Dielectric and Attraction Between Oppositely Charged Plates

[iVenky]

I am not able to intuitively understand the reasoning behind why the presence of dielectric between oppositely charged plates, let's say, reduces the force of attraction between the plates. I understand to some extent that electric field lines prefer to flow through dielectric (or insulator) than a vacuum, but I am not able to grasp clearly how this drops the force of attraction between the charged particles. Shouldn't the number of field lines be the same in both instances (i.e. different relative permittivities)? I am missing something fundamental here.

 

[Baluncore]

Maybe, charge the plates to a known voltage. C = Q / V.
Disconnect the capacitor so charge is fixed.
Then slide a dielectric into the gap.
The capacitance is increased, so the voltage must fall.

 

[sophiecentaur]

This effect has to be compared with the Force between plates on a capacitor (connected to a battery) as the dielectric is changed. The PD between the plates is kept the same so the dielectric doesn't alter it.

 

[alan123hk]

but I am not able to grasp clearly how this drops the force of attraction between the charged particles

I think there are two different situations.

The first situation is that when the dielectric slides into the gap, the voltage of the capacitor remains constant ( connected to a battery ), so both the energy and the attractive force will increase.

But in the second situation, the voltage source of the capacitor is removed, and then the dielectric slides into the gap, so the energy and attractive force will be reduced.

 

[iVenky]

I think there are two different situations.

The first situation is that when the dielectric slides into the gap, the voltage of the capacitor remains constant ( connected to a battery ), so both the energy and the attractive force will increase.

But in the second situation, the voltage source of the capacitor is removed, and then the dielectric slides into the gap, so the energy and attractive force will be reduced.

Thanks but I still don't get the intuition behind this (instead of math). Can you explain how the force changes in terms of field lines?

 

[eq1]

Can you explain how the force changes in terms of field lines?

I think this explanation is quite good.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dielec.html#c1

increased dielectric -> decreased voltage -> increased space between fields lines

 

[sophiecentaur]

. Can you explain how the

You want intuition? OK. If the Energy stored changes then you can think in terms of work being done or Energy flowing in or out of an electrical.
If your intuition will allow you to think in terms of
Energy = CV²/2 or E = QV and consider whether the V or the Q are constant.
Then the work done will either be positive or negative, depending and any slight asymmetry will cause the dielectric to be pulled in or pushed out. And you don't have to work out any actual forces for the above argument to work.

 

[alan123hk]

Can you explain how the force changes in terms of field lines?

First of all, I want to correct what I said. When the voltage source of the parallel plate capacitor is removed and then the dielectric is slid into the gap, the attractive force between the two plates of the capacitor will not decrease. In fact, this attraction remains the same.

In addition, I didn't have time yesterday, but today I finally have time to think carefully about this thing and draw the intuitive electric field line diagram. Hope this diagram is of reference value for you.

 

[iVenky]

First of all, I want to correct what I said. When the voltage source of the parallel plate capacitor is removed and then the dielectric is slid into the gap, the attractive force between the two plates of the capacitor will not decrease. In fact, this attraction remains the same.

In addition, I didn't have time yesterday, but today I finally have time to think carefully about this thing and draw the intuitive electric field line diagram. Hope this diagram is of reference value for you.View attachment 278482

I really appreciate you spent some time drawing the diagram (the diagrams look amazing by the way). Thanks so much!

 

[AlexCaledin]

The attraction might increase a little because the field becomes more gathered in between the plates. The dipole-like curved field around the edges becomes weaker with the dielectric, right?

 

[alan123hk]

The attraction might increase a little because the field becomes more gathered in between the plates. The dipole-like curved field around the edges becomes weaker with the dielectric, right?

It seems that this is very possible.