# Calculate Electric field from two point charges

1. Feb 12, 2008

### mikehibbert

[SOLVED] Calculate Electric field from two point charges

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

Two point charges with q1=2x10^-5 and q2=-4x1-^-3 are located in free space at (1,3,-1) and (-3,1,-2), respectively, in a cartesian coordinate system. Calculate the electric field E at (3,1,-2)

2. Relevant equations

E=(1/4piE0)(-qr1/r1^3+qr2/r2^3)

3. The attempt at a solution

I understand the above equation but am unsure of how to implement it using cartesian coords?

Last edited: Feb 12, 2008
2. Feb 12, 2008

### Kurdt

Staff Emeritus
The distance between two points is:

$$s = \sqrt{(x_1-x_2)^2+(y_1-y_2)^2+(z_1-z_2)^2}$$

3. Feb 12, 2008

### mikehibbert

how do i use the vector in this part of the equation though?

q1r1/r1^3

the r1 on top is still a vector...

and then i presume for the bottom part i just take the magnitude of r1 and cube it?

4. Feb 12, 2008

### Kurdt

Staff Emeritus
Well I've given you the magnitude of the vector so what do you imagine its components to be?

5. Feb 12, 2008

### mikehibbert

just (x,y,z) surely?

6. Feb 12, 2008

### Kurdt

Staff Emeritus
Make that: $((x_1-x_2) \mathbf{\hat{i}}+.....)$ etc.

7. Feb 12, 2008

### mikehibbert

Awesome, thanks!