Charges A through E have a charge of either + or - 5mC. They are placed at regular intervals along a semicircle of radius 7cm. Find the magnitude and direction of the force exerted on an electron placed at the center of the circle. Find the electric potential at the center of the circle

Answer 1

Are they All the same charge? All + or All -?
If so, you know by symmetry that the force will be straight toward the center of the semicircle if the charges are + and straight away from C if the charges are -
You do not have to do A and E because they are equal and opposite.
You only have to do the vertical component of B and D because the horizonal components are equal and opposite and just do one of them and double.
So you really only have to do two force computations,
Do B and double it to include D
Do C
add those three forces.
then for the potential:
Do the potential for ONE of the 5.
The potential depends only on the charges and distance apart, 7 cm for all five of them.
POTENTIALS ADD !!!
so multiply your potential result for the first one by 5
because they are all the same charge and the same distance.
Then use

Answer 2

Forget the last line. I was going to say use Coulomb's law, but you know that.

Answer 3

There will be five charges spaced 45 degrees apart along the semicircle. In order to compute the net force by vector addition, you will need to know which have + and which have - charges. If they all have the same charge, or if the charges alternate +, -, + , -, + around the semicircle, you can use symmetry arguments to conclude that the force at the center of the semicircle is perpendical to the diameter.

Use Coulomb's Law and do the vector addition.

You might find out that the net force is zero if the charges alternate. Note that the forces due to the two charges at the end of the semicircle cancel out.

Since you have not said what the sign of the charges is, that is as far as I can go with this question