# Problems that I got wrong and still don't understand

1. Feb 15, 2007

### jst

Before my discovery of this forum, I tried to do some homwork with not help....well, I got a few wrong and will be having a quiz soon...so I gotta correct a few of my old mistakes

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

Given a uniformly charged arc with radius R and total charge Q. The arc subtends a total angle of 90 degrees. Derive an equation for the total electric field at the center of curvature of this arc in terms of R,Q and the Coulomb constant k.

2. Relevant equations

I'm not sure

3. The attempt at a solution

I put: (kQ^2)/R^2 , which was not the correct answer

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

Given: two charge diagrams each consisting of three charges arranged in an equilateral triangle. Each charge has the same magnitude of charge, q. Force directions are labeled A through L spaced at 30 degree intervals. What is the direction of the Coulomb force on charge #3 in Diagram A?

2. Relevant equations

3. The attempt at a solution

I put "B" and was correct, but I'm not sure my logic was correct.

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

Given two charges aligned horizontally. Initially both charges have an equal charge -q. The electric force on the right charge is shown having a magnitude of one. In the diagrams below changes have been made to the situation. For each diagram A,B, and C, state the relative length of the electric force vector on the right charge.

2. Relevant equations

3. The attempt at a solution

I put: A half, B 4, C 4

The correct was: A 8, B 4, C half
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1. The problem statement, all variables and given/known data

Given: Two point charges of charge +q and - q aligned horizontally. The positive charge feels an electrostatic force F. Five points along the horizontal axis are labeled as shown A through E. To what point could a third charge +q be placed so that the net electrostatic force on the positive charge is 3F?

2. Relevant equations

3. The attempt at a solution

I put A, but the correct answer was B

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

Given: Two point charges of charge 7.00 nC and -7.00 nC aligned horizontally. The charges are separated by a distance of 8.00 cm. What is the electric field strength at point B halfway between the charges?

2. Relevant equations

3. The attempt at a solution
I did:

(8.988*10^9)*(7*10^-9)/(.08/2) +
(8.988*10^9)*(-7*10^-9)/(.08/2)
=0

Which was wrong, the solution was:
.0788 MN/C

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

Given a uniformly charged vertical line and a uniformly charged thin half ring with their axes of symmetry aligned horizontally as shown. The total charge on the line is Q. The total length of the line is 2.00 m and the half-ring radius is R = 1.00 m. At a distance x = 1.00 m to the right of the line charge is the center of curvature of the half ring. What is the total charge on the half ring if the total electric field at the center of curvature is zero? (You will need to integrate over the half ring. S = R(theta) and dS = R(dtheta) and dq = (charge per length)(dS)

2. Relevant equations

3. The attempt at a solution

I some how came up with: 3.84Q

But the solution was: 1.11Q

I appreciate any help with any/all of these.

Thanks,

Jason