What is the Charge on a Capacitor with Given Area and Plate Separation?

[gracy]

How can we know this time Earth will give charges and at the other instance it won't?

 

[gneill]

Should not q transfer direction be other way around in #54?

How so? The batteries are driving charge in the direction that a current would flow.

 

[gracy]

I am taking q to be electron because it is electron which actually travels.

 

[gracy]

My bad that would be -q then not q,right?

 

[gneill]

How can we know this time Earth will give charges and the other it won't?

The charges are available if needed. If they are not needed, none are "borrowed".

 

[cnh1995]

How can we know this time Earth will give charges and the other it won't?

When there is a potential difference between earthed plate and other plate(s), Earth will give charge.

 

[gracy]

What is called " need" in this context?

 

[cnh1995]

What is called " need" in this context?

I believe its the potential difference.

 

[gneill]

I am taking q to be electron because it is electron which actually travels.

I've assumed conventional current, where charge carriers are assumed to be positive. It's less likely to confuse others if we stick to the convention.

 

[gracy]

When there is a potential difference between earthed plate and other plate(s), Earth will give charge.

And when Earth takes charge?

 

[cnh1995]

And when Earth takes charge?

When Earth is at higher potential then the other plate(s).

 

[gneill]

What is called " need" in this context?

A potential difference that requires charges to move. They have to come from somewhere. If they are available from an external connection then that's where they come from. If the plate were isolated then existing charges could only be re-arranged, and charge separation would occur.

 

[gracy]

Why there is no potential difference between earthed plate and other plate(s) in #39

 

[gracy]

When Earth is at higher potential then the other plate(s).

You meant when earthed plate is at higher potential then the other plate(s)?

 

[gracy]

When Earth gives electron?

when earthed plate is at lower potential then the other plate(s)?

 

[cnh1995]

You meant when earthed plate is at higher potential then the other plate(s)?

Yes. When 'ground' is assumed at +ve terminal.

 

[gracy]

How can we know earthed plate is at higher or lower potential than the other plate(s)?

 

[cnh1995]

When Earth gives electron?

when earthed plate is at lower potential then the other plate(s)?

Yes.

 

[cnh1995]

How can we know earthed plate is at higher or lower potential than the other plate(s)?

By convention, +ve side is considered as higher potential side.

 

[gracy]

By convention, +ve side is considered as higher potential side.

Here side means?

 

[cnh1995]

Here side means?

Sorry, I meant +ve charged part of the circuit like +ve terminal of the battery.

 

[gracy]

Sorry, I meant +ve charged part of the circuit like +ve terminal of the battery.

plate with net positive charge is at higher potential and plate with net negative charge is at lower potential,right?

 

[cnh1995]

plate with net positive charge is at higher potential and plate with net negative charge is at lower potential,right?

Yes, by convention.

 

[gneill]

Why there is no potential difference between earthed plate and other plate(s) in #39

There is. It's established by the two batteries. Plate a is at +V with respect to the middle plate, and plate c is at -V with respect to the middle plate.

 

[gracy]

Then why no charge transfer occur from Earth as charge are transferred from Earth if there is potential difference between earthed plate and the other plate(s)?

 

[cnh1995]

Then why no charge transfer occur from Earth as charge are transferred from Earth if there is potential difference between earthed plate and the other plate(s)?

I believe that's because it is connected to +ve terminal of one battery and -ve terminal of the other battery.

 

[cnh1995]

I believe that's because it is connected to +ve terminal of one battery and -ve terminal of the other battery.

Formation of net -ve charge is avoided by the +ve terminal of one battery and formation of +ve charges is avoided by the -ve terminal of the other battery. However, if the two battery voltages were different, there would be net charge on the middle plate.

 

[jbriggs444]

Yes, by convention.

This is not correct as I have pointed out previously. The charge on a plate does not determine its potential. The potential of a plate does not determine its charge.

 

[cnh1995]

This is not correct as I have pointed out previously. The charge on a plate does not determine its potential. The potential of a plate does not determine its charge.

I agree. But in case of a battery, isn't +ve terminal considered as higher potential conventionally? Also, conventional current is assumed to be from higher potential to lower potential and its direction is shown from +ve to -ve terminal of the battery. So, if a capacitor is connected across a battery, its +vely charged plate will be at higher potential than the -ve plate. That's what I meant in that post.

 

[jbriggs444]

I agree. But in case of a battery, isn't +ve terminal considered as higher potential conventionally? Also, conventional current is assumed to be from higher potential to lower potential and its direction is shown from +ve to -ve terminal of the battery. So, if a capacitor is connected across a battery, its +vely charged plate will be at higher potential than the -ve plate. That's what I meant in that post.

Agreed, the relatively more positively charged plate will be at a higher potential than the relatively less positively charged plate.