Potential energy (1 Viewer)

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

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

Two point charges are arranged as shown on the x axis. q1 = +7.00 nC,
and q2 = −5.00 nC. a = 25.0 cm.
Calculate the position on the x axis, outside of the charges, where the
electric potential is zero


(ignore the periods)

3. The attempt at a solution


kq1/d = -kq2/d

k(7nC)/d = -k(-5nC)/(d-25)

then solve.

I get the correct answer but my question is, why is it d-25 and why is it under q2? My first approach I had.

k(7nC)/(d+25) = -k(-5nC)/d
Last edited:
It depends on what d is. your first approach used d as the distance from q2, and the secontd approach used d as the distance from q1. And in the equation kq1/d = -kq2/d you use both at the same time. This should hae been kq_1/d_1 = -kq_2/d_2

I would have used x for the position of the point with zero potential, the positions of the charges are x_1 = 0, and x_2 = 25
would have d_1 = |x - x_1| and d_2 = |x - x_2| (|x| is the absolute value of x)
if you look for a point to the right of q_2 you can simply have
d_1 = x - x_1 = x
d_2 = x - x_2 = x - 25

The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving