# Calculate the distance r where the electrostatic potential is zero between charges.

1. Jan 13, 2013

### mrcotton

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

The diagram shows two charges, +4 μC and –16 μC, 120 mm apart. What is the distance from
the +4 μC charge to the point between the two charges where the resultant electric potential is
zero?
A 24mm
B 40mm
C 80mm
D 96mm

2. Relevant equations

potential equation

3. The attempt at a solution

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

The correct answer is a 24mm
I get that we could simply use a 1:4 ratio and therefore r must be a fifth of the way from the 4 micro coulomb charge
However when I try and equate the formula and solve algebraicaly I seem to get the wrong answer if I put in the sign of the charges.

So my question is why do we just use the magnitude of the charges?

2. Jan 13, 2013

### Staff: Mentor

Re: Calculate the distance r where the electrostatic potential is zero between charge

You seem to have written U1 = U2 when you should have used U1 + U2 = 0.

3. Jan 13, 2013

### mrcotton

Re: Calculate the distance r where the electrostatic potential is zero between charge

Thank you, I am a fool
I must take a break and do some maths in between my physics