# Two charges are placed at the corners of a square

1. Jan 29, 2012

### remchu123

Electric Potential !!

1. The problem statement, all variables and given/known data
Two charges are placed at the corners of a square. One charge, 4.0μC, is fixed to one corner and another, -6.0μC, is fixed to the opposite corner. What charge would need to be placed at the intersection of the diagonals of the square in order to make the potential difference zero at each of the two unoccupied corners?

2. Relevant equations

V= kq /r

3. The attempt at a solution

Vtot = V1 + V2 + V3

but then i realized that 3rd charge is unknown where it is, i just don't know how i should approach this problem? help. =[

2. Jan 29, 2012

### tiny-tim

welcome to pf!

hi remchu123! welcome to pf!
call the 3rd charge (the one in the centre) q, then proceed as usual …

show us what you get

3. Jan 29, 2012

### SammyS

Staff Emeritus
Re: Electric Potential !!

Hello remchu123 . Welcome to PF !

That's why there are variables.

Let the charge at the center have a charge of Q or q.

4. Jan 29, 2012

### remchu123

Re: Electric Potential !!

V1 + V2 + V3 =0

kq1 /d + kq2 /d + kq3 /d (route)2 over 2= 0

then i solved for q3 and i got -2.8x 10^-6 C

but the answer is -1.4 x 10^-6C

which means i forgot half somewhere..

thanks though! i think i can handle it!

wish me luck in physics 12 exam tomorrow! ahha

i'll very likely need all of your "ki"s ( DBZ reference? lol)

5. Jan 29, 2012

### tiny-tim

hi hremchu123!

(btw, it's "root" not "route" )

you just got confused about the distance …

it's d/√2, not d√2