BF2S Forums Physics Electricity Help

The leaves are still apart. When you move the rod near, the negative rod brings the positive charge towards the top, and then when you touch the neutral object the positive charge that is at the top leaves the electroscope. The electroscope will have an overall negative charge, so the leaves will repel each other.

Wow, up until just now I'd never even heard or come across this instrument

Mekstizzle wrote:

Wow, up until just now I'd never even heard or come across this instrument

i come across your mum.

Mek it's because the only purpose of this "instrument" is to teach kids how charge and grounding works.

Ryan wrote:

Flaming_Maniac wrote:

The leaves are still apart. When you move the rod near, the negative rod brings the positive charge towards the top, and then when you touch the neutral object the positive charge that is at the top leaves the electroscope. The electroscope will have an overall negative charge, so the leaves will repel each other.

...

Lemme try and figure this out. This is what I have so far (for the previous questions):

When the rod is brought near the sphere, the electrons in the sphere are repelled toward the bottom of the electroscope and accumulate in the leaves. Therefore, the leaves will repel each other.

Once the electroscope is grounded, the electrons in the leaves will exit through the finger into the ground (because they are attracted to the positive charge in the earth).

So when both the ground and the rod are removed (at the same time I'm assuming), wouldn't the electroscope be left with an overall positive charge (because most of the electrons left through the finger), causing the leaves to remain apart (repulsion)?

A ground is a sink or a source.

Imagine you have a positively charged sphere and an uncharged sphere of equal size. When you touch them, they are both positively charged with half the magnitude of the original positively charged sphere right? Now imagine the uncharged sphere is HUGE in comparison. Charge flows the same way, but because the uncharged sphere is so much larger nearly all the charge leaves the original sphere. That is essentially what a ground is. It doesn't have a charge either way, but it is so large that it soaks up practically all the charge present.

So you got the first part right. But when you touch the ground to the sphere of the electroscope, the rod is still present. The electrons at the bottom don't just ignore the negatively charged rod that is what forced them to the bottom in the first place. The ground makes everything under the influence of the negatively charged rod zero out, meaning electrons actually flow from the ground into what was previously positively charged until the sphere has no charge. When you remove the ground, now the top of the rod is still neutral, but it still has a more positive charge in relation to the bottom of the electroscope because of the negatively charged rod. When you remove everything the electroscope has an overall negative charge as a result.

Peter wrote:

Flaming_Maniac wrote:

The leaves are still apart. When you move the rod near, the negative rod brings the positive charge towards the top, and then when you touch the neutral object the positive charge that is at the top leaves the electroscope. The electroscope will have an overall negative charge, so the leaves will repel each other.

Pretty sure positive charge can't just get up and 'leave'. Protons are pretty heavy you know.

Negative rod pushes electrons away from the rod. When it is grounded electrons escape, leaving an overall positive charge and the leaves repel. No?

Charge has nothing to do with protons outside chemistry. It is all about the presence or absence of electrons.

In the scenario you and Ryan are describing, what is the point of the rod at all? If all the electrons are going to "escape" to ground anyways, why is the rod included in the problem?

ah I see your second post now

The electroscope can also be charged without touching it to a charged object, by electrostatic induction. If a charged object is brought near the electroscope terminal, the leaves also diverge, because the electric field of the object causes the charges in the electroscope rod to separate. Charges of the opposite polarity to the charged object are attracted to the terminal, while charges with the same polarity are repelled to the leaves, causing them to spread. If the electroscope terminal is grounded while the charged object is nearby, by touching it momentarily with a finger, the same polarity charges in the leaves drain away to ground, leaving the electroscope with a net charge of opposite polarity to the object. The leaves close because the charge is all concentrated at the terminal end. When the charged object is moved away, the charge at the terminal spreads into the leaves, causing them to spread apart again

Flaming wrote:

The ground makes everything under the influence of the negatively charged rod zero out, meaning electrons actually flow from the ground into what was previously positively charged until the sphere has no charge.

Huh?

I'm not sure I'm following anything you said after that lol.

I thought the sphere would be positive. But the point you made about how the electrons don't ignore the negative rod makes sense.
My teacher is a super hard marker and if I don't have every frickin' detail, it'll be wrong.

Well what stubbee quoted is what I said earlier that the electrons in the leaves drain to ground leaving the electroscope positive.

You're still saying there is a surplus of electrons.

Okay then, I was just blindly trusting it.

Last edited by Peter (2010-04-10 13:37:49)

Okay, I'm bad, if you can trust me at all I think this is a good explanation why.

http://www.physicsclassroom.com/mmedia/estatics/esn.cfm wrote:

Once charge within the electroscope has been polarized (i.e., separated into opposite types), the bottom of the electroscope is touched by a finger. Being repelled by the negatively charged balloon, electrons from the electroscope exit and enter into the ground. Once more, this process is driven by the principle that like charges repel. The electrons, having a mutual repulsion for one another and for the negatively charged balloon, choose to exit the electroscope and enter into the larger region. By doing so, the electrons are able to distance themselves and so minimize the repulsive interactions. It is at this point in the induction process that the electroscope acquires an overall charge. Since electrons have left the electroscope, the overall charge on it is positive. In general, the induction process will always place a charge on the object which is the opposite type of charge possessed by the object used to charge it.

The resistance between electrons moving across inductors is so low it is nothing compared.

That wiki link is right for the wrong reasons. The charge being in the leaves has nothing to do with it, the point is the overall charge in the electroscope wants to be opposite the charge in the rod.

Again, sorry I messed this one up pretty bad. Makes me sad considering I raped this shit two years ago.

edit: Make sure you understand grounding though. That part I didn't mess up the explanation of - ground is neutral and can be a sink or a source. The charge doesn't leave because it is attracted to the ground, it leaves because it is repelled from the charged rod.

Hang on a minute, so

Peter wrote:

Negative rod pushes electrons away from the rod. When it is grounded electrons escape, leaving an overall positive charge and the leaves repel.