if you put your fingers into the socket and what would happen if they put their fingers in 200.000V you can put your hands on 200.000V and survive “???
If you put your fingers in the two poles of a conventional battery or a 9V battery, apparently, nothing will happen, you will not feel anything. And the reason is that the voltage and current are too low. However, by placing your fingers in 110V socket is enough to cause a dangerous electric shock. And for the human body, the higher the voltage, the greater the current, so if the tension in your house is 220V, it is even more dangerous. Thinking of 200.000V the situation is even more tragic. Unless you use a generator as the Van de Graaff’s.
The great secret of the Van de Graaff generator is that although the tension between the ball and the floor can be up to 200,000 volts, the amount of charge is very small, so that we can touch the ball without any problems. Being off the electrical current involved, we are susceptible to a minimum electric shock, something more like a small bite.
Suggested Experiments
Depending on the content that is being explained, the Van de Graaff generator. It can be used to enhance a performance and learning. The proper functioning of the generator is an interesting example of electrification by friction. With the electrified metal ball, and a person with both hands in contact with the ball, we can demonstrate the electrification by contact, charged points and to demonstrate that the electric shock does not depend on potential, but a potential difference. The student, to be isolated from the ground and in contact with the electrified sphere, is at the same potential of the sphere, the same effect that occurs when a bird lands on a high-voltage wire and doesn’t get hurt.
Another way to explore the generator is approaching the smaller metal sphere to the largest, causing the neutralization of this surface and the formation of a small lightening. Whereas the dielectric strength of air, under normal conditions, is somewhere around 10.000V / cm, when measuring approximately the distance of the smaller sphere to the largest so that the lightening occur, students can roughly calculate the potential of the sphere.
Also, when a fluorescent lamp is approximately the electrified sphere, without touching, it tends to light up. This is another demonstration of the electric potential formed by the ball. The potential difference between the two ends of lamp generates inside a small current capable of light it up.
There are many possibilities for use in the classroom of the Van de Graaff generator, some of the cited applications and other more are addressed in our experimental manual that comes with the product kit. Are activities that will bring enchantment and learning for one of the most dense contents of physics. It is worth checking!