# Simple capacitor problem

I asked the relation between the new and the initial surface charges.
Oh okay, well if
σ=ε0E1
σ'=0.7σ=0.7ε0E1

Or would it be by a factor of 0.3?

ehild
Homework Helper
Oh okay, well if
σ=ε0E1
σ'=0.7σ=0.7ε0E1

Or would it be by a factor of 0.3?
What do you think, is the new charge more or less than the initial one? Why we use dielectrics in the capacitors? To increase their ability to store more charge.
Look at your picture. The actual E is E = E1-E2, and E2=0.3 E.

What do you think, is the new charge more or less than the initial one? Why we use dielectrics in the capacitors? To increase their ability to store more charge.
Look at your picture. The actual E is E = E1-E2, and E2=0.3 E.
Right, so then it would be σ'=0.7σ=0.7ε0E1.

ehild
Homework Helper
Is the new charge less than the initial one?

Is the new charge less than the initial one?
Well logically I would suppose not then, since C=q/V and we already know V is constant because of the battery, so q must change. Adding dielectrics help increase C, so that means q increases as well.

So before, were you saying that E=E1-(-E2)? Or E=E1+(-E2), because it seems like you're hinting that it's the first, which in that case it would be by a factor of 1.7.

ehild
Homework Helper
Well logically I would suppose not then, since C=q/V and we already know V is constant because of the battery, so q must change. Adding dielectrics help increase C, so that means q increases as well.

So before, were you saying that E=E1-(-E2)? Or E=E1+(-E2), because it seems like you're hinting that it's the first, which in that case it would be by a factor of 1.7.
It is the first, E1=E+E2, but E2 is the field of the dipoles, which is 0.3E, if the E2 means the magnitude of the dipole field.

It is the first, E1=E+E2, but E2 is the field of the dipoles, which is 0.3E, if the E2 means the magnitude of the dipole field.
Right, so then it would increase by a factor of 1.7, so
σ'=1.7σ=1.7ε0E1.
And that is the surface charge density on the dielectric?

ehild
Homework Helper
Right, so then it would increase by a factor of 1.7, so
σ'=1.7σ=1.7ε0E1.
And that is the surface charge density on the dielectric?
Why 1.7? It is wrong.

Last edited: