# Griffith's Figure 2.24

1. Mar 9, 2008

### ehrenfest

[SOLVED] Griffith's Figure 2.24

1. The problem statement, all variables and given/known data
This question refers to Griffith's E and M book.

In the paragraph above this figure, Griffith's says that the E-fields cancel in regions i and iii. He does NOT mean cancel completely, correct? That is, the field is NOT 0 in those two regions, correct?

2. Relevant equations

3. The attempt at a solution

2. Mar 9, 2008

### kdv

Yes they cancel completely if you assume infinite plates. Of course in real life there are no infinite plates.

3. Mar 9, 2008

### ehrenfest

Oh I see. It is because equation 2.17 tells us that the E-field is independent of the distance from the plates.

4. Mar 9, 2008

### kdv

Exactly. This is the special property of an infinite surface charge distribution.

A point charge has an E field that goes like 1/r^2. A uniform line of charge (infinite) has an E field going like 1/r. A uniform infinite surface charge distribution has a constant E field