# Electric Field and Force between two plates [homework question]

• Samurai44
In summary: Basically, if you have a block of metal with a zero potential on one side, the electric field strength between the metal and the table will be the same on the other side.
Samurai44

## Homework Statement

*Is the a difference when calculating the electric field , or a force exerted on a charge between two plates , if the two plates were :
a) both plates are charged ( connected to a power source) ,
b) both plates are charged ( connected to a power source), but the negative plate is earthed , i.e 0 V.

*Do we consider the sign of the charge when calculating the electric field ?

E=F/Q or F=EQ
E=V/d
F=e V/d

## The Attempt at a Solution

That blank attempt at the solution is somewhat bothersome. What's the voltage across the gap between the plates in either case?

BiGyElLoWhAt said:
That blank attempt at the solution is somewhat bothersome. What's the voltage across the gap between the plates in either case?
Im just asking a general question, not a specific problem ,, take any voltage you like

Well I should really treat this like a homework problem. So the question becomes, in case a (nongrounded) let's say it's -3 and +3V. Is the second case 0 and +3 or 0 and +6?

BiGyElLoWhAt said:
Well I should really treat this like a homework problem. So the question becomes, in case a (nongrounded) let's say it's -3 and +3V. Is the second case 0 and +3 or 0 and +6?
exactly
i uploaded a pic as an example for the second case , how do find the the electric field strength , and force on the electron if it was ,lets say, at mid point between the plates ?

Np, this is physics forum not art forum. If you look at your relevant equations, and then look at your picture, you have a voltage and a distance; you're looking for the electric field. You have the equation you need, with one (reasonable) assumption. First off, 2m is pretty large for a gap in capacitors, but it doesn't really matter, as long as we can assume that the length of the plates are long with respect to the gap, the field will be approximately uniform throughout the region between the plates. Then it's a matter of applying coloumbs law once you find the field. This will give you the force on the electron.

With our assumption, it doesn't matter if the electron was mid point, quarter way or pi/32 of the way through, the force would be (for all intensive purposes) the same.

BiGyElLoWhAt said:
Np, this is physics forum not art forum. If you look at your relevant equations, and then look at your picture, you have a voltage and a distance; you're looking for the electric field. You have the equation you need, with one (reasonable) assumption. First off, 2m is pretty large for a gap in capacitors, but it doesn't really matter, as long as we can assume that the length of the plates are long with respect to the gap, the field will be approximately uniform throughout the region between the plates. Then it's a matter of applying coloumbs law once you find the field. This will give you the force on the electron.

With our assumption, it doesn't matter if the electron was mid point, quarter way or pi/32 of the way through, the force would be (for all intensive purposes) the same.

oh lol i meant 2 cm ,nevermind ,,
so the position doesn't matter i see , what about if the electron was moving , for example from the negative plate to mid point ,, is it the same case ?

Essentially, yes. If you want to find out for sure. Assume uniform charge density on either plate, the integrate the field contribution from each dL along the length of the plate for some arbitrary point (x,y) (you only need to do this in 2d to meet your picture). If you do that for both plates, then apply superposition and sum the contributions from both plates for all points (x,y) you will see that all points are approximately the same, especially far from the edges.

BiGyElLoWhAt said:
Essentially, yes. If you want to find out for sure. Assume uniform charge density on either plate, the integrate the field contribution from each dL along the length of the plate for some arbitrary point (x,y) (you only need to do this in 2d to meet your picture). If you do that for both plates, then apply superposition and sum the contributions from both plates for all points (x,y) you will see that all points are approximately the same, especially far from the edges.
that explains ,thanks
when I learned about equipotentials , i got confused ..
so even if one plate was at 0V , the electric field strength stays constant at all points - (just to make sure)

Yea, zero potential doesn't really mean anything. It's kind of like me saying that a block sitting on a table has 0 GPE or me saying that it has GPE mg(height of the table from the floor). Very analagous.

BiGyElLoWhAt said:
Yea, zero potential doesn't really mean anything. It's kind of like me saying that a block sitting on a table has 0 GPE or me saying that it has GPE mg(height of the table from the floor). Very analagous.
alright ,, thanks a lot for helping :)

## 1. What is an electric field and how is it created?

An electric field is a physical field that surrounds charged particles and exerts a force on other charged particles within its range. It is created when a charged particle (such as an electron) is placed near another charged particle or object, causing a repulsive or attractive force between them.

## 2. How is the strength of an electric field between two plates calculated?

The strength of an electric field between two plates is calculated using the formula E=V/d, where E is the electric field strength, V is the potential difference between the plates, and d is the distance between the plates. This formula assumes that the plates are parallel and have a uniform charge distribution.

## 3. What is the relationship between electric field and force between two plates?

The force between two plates is directly proportional to the strength of the electric field between them. This means that as the electric field strength increases, the force between the plates also increases. The direction of the force is determined by the charges on the plates; like charges repel and opposite charges attract.

## 4. Can the electric field between two plates be manipulated?

Yes, the electric field between two plates can be manipulated by changing the potential difference (voltage) between the plates or by changing the distance between them. The strength of the electric field can also be affected by the material and geometry of the plates.

## 5. How does the electric field between two plates affect charged particles?

The electric field between two plates exerts a force on charged particles within its range. If the charged particles have the same charge as the plates, they will experience a repulsive force and will be pushed away from the plates. If the charged particles have the opposite charge, they will be attracted towards the plates. The strength of the electric field will determine the magnitude of this force.

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