# Three Kind of electrostatics problem(quite )

1. Mar 18, 2008

### sunumen

Three Kind of electrostatics problem(quite urgent)

1. The problem statement, all variables and given/known data
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?

2. Relevant equations
Q1:
(eqn1)
$$\frac{tan\alpha1}{tan\alpha2}$$ = $$\frac{K1}{K2}$$
where alpha1 is incidence angle
K is dielectric coefficient

D=$$\epsilon$$E
where D is electric displacement
E is E-field

K=$$\epsilon$$/$$\epsilon$$o

where $$\epsilon$$o is the permittivity of dielectric
$$\epsilon$$ is the permittivity of vaccum

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1. The problem statement, all variables and given/known data
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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1. The problem statement, all variables and given/known data
The last kind of question is about the leakage and capacitance.
http://myweb.polyu.edu.hk/~07190086d/phy/q3.zip

The equation related:
RC=$$\epsilon$$/$$\sigma$$
where $$\sigma$$ 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