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Electric Potential between two charges
I've been doing some past exam papers for my AQA module 4 exam paper on Friday and I'm a bit stuck on one question. Unfortunately it's multiple choice and so the mark scheme only says the letter... so no help there.
Anyway, the question is;
Two charges, P and Q, are 100mm apart.
P has a charge of +4µC
Q has a charge of -6µC
X is a point on the line between P and Q. If the potential at X is 0V, what is the distance from P to X?
I've tried balancing all kinds of things with r's and 0.1 - r's but I'm not getting ANY of the answers, never mind the right one... I'm not really sure how to deal with potentials when theres more than one charge.
Any help?
Anyway, the question is;
Two charges, P and Q, are 100mm apart.
P has a charge of +4µC
Q has a charge of -6µC
X is a point on the line between P and Q. If the potential at X is 0V, what is the distance from P to X?
I've tried balancing all kinds of things with r's and 0.1 - r's but I'm not getting ANY of the answers, never mind the right one... I'm not really sure how to deal with potentials when theres more than one charge.
Any help?
potentials obey the rules of superposition, so you really do just add them up:
the distance between p and q is d = 100mm, the distance from p to x is x - the unknown. q1 = 4uC and q2=-6uC (the u's are meant to be mu's but i'm lazy):
V(x) = 0 = 1/4pi*epsilon_0 * (q1/x + q2/(d-x))
=> q1/x + q2/(d-x) = 0
=> q1(d-x) + xq2 = 0
=> x = (-dq1)/(q2-q1)
= (100mm * 4uC)/(10uC) = 40mm
that the answer?
the distance between p and q is d = 100mm, the distance from p to x is x - the unknown. q1 = 4uC and q2=-6uC (the u's are meant to be mu's but i'm lazy):
V(x) = 0 = 1/4pi*epsilon_0 * (q1/x + q2/(d-x))
=> q1/x + q2/(d-x) = 0
=> q1(d-x) + xq2 = 0
=> x = (-dq1)/(q2-q1)
= (100mm * 4uC)/(10uC) = 40mm
that the answer?
Reply 2
Nickf
OP
Yes! Thank you!
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