- #1

dHannibal

- 10

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## Homework Statement

Two discs are each charged +q. A third metal disc of radius R∗ > 5

cm is carefully inserted between the two discs; the third disc is neutral and is suspended by

an electrically insulating thread. The three discs are all parallel to each other and their

centers lie along the same horizontal line ,so that when viewed head-on the discs are

concentric circles. The resulting set-up is shown in the figure.

Find the radius R∗ of the third disc such that the net electrostatic force acting on each

charged disc is zero. (The fringing effect is neglected in this problem).

[PLAIN]http://img89.imageshack.us/img89/4135/vdcx.png

## Homework Equations

The charge distribution on a single charged disc is

[tex]\sigma _{\text{disk}} = \frac{Q}{4 \pi R \sqrt{R^2 - r^2}}. [/tex]

## The Attempt at a Solution

For starters,I can't figure out how the charges will be distributed on the neutral disc.

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