Distance from +4μC to Zero Potential: 24mm

[mrcotton]

Homework Statement

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

Homework Equations

potential equation

The Attempt at a Solution

Homework Statement

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?

 

[Doc Al]

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

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

 

[mrcotton]

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