room topic changed to Physics noob tries to get better (ft.John rennie): (no tags)

@satan29 hi :-)

i figured ill jump on the bandwagon, if its okay with you

Yes, that's fine :-)

Is it late right now, or can i ask something?

Though by this time of day I'm pretty burned out. You need to catch me in the morning really.

@satan29 you can ask ...

so my question is about the earthing of metal plates

OK ... ?

I am quite familiar with the charge distribution on metal plates

However, my textbook has given this result: Consider a system of "n"plates.When we earth any of the plates, The charge on the two outermost plates beceomes zero

I asked about this on the main site a while back:

and I didn't get an answer.

^yes, i am aware of this

It's probably one of those things that's obvious once you know the trick for proving it, but I have to confess that it is not obvious to me.

The problem starts when we earth one of the plates

@satan29 in my question I wrote:

> For bonus marks another of the starting points students are taught is that if any plate in the setup is earthed then the exterior charges are zero. If this doesn't emerge naturally from whatever approach is used to answer my first question I would also be curious to see how this could be proved.

But got no answer.

ohhh right, I just saw this, apologies.

Actually, @JohnRennie , I kind of came up with an explanation

Yes ... ?

just give me 2 minutes

@JohnRennie

You're going to argue that the only way the field can be zero outside the plates is if the external charges are zero?

What i am saying is

Consider the field lines on the extreme left, and extreme right

(consider only one atm)

They terminate at Infinity

and upon earthing, We assert that there is no potential difference between the earth and the system

earth----->infty

so we assert that there is no PD between Our system, and infty.

Wich is equivalent to saying that no field lines should exist between our system and infty

And once we make c zero, the field lines on the extreme right and left dissapear

And another thing, this explanation holds regardless of what plate we earth

@JohnRennie makes some sense , or this sounds like a load of steaming hot BS?

Yes it makes sense, and it's along the lines I was thinking. It's just that for infinite parallel plates I'm not sure if the potential at infinity is defined.

It is certainly true that if we want the potential at infinity to be zero then the only way to achieve this is if the field outside the plates is zero.

And that requires the outermost charges to be zero.

@JohnRennie Thats why i decided to think of an arguement regarding the field instead of potentials

@JohnRennie May i ask, WHat were you thinking?

Yes

its very hard to find literature on this...anywhere.My textbooks simply state the result

Yes :-)

Maybe you should post that as an answer to my question.

what was your approach to this?

@JohnRennie Yes, i will now :)

@satan29 same as yours i.e. the field outside the plates must be zero.

ok, thats nice to hear :).

Thanks for your time sir, I will see you tomorrow :-)

Bye :-)

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