# Homework Help: Electric field problem

1. Jan 24, 2010

### look416

1. The problem statement, all variables and given/known data
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

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

3. 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 im doing so
then think of electric field, E is applicable to everywhere, thinking of something else
conclusion : cant solve it

2. Jan 24, 2010

### tiny-tim

Hi look416!
uhh?

just find the individual potentials, and add them.

3. Jan 24, 2010

### 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 thats not the answer XD
aw probably i need more detailed instruction =.=

4. Jan 25, 2010

### look416

push push

5. Jan 25, 2010

### tiny-tim

Shouldn't the potential for q2 be negative?

6. Jan 26, 2010

### look416

even if the q2 is negative
Enett=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

7. Jan 26, 2010

### 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

8. Jan 26, 2010

### look416

impressive, coz in my memory what i have alrdy learnt is all about uniform parallel electric field