Calculating Electric Field and Distance for a Point Charge

[WeiLoong]

Homework Statement

https://s.yimg.com/hd/answers/i/c7a04ad5e8d74b57a246295c815e183d_A.jpeg?a=answers&mr=0&x=1452518295&s=c54275383477f3e40a56996127dfee96

Homework Equations

Electric field, electrostatic

The Attempt at a Solution

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

V=Ed
=800(0.125+0.4)
=420V
[/B]

 

[blue_leaf77]

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

That one is correct.

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

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.

 

[WeiLoong]

that mean ed = 800x0.40 only?

 

[blue_leaf77]

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

 

[WeiLoong]

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

neeed help about 15b.
W= kq/r + kq/r ?
no idea about it.hehe

 

[blue_leaf77]

that mean ed = 800x0.40 only?

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.

neeed help about 15b.
W= kq/r + kq/r ?
no idea about it.hehe

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.

 

[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 that's the only thing i can think about it.

 

[blue_leaf77]

i guess it should be -320 since V is decreasing.

Yes, it should be -320, otherwise the E field vector cannot be uniform everywhere.

Q1= k(40uC)/12cm

No, that's still not correct. Please look up "Coulomb's law" either in your book or in the internet.

 

[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.

 

[blue_leaf77]

Perhaps it should be kqq/r

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.