# Magnetic field from Van De Graff

• arydberg
In summary, there is a magnetic field generated from the charges on a moving Van De Graff belt, as any moving charge generates a magnetic field. The charges on the belt do not move at relativistic speeds, so the relativistic effects on the magnetic field are negligible. It is possible to detect this magnetic field, but it would most likely be too small to measure by practical methods.
arydberg
Is there a magnetic field generated from the charges on a moving Van De Graff belt?

If yes how come they do not move at relativistic speeds.

If no doesn't this represent a current and doesn't a current produce a magnetic field.

arydberg said:
Is there a magnetic field generated from the charges on a moving Van De Graff belt?
Yes. Any moving charge generates a magnetic field.

arydberg said:
If yes how come they do not move at relativistic speeds.
Why would you expect that they would?

arydberg said:
Is there a magnetic field generated from the charges on a moving Van De Graff belt?
As BCrowell said, yes, of course.

If yes how come they do not move at relativistic speeds.
A very strange question. You just said the charges are on the Van De Graff belt and it is not moving at relativistic speeds.

[If no doesn't this represent a current and doesn't a current produce a magnetic field.

I assumed relativistic velocities are necessary because just as space and time take on different values depending on the velocity of the viewing frame so do electric and magnetic fields change values depending on being viewed from a moving frame.

If you look at the expression for magnetic force, F = q v x I, you can rewrite it as

I = (rho)*A*v', where rho is the density of charge per meter's in the wire and v' is the velocity of the charges in the wire. Typically v' is very small, by the way. So you get something like

F is proportional to q rho A v v', where v is the velocity of your test charge and v' is the velocity of the charge in the current.

If v <<c and v' <<c, one might expect this expression to be small, but it turns out to be easily measurable in practice, in part because the electromagnetic field is so strong.

I am assuming that the very existence of a magnetic field is a relativistic effect. This idea comes from Lectures in Physics by Feynman ( section 13-6 and 13 -7)
If this is so then is there a difference between the field formed by two Van De Graff belts one carrying half the charge of the second but running at twice the speed. Of course the equation says there is no difference as the moved charge per second is the same but it is hard to imagine relativistic effects arising from such slow velocities.

arydberg said:
I am assuming that the very existence of a magnetic field is a relativistic effect. This idea comes from Lectures in Physics by Feynman ( section 13-6 and 13 -7)

It is, but that doesn't mean you need relativistic velocities. All ordinary magnetism comes from charge flow rates extremely small compared to c. The strength is due to the amount of charge, and the strength of E/M interaction. You don't feel how incomprehensibly strong the coulomb field would be for the amount of charge in ordinary matter because of neutrality.

For example, if magically 1 kg of pure protons in a 1 foot ball materialized near you, I think you would essentially instantly be turned into plasma from the combination of electrons stripped from you, and rapidly expanding protons tearing through you.

OK

Thanks for the help.

OK so if i have a Wimshurst machine with charged spinning disks as shown here

http://www.scientificsonline.com/wi...nerator.html?gclid=CN3NiKqy26kCFSN5gwodJH-naw

And if we assume each metallic element is 10 pF and the voltage is 10 KV then the charge per plate is Q = C X V or 10 X exp -12 x 10 exp 3 = 100 exp -9 coulombs

assuming 500 rpm or 3.14159 feet x 500 = 1570 feet per minute or 26 feet per second or 8 meters per second

now the current = q x velocity = 1 x exp -9 coulombs x 8 meters per second = 8 * exp -7 amps or about 1 micro amp.

So can we detect this magnetic field?

arydberg said:
So can we detect this magnetic field?

I suggest you try estimating it numerically before you try to plan how to measure it. The method of detection is going to depend on the order of magnitude of the effect you're trying to measure. Find the magnetic field of a straight wire carrying a micro-amp of current, at the relevant distance. I suspect it will be much too small to measure by any practical method that you have available.

## What is a Van De Graff generator?

A Van De Graff generator is a device that uses static electricity to produce a high voltage electric field. It typically consists of a large metal sphere connected to a motorized belt that creates friction and transfers electric charge to the sphere.

## How does a Van De Graff generator create a magnetic field?

A Van De Graff generator generates a magnetic field through the movement of electric charge. As the motorized belt creates friction and transfers electric charge to the sphere, it also produces a flow of electrons. This flow of electrons creates a magnetic field around the sphere.

## What is the direction of the magnetic field from a Van De Graff generator?

The direction of the magnetic field from a Van De Graff generator is determined by the direction of the flow of electrons. Since the electrons move from the motorized belt to the sphere, the magnetic field will be directed outward from the sphere.

## How strong is the magnetic field from a Van De Graff generator?

The strength of the magnetic field from a Van De Graff generator depends on the voltage produced by the generator. The higher the voltage, the stronger the magnetic field will be.

## What are the practical applications of the magnetic field from a Van De Graff generator?

The magnetic field from a Van De Graff generator has various practical applications, such as in particle accelerators and electrostatic motors. It is also used in experiments and demonstrations to illustrate the principles of electromagnetism.

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