Finding magnitude of two charge given midpoint

[paulie]

Homework Statement

Two positive point charges, A and B are 6m apart. The electric field intensity at point C, midway between the two charges is 5 N/C directed towards B and the potential at the same point is 45V. Find the magnitude of A and the magnitude of B.

Homework Equations

E = kQ/r²

The Attempt at a Solution

From what I visualize it:

(+A)<---3m--->(-C)<---3m--->(+B)

Using E = kQ/r² where Q=Er²/k

Q = (5 N/C)(3 m)² / (9x10⁹ Nm²/C²) = 5x10^-9C

which is the answer for A and B I suppose? But the answer key tells me that the answer in A is twice the B. How come? I'm quite confuse on how this works.

 

[ehild]

Homework Statement

Two positive point charges, A and B are 6m apart. The electric field intensity at point C, midway between the two charges is 5 N/C directed towards B and the potential at the same point is 45V. Find the magnitude of A and the magnitude of B.

Homework Equations

E = kQ/r²

The Attempt at a Solution

From what I visualize it:

(+A)<---3m--->(-C)<---3m--->(+B)

Using E = kQ/r² where Q=Er²/k

Q = (5 N/C)(3 m)² / (9x10⁹ Nm²/C²) = 5x10^-9C

which is the answer for A and B I suppose? But the answer key tells me that the answer in A is twice the B. How come? I'm quite confuse on how this works.

The equation you applied is valid for the electric field of a single point charge. You have two charges, Q_A and Q_B, both contributing to the electric field and to the potential at point C.