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**Three Kind of electrostatics problem(quite urgent)**

## Homework Statement

The first kind of prbolem is about the dielectric coefficient

http://myweb.polyu.edu.hk/~07190086d/phy/q1.zip

I know the equations below

but I don't know how to relate the E-field into eqn1

The main reason is that I don't know is the D related in this kind of problem?

## Homework Equations

Q1:

(eqn1)

[tex]\frac{tan\alpha1}{tan\alpha2}[/tex] = [tex]\frac{K1}{K2}[/tex]

where alpha1 is incidence angle

K is dielectric coefficient

D=[tex]\epsilon[/tex]E

where D is electric displacement

E is E-field

K=[tex]\epsilon[/tex]/[tex]\epsilon[/tex]o

where [tex]\epsilon[/tex]o is the permittivity of dielectric

[tex]\epsilon[/tex] is the permittivity of vaccum

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## Homework Statement

The second type of problem is about the simplest electrostatic problem!!!!!!!

http://myweb.polyu.edu.hk/~07190086d/phy/q2.zip

I don't really understand the meaning of "potential" is zero.

Doesn't the potential is set by us?

I know that when the net force is zero, then the PE should be minimum.

But these problems make me very confusing.

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## Homework Statement

The last kind of question is about the leakage and capacitance.

http://myweb.polyu.edu.hk/~07190086d/phy/q3.zip

The equation related:

RC=[tex]\epsilon[/tex]/[tex]\sigma[/tex]

where [tex]\sigma[/tex] is lossy dielectric of conductivity

I cannot really remember the previous concept of capacitance very well

but I know that the time constant(decay to half ) is RC

this problem are something new for me

and I wanna know how to use the idea of lossy dielectric of conductivity