# I The effect of an external substance on the electric force between two charges

#### hokhani

How does the force between the two opposite charge change if we place a substance (conductor, insulator, ...) between them? It seems that the force between the two charges doesn't change but the total force on each charge increases because of the superposition of the original field of the other charge and the extra electric field due to the induced charge on the substance. I would like to know if I am wrong.

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#### mfb

Mentor
That depends on how you count the forces. What you described is one possible way to view the problem.

#### hokhani

That depends on how you count the forces. What you described is one possible way to view the problem.
If we had for example a thick wood between the two point charges, $q_1$ and $q_2$, can we say that:
the force between the two charges is $F=kq_1q_2/r^2$ and the resultant force (sum of the this force and the effects of other charges inside the wood) on each charge is zero? In other words, is the force between the two charges independent of any other effect?

#### mfb

Mentor
and the resultant force (sum of the this force and the effects of other charges inside the wood) on each charge is zero?
Why would it be zero?
In other words, is the force between the two charges independent of any other effect?
That is one way to consider forces. It is not necessarily always the easiest approach.

#### thierrykauf

Gold Member
How does the force between the two opposite charge change if we place a substance (conductor, insulator, ...) between them? It seems that the force between the two charges doesn't change but the total force on each charge increases because of the superposition of the original field of the other charge and the extra electric field due to the induced charge on the substance. I would like to know if I am wrong.
If you put a conductor between two charges, doesn't the electric field vanish inside ?

#### ZapperZ

Staff Emeritus
2018 Award
How does the force between the two opposite charge change if we place a substance (conductor, insulator, ...) between them? It seems that the force between the two charges doesn't change but the total force on each charge increases because of the superposition of the original field of the other charge and the extra electric field due to the induced charge on the substance. I would like to know if I am wrong.
Maybe you need to look at what happen to the electric field when you insert a dielectric in between the plates of a capacitor.

Zz.

#### hokhani

Why would it be zero?
I think a thick wood shields the electric field and so each charge doesn't feel the effect of the other charge.

#### hokhani

If you put a conductor between two charges, doesn't the electric field vanish inside ?
Right, It does. But I want to know the force between the two charges and compare it with the total force (the resultant force due to the two charge and the induced charge on the conductor surface) each charge.

#### hokhani

Maybe you need to look at what happen to the electric field when you insert a dielectric in between the plates of a capacitor.

Zz.
The main question is:
How does the force between the two point charges change if we put a conductor between them? (increase, decrease or no change)

#### olgerm

Gold Member
How does the force between the two opposite charge change if we place a substance (conductor, insulator, ...) between them?
conductor between the charges gets polarized.

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#### ZapperZ

Staff Emeritus
2018 Award
The main question is:
How does the force between the two point charges change if we put a conductor between them? (increase, decrease or no change)
Maybe you should re-read your own post. Here's what you wrote in the original first post:

How does the force between the two opposite charge change if we place a substance (conductor, insulator, ...) between them? It seems that the force between the two charges doesn't change but the total force on each charge increases because of the superposition of the original field of the other charge and the extra electric field due to the induced charge on the substance. I would like to know if I am wrong.
I simplified one part of the question by suggesting something that you can look up easily as an analogous situation. but I guess learning something is not what you're interested in.

Zz.

#### hokhani

Maybe you should re-read your own post. Here's what you wrote in the original first post:
Although the main question was about placing a conductor between the two point charges, I raised the question generally (for conductors and insulators and ...), and exactly stated what I like to know.

I simplified one part of the question by suggesting something that you can look up easily as an analogous situation.
My question is beyond the well-known dielectric effect which decreases the electric field between the two capacitor plates. This clear effect is explained exactly in the elementary text books.
but I guess learning something is not what you're interested in.

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#### Lord Jestocost

Gold Member
2018 Award
How does the force between the two opposite charge change if we place a substance (conductor, insulator, ...) between them?
No liability assumed. Your question is sometimes addressed in terms of an “effective distance between charges” in case a dielectric slab of a given thickness is placed in between two charges in vacuum.

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"The effect of an external substance on the electric force between two charges"

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