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**1. Homework Statement**

From Resnick and Halliday:

A metal plate 8 cm on a side carries a total charge of 60 microC. Using the infinite plate approximation, calculate the electric field 0.5 mm above the surface of the plate near the plate's center.

**2. Homework Equations**

(1) [tex]E = \frac{\sigma}{\epsilon_0}[/tex]

or

(2) [tex]E = \frac{\sigma}{2\epsilon_0}[/tex]

[itex]\sigma[/itex] being charge density.

**3. The Attempt at a Solution**

I got a bit confused here. Equation (1) should be used when the plate is a conductor, and equation (2) should be used when the plate is an insulator, according to the explanation in the book.

However, when I use equation (1) to calculate, I get 106MN/C, and in the answers in the back of the book it says it should be 53 MN/C. Either I used the wrong equation, or I should have used half of the charge.

What went wrong?

And honestly, I really did not understand why there are 2 different equations for each situation (conductor, insulator). I tried deriving them on my own but no success in gaining real understanding, so I'd be really glad if someone could shed some light on the subject.