How to find potential at center of square of point charges?

[look416]

Homework Statement

what is the potential at point P, located at the centre of the square of the point charges as shown in the Fig 10? Assume that d = 1.3m and that the charges are q1 = +12 nC,
q2 = -24 nC, q3 = +31 nC and q4 = +17 nC

http://i923.photobucket.com/albums/ad73/look416/tege.jpg

Homework Equations

F = Eq
E = V/d
F = $$\frac{q1q2}{4\pi\epsilon r^2}$$

The Attempt at a Solution

at first i thought of getting resultant force at the center then using f=Eq to find the E, later using E=V/d to find the V
however, i have to find the charges of P 1st if I am doing so
then think of electric field, E is applicable to everywhere, thinking of something else
conclusion : can't solve it

 

[tiny-tim]

Hi look416!

what is the potential at point P, located at the centre of the square of the point charges as shown in the Fig 10?
…
at first i thought of getting resultant force …

uhh?

just find the individual potentials, and add them.

 

[look416]

lolz, calculated ans wrong =.=
i found out tha for q1 = 63.8, q2 = 127.7, q3 = 164.9, q4 = 90.5
and totaled up to 446.9,
using E = V/d
V = Ed
V = 446.9 x 1.3
= 580.97
omg that's not the answer XD
the answer is 352V
aw probably i need more detailed instruction =.=

 

[look416]

push push

 

[tiny-tim]

what is the potential at point P, located at the centre of the square of the point charges as shown in the Fig 10? Assume that d = 1.3m and that the charges are q1 = +12 nC,
q2 = -24 nC, q3 = +31 nC and q4 = +17 nC

lolz, calculated ans wrong =.=
i found out tha for q1 = 63.8, q2 = 127.7, q3 = 164.9, q4 = 90.5
and totaled up to 446.9,

Shouldn't the potential for q₂ be negative?

 

[look416]

Shouldn't the potential for q₂ be negative?

even if the q2 is negative
E_nett=63.8+(-127.7)+163.9+90.5
=191.5
Convert it into V using E=V/d
V = 248.95
it still not correct

 

[watermlon]

The relation V=Ed is true only for a region of uniform electric field and does not apply here. Assume a test charge Q at the centre of the arrangement, find the potential energy of this test charge , a multiple of Q. Divide by Q to get the potential at the centre. Use
potential energy =
(Qq₁ + Qq₂ + Qq₃₎ + Qq₄$$/$$4$$\pi$$$$\epsilon$$(d$$/$$$$\sqrt{2}$$ )

I'm getting 352.4 V

 

[look416]

The relation V=Ed is true only for a region of uniform electric field and does not apply here. Assume a test charge Q at the centre of the arrangement, find the potential energy of this test charge , a multiple of Q. Divide by Q to get the potential at the centre. Use
potential energy =
(Qq₁ + Qq₂ + Qq₃₎ + Qq₄$$/$$4$$\pi$$$$\epsilon$$(d$$/$$$$\sqrt{2}$$ )

I'm getting 352.4 V

impressive, coz in my memory what i have alrdy learned is all about uniform parallel electric field
too sad no idea you are talking about the equation given
mind giving some website that can clearly express out what you have meant?

Btw, since we don't know the charge of Q,how are we calculating it?