# Electric field strength between two points.

1. Aug 1, 2012

### Dodgers31296

1. The problem statement, all variables and given/known data
Find the electric field strength at point P, which is located midway between charges A and B. The charges are separated by 20 cm. The first charge has a charge of 20 micro coulombs. The second charge has a charge of -30 micro coulombs.

2. Relevant equations
Electric field strength E=Kq/r K is coulomb's constant 9x10 raised to the ninth power. Q is charge. Ris distance from charge.

3. The attempt at a solution
I'm not sure where to start. You need the charge at point P to use Coulomb's law.

2. Aug 1, 2012

### CFede

Hi.

You don't need the charge at the point P. As you well said, the formula for electric Field is E=kQ/R. Only one charge appears there and it is the charge that produces the field.

You are probably confused with Coulomb's law that says the force excerted by a charge Q on a charge q is F=kqQ/R. Since F=q E then E=kQ/R. So you don't need to know the charge at the point P.

3. Aug 2, 2012

### Dodgers31296

Ok so then what do i plug into the equation?

4. Aug 2, 2012

### voko

You have two sources of the field. You can determine the field generated by each one. But you need to know the combined field at a given point. How could you do that?

5. Aug 2, 2012

### Dodgers31296

So i get the field of both and combine them?

6. Aug 2, 2012

### voko

Yes, but the field is directional. You need to take this into account when you combine!

7. Aug 2, 2012

Thank you!