What is the distance from Q1 for a zero electric field at point P?

[jena]

Hi,

My question:

Two point charges, Q1=- microcolumbs and Q2= microcolumbs are separated by a distance of 12cm. The electric field at the point P is zero. How far from Q1 is P?

I know that E=F/q, but I'm not sure if I equal this to zero. To figure out this problem, do I first devise a portion to help me find the answer? Not sure

Thank You

 

[HallsofIvy]

Hi,

My question:

Two point charges, Q1=- microcolumbs and Q2= microcolumbs

Aren't there supposed to be numbers in here?

are separated by a distance of 12cm. The electric field at the point P is zero. How far from Q1 is P?

I know that E=F/q, but I'm not sure if I equal this to zero. To figure out this problem, do I first devise a portion to help me find the answer? Not sure

Thank You

 

[jena]

Yep sorry

Two point charges, Q1=-25 microcolumbs and Q2= 50 microcolumbs are separated by a distance of 12cm. The electric field at the point P is zero. How far from Q1 is P?

Thank You

 

[Doc Al]

The first thing for you to figure out is where is point P. Is it between the two charges? Or on the far side of one of them? (Which one?) This will help you visualize what's going on.

To solve for the position of point P, try this: Call the distance between point P and Q1 by the variable "x". Now write the equations for the field from each charge at point P. The fields must be equal and opposite to cancel, so set the magnitudes of the fields from each charge equal to each other. Solve for x.

 

[lightgrav]

jena, they're asking questions about the E-field because
they want you to get used to using it in its own right,
not just a "math trick" computed from Force per charge.

an E-field surrounds any source charge Q, as E=kQ/r^2 ,
even when there's no other charge *at* the distant place.
(like gravity field "g" exists even where it's not pulling on a rock)

Once you find the E-field at some place, caused by source charges Q,
then you get the Force on another charge there by F = qE . (like mg)

 

[jena]

Hi,

I think I figured it out I have to make a equation like

Kq/r^2 + kq/(r+12)^2= E

This will allow me to solve the problem.

Thank You

 

[Doc Al]

I think I figured it out I have to make a equation like

Kq/r^2 + kq/(r+12)^2= E

Good! Be careful with units.
$E_{total} = k q_1/ r^2 + k q_2/(r + 0.12)^2$