# Attraction of metal plates and ball

paperdoll

## Homework Statement

I'm having trouble understanding charges on Earthed plates I think.
I know the answer for this question is that the ball will be attracted to the high voltage plate and stay there but I do not know why is that.

## Homework Equations

A light metal ball is suspended between two metal plates. One plate is earthed and the other is connected to the positive terminal of a high voltage supply. The ball will: ?

## The Attempt at a Solution

I've been thinking about it and I'm not sure why I'm wrong about this question. In my opinion, the earthed metal plate would be devoid of electrons (since they all escaped to the Earth?) so it would be positively charged. So wouldn't the ball be more attracted to the earthed plate rather than the voltage positive plate? I'm not really sure

## Answers and Replies

Homework Helper
MHB
Hi paperdoll!

In an electric circuit the negative electrons flow from the negative pole to the positive pole.
In other words, a positive pole attracts and absorbs electrons.

An Earth connection allows electrons to flow in or out freely.
In the presence of a positive charge, electrons will be drawn in from the earth.

So the electrons will be drawn away from high voltage plate, leaving a positive charge.

The positive charge will attract electrons to the earthed plate and it will also unbalance the electrons in the metal ball.
In particular one side of the metal ball will become negatively charged with electrons (on the side of the positive plate), and the other side will becomes equally positive charged.

Which forces will come into play?

paperdoll
Hi paperdoll!

In an electric circuit the negative electrons flow from the negative pole to the positive pole.
In other words, a positive pole attracts and absorbs electrons.

An Earth connection allows electrons to flow in or out freely.
In the presence of a positive charge, electrons will be drawn in from the earth.

So the electrons will be drawn away from high voltage plate, leaving a positive charge.

The positive charge will attract electrons to the earthed plate and it will also unbalance the electrons in the metal ball.
In particular one side of the metal ball will become negatively charged with electrons (on the side of the positive plate), and the other side will becomes equally positive charged.

Which forces will come into play?

ok, I've drawn a diagram of the situation but I still cannot figure out why the ball will be attracted to the voltage plate. Why shouldn't the ball just stay put where it is? or maybe swing around between the two plates

here is my diagram http://imageshack.us/photo/my-images/819/screenshot20111023at721.jpg/"

Last edited by a moderator:
Homework Helper
MHB
ok, I've drawn a diagram of the situation but I still cannot figure out why the ball will be attracted to the voltage plate. Why shouldn't the ball just stay put where it is? or maybe swing around between the two plates

here is my diagram
[PLAIN]http://img819.imageshack.us/img819/7204/screenshot20111023at721.jpg[/QUOTE] [Broken]

Nice picture!
(I still like pictures! )

Well, initially the ball is more or less in balance.
I think it will have a slight tendency toward the high voltage plate, since that one is actively charged positively, while the earthed plate is only charged negative as a response.

It becomes interesting when the ball touches either plate however.
In your problem statement you're suggesting that the ball will stay at the high voltage plate.
This is not the case.

What do you think will happen when the metal ball touches the positively charged high voltage plate?

Last edited by a moderator:
paperdoll
Nice picture!
(I still like pictures! )

Well, initially the ball is more or less in balance.
I think it will have a slight tendency toward the high voltage plate, since that one is actively charged positively, while the earthed plate is only charged negative as a response.

It becomes interesting when the ball touches either plate however.
In your problem statement you're suggesting that the ball will stay at the high voltage plate.
This is not the case.

What do you think will happen when the metal ball touches the positively charged high voltage plate?

umm when the metal ball touches the positively high voltage plate I would imagine that electrons will move towards the plate and the ball will become more positively charged? so then it will be repelled and swing towards the earthed plate :\ I think

Homework Helper
MHB
Yep! :)

When the ball touches the high voltage plate, its voltage with be equalized to the voltage of the plus pole, meaning it becomes positively charged.

So the ball swings to the earthed plate.
What will happen when the ball touches the earthed plate?

paperdoll
Yep! :)

When the ball touches the high voltage plate, its voltage with be equalized to the voltage of the plus pole, meaning it becomes positively charged.

So the ball swings to the earthed plate.
What will happen when the ball touches the earthed plate?

ok so the ball will now be positive and this means that electrons will be drawn up from the ground on the earthed plate. So when the ball touches this plate, electrons, move from the earthed plate to the ball and then the ball becomes the same charge as the the plate. So it repels and swings back??

I'm imaging the ball swinging back forth, back forth, back through now haha :uhh:

paperdoll
Homework Helper
MHB
Till next time then!