Static electricity?

Question

A negatively charged sphere and an insulating thread is lowered into the uncharged metal can standing on an insulating slab.

My question is: why is a metal can used and not like an insulating can?

the question in my book just says: state witha areason the effect of touching the outside of the can with a wire which is connected to earth.

Answer 1

when we lower negatively charged sphere into a can , it attracts positive charges and remove negative charges into the earth . an insulating can , can't conduct electrical charges.

Answer 2

First, you DON'T need to know why and second, electrons DO NOT carry a positive charge.

The negative electrons in an atom whirl around a positively charged nucleus. In a metal, a few are loosely bound and can move about from atom to atom with ease. If a piece of metal has too many electrons (a static charge), the extra electrons will repel each other and move as far from each other as possible. An insulating material does not allow them to flow like this within itself. So the negatively charged sphere doesn't have excess electrons distributed evenly within itself; they have all moved as far from each other as possible and sit on the outside surface.

The metal can is used because someone wants to move the charge around, and an insulating can won't let the charge move. When the sphere is lowered into the can and touches it, it becomes electrically one with the can. Now the can-and-sphere has a surplus charge, and where does it go? The electrons again repel each other and move to the outside surface of the can-and-sphere. If you remove the sphere, you'll find it has no charge left on it. The entire charge has moved to the surface of the can. The inside of the can ALWAYS HAS NO CHARGE because all excess electrons will repel each other to the outside surface--just like the charge on the sphere only existed on the surface and not within it. So you can do this again and again and again, and the bucket will accept the additional charge until it eventually sparks over to something. This is the principle behind a Van de Graaf generator. Google that up, and also try Google images because they look really cool in action.

But the charge always resides on the outside of the can which, standing on an insulating slab, is where it has to stay until you approach it with a grounded wire which allows it a path to leave. Notice that if you touch the wire to the INSIDE of the can, nothing will go anywhere because that isn't where the electrons are hanging out. They're busy repelling each other and wouldn't be caught dead inside that can.

So the metal can is used to allow the electrons to move around.

Answer 3

EDIT: I meant positive charges, not positive electrons.

Your additional details explains the reasons for the metal can. It needs so be metal (an electrical conductor) so that charges can move freely. Positive charges in the metal will move to the inner surface, and negative charges to the outer surface, but the overall charge on the metal can is still neutral. When you connect the can to earth, the negative charges in the can will be "pushed out" through the wire to the earth. Only metal will allow the electrons to travel from every part of the can to the wire. The end result is that the can will have an overall positive charge equal to that of the charge on the sphere.


MORE: It doesn't matter if you ground the can on the outside or inside, the electrons will still be drawn out of the metal can, leaving the can with a positive charge. It is easier to explain this by looking at potential differences rather than charge location. The charged sphere creates a potential field around it; when dropped into the isolated can, the can will reach a potential (different from ground) that depends on the geometry of the situation. In any case, the potential difference between the can and ground will cause a current to flow when a grounded wire touches it anywhere, and will continue to flow until the can is at ground potential. At that point the can will have a net positive charge.

Answer 4

To answer this question, we would have to know why someone is lowering a sphere into a can in the first place. Since you didn't specify the result they were trying to obtain, we can't help you.