# Parallel plates & Point charge ?

1. Feb 28, 2005

### n3ro

1. parallel plate with capacitor of 20uF, plate is 0.5mm apart.

Find
1. Area of each plate
2. potential difference if there's a change of magnitude of 30uC on each plate.
3. stored energy
4. electric field between the two plates
5. charge density on the plate.

|_| |_|
|_____|
.5mm

I know $$C=\frac{Eo*A}{d}$$

so would $$A=\frac{Eo*C}{D}$$ or A=Q / Eo *E

A= (8.85*10^ -12 ) * 20 / (.5 * 10^ -3) = 3.54 * 10 ^7 ?

2. three point charges on the x axis q1= (10uc) at x= -4m, q2= (-5uC) at the origin, q3= (-20uC) at x= 4m.

Q1(-4,0)...... Q2(0,0)...... Q3(4,0)
(+)----------(-)-----------(-)
10uC........ -5uC........ -20UC

Find
1. electric field at origin
2. force on q2

would electric field on orgin = Zero & force on q2 equal

$$F1= Ke\frac{q1|q2|}{r^2}$$ , $$F2=Ke\frac{|q3|q2|}{r^2}$$

$$F1=\frac{(8.99* 10^9)* (10 * 10^ -6)*( 5*10^-6)}{(-4)^ 2}$$

$$F2=\frac{(8.99* 10^9)* (20 * 10^ -6)*(5*10^-6)}{(4)^ 2}$$

After this would i add components ( x i + y j )

2. Feb 28, 2005

### dextercioby

For the first problem,the area u computed is incorrect.You began with the correct formula,but somehow screwed up the arithmetics.What about the other 4 points...?

Daniel.