Electric Field and distance

1. Jan 11, 2016

WeiLoong

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

2. Relevant equations
Electric field, electrostatic
3. The attempt at a solution

E=v/d
d=100/800
d=0.125m

V=Ed
=800(0.125+0.4)
=420V

Last edited by a moderator: May 7, 2017
2. Jan 11, 2016

blue_leaf77

That one is correct.
By potential at X, the question must refer to the potential difference between point X and the 0 V. So, 0.125+0.4 is not the correct value for the corresponding distance.

3. Jan 11, 2016

WeiLoong

that mean ed = 800x0.40 only?

4. Jan 11, 2016

blue_leaf77

Yes. Just, be careful with the proper sign of the potential.

5. Jan 11, 2016

WeiLoong

https://scontent-hkg3-1.xx.fbcdn.net/hphotos-xfp1/v/t1.0-9/12509720_10205411857847476_602511770052026952_n.jpg?oh=66b58b8024f86e46c7a9c860c4d89bcf&oe=56FB9605

W= kq/r + kq/r ?

6. Jan 11, 2016

blue_leaf77

Just want to remind you, in case you are not yet aware of, that that answer is still missing something to be justified according to the physical situation given in the problem.
Write down the formula of electric field due to each charge along the connecting line, define this line as $x$. Denote $x_1$ and $x_2$ as the distances of a given point from $Q_1$ and $Q_2$, respectively. The equation you have there is wrong.

7. Jan 11, 2016

WeiLoong

Whats the missing part actually? i got 320 V. I am not sure it is -320 or +320V actually. but i guess it should be -320 since V is decreasing.

Q1= k(40uC)/12cm
Q2 =k(80uC)/12cm thats the only thing i can think about it.

8. Jan 11, 2016

blue_leaf77

Yes, it should be -320, otherwise the E field vector cannot be uniform everywhere.
No, that's still not correct. Please look up "Coulomb's law" either in your book or in the internet.

9. Jan 11, 2016

WeiLoong

Perhaps it should be kqq/r
can u explain this concept to me? i thought product of charges over distance is only used to determine the force, never thought u can be express as electric potential too.

10. Jan 11, 2016

blue_leaf77

There is no "perhaps" in this matter because the related equation which is sometimes called "Coulomb's law" had been agreed upon among the physicists long time ago. You will gain much more knowledge by searching/reading on your own about this basic concept of electric field by a point charge in electrostatics than you will if someone were to explain that to you.