## Sunday, 2 June 2013

### Van Der Graaf Machines - Air Discharge and Identified Flying Objects

This is the last post I have on Van Der Graaf machines for the moment.
In this post, I describe demonstrations that show the charge on objects is restricted to the surface of the object, and objects with large radii, can store more charge than objects with small radii.

This explains why the Van Der Graaf machine has a large hollow sphere, and why dust (and the Hair Extension) reduces the charge on a Van Der Graaf machine.

### Airplane Attachment

There is another attachment that you can get for a Van Der Graaf machine which consists of a little pin that fits into the top of the Van Der Graaf main sphere, and a little model airplane that balances on top of the pin's head.
Once you start up the Van Der Graaf machine, the little airplane will start to spin around.
If that doesn't work, hold the discharge sphere over the airplane to give it a kick.

### Theory

Don't explain the full theory until you complete the final Pie Plate demonstration. The main points you will want to present now are:
• The airplane has a sharp point at it's tail. As the electrons build up on the main sphere, they flow into the airplane.
• The sharp point of the tail loses charge to the air fast. (You can remind the students about the lower charge held on the Van Der Graaf machine when the Hair Extension was used.)
• This leakage of charge causes the air behind the plane to become negatively charged.
• The plane and the air now repel each other and the plane spins around.

### Pie Plate Demonstration

Otherwise known as the "Identified Flying Object" demonstration. This is an ideal demonstration - it demonstrates the scientific principle of charge spread in the simplest possible way I can think of.
1. Discharge the Van Der Graaf and loosely stack the pie plates on top of the main sphere. If you've got perfect weather conditions, the stack can be as high as 20 plates tall.
2. Fire up the Van Der Graaf machine. Pretty soon the pie plates should fly off the Van Der Graaf from the topmost plate to the bottom.

### Theory

• Like charges repel, including the negatively charged electrons that build up in the Van Der Graaf machine.
• The quickest way for the electrons in the Van Der Graaf sphere to get as far away from each other as possible, is for them to spread themselves out over the external surface of the Van Der Graaf sphere.
• If there is a point that is further away than the rest, the electron density will build up at that point to a greater extent than elsewhere.
• This is why the Van Der Graaf generator uses a sphere - all points are the same distance away from the center.
• This is also why the sphere can be hollow - only the outside of the sphere matters.
• This is why the top plate flies off first - it is at the furthest point on the sphere. The most charge density builds up there, causing the greatest repulsion between this plate and the one below. Once the top plate is flicked off, the furthest point is now the second top plate. The charge density builds up there and then it is flicked off.
• Finally, this externalization of charge explains why the point of the airplane loses charge easily - it's tail becomes the furthest external point on the sphere, the charge density builds up there very quickly, and jumps off into the air rapidly.
• Such points gather charge density very quickly, but because they're so small, they don't built up much total charge. And because they leak charge so rapidly, they prevent anything connected to them from building up much total charge.
• Finally this is the basic theory behind lightning rods - they don't attract lightning. Rather they discharge the ground below lightning storms, preventing a lightning strike from occurring.