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fluidistic
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Homework Statement
Say I have a parallel plates capacitor. The length of the plates are worth l. Both plates are separated by a distance d. Say there's a dielectric material whose permissivity is [tex]\varepsilon _1[/tex] such that it's between the plates from 0 to x, if 0 is the left side of the capacitor, that is if you draw a sketch such that the 2 plates are horizontal.
I want to know if I have to do some work in order to displace the dielectric material on a distance x'. I'd like to know the answer if the dielectric material is semi infinite (i.e. if I pull it, I will gradually fill the capacitor with this material) and if the material is finite (i.e. if I pull it, it will only cover a distance x at all times, although it moves).
Homework Equations
[tex]Q=CV[/tex], [tex]U=\frac{CV^2}{2}[/tex].The Attempt at a Solution
I've done some attempt on my draft. However I don't know why I struggle with the E field. I will detail a bit more. I've said that the problem is equivalent to sum up 2 capacitances. One of a capacitor [tex]C_1[/tex] filled with the dielectric material, such that the length of the capacitor is x. The other capacitor [tex]C_2[/tex] such that it is not filled with anything (i.e. vacuum) and whose length is l-x. I said both capacitors are considered in parallel, so I have that [tex]C=C_1+C_2[/tex].
So my task is to calculate [tex]C_1[/tex] and [tex]C_2[/tex].
For [tex]C_1[/tex], [tex]V=\frac{Q}{C_1}[/tex].
[tex]V=-\int _0 ^d \vec E \cdot d\vec l[/tex]. I know that [tex]\vec E[/tex] and [tex]\vec l[/tex] are parallel so the integral simplifies a lot, but I'm stuck at what value to plug for E. Of course I have to consider E as constant, but what value do I choose?
I've been stuck on a similar question last time... I'd like to continue alone, but I need someone to help me.
By the way, I'm planning on calculating the energy stored in the capacitor before and after having displaced the dielectric material. The difference of these energy would represent the work I have done by displacing the material. Am I right thinking like this?
Also, as [tex]U=\frac{CV^2}{2}[/tex], if the dielectric material is semi infinite then C will increase, but I'm unsure about Q and V, so I can't say that the energy stored in the capacitor increases and thus [tex]\Delta U >0[/tex] so to make this increment of energy, I had to do some work.
I also believe that if there's an emf connected to the capacitor, the problem changes completely because I've heard it, but I don't realize it. If someone could enlighten me, I'll be extremely glad.