electric field, charges...

[Ice-cream]

Four point charges, each of magnitude q are located at the corners of a sqaure with sides of length a. Two of the charges are +q and two are -q. The charges are arranged in one of the following two ways: 1. The charges alternate in sign (+q,-q,+q,-q) as you go around the sqaure. 2. The top two corners of the square have positive charges (+q,+q) and the bottom two corners have negative charges (-q,-q).

 

a) Which case will the electric field at the centre of the square have the greatest magnitude?

 

b) Calculate the electric field at the centre of the square for each of these two cases.

 

I really have no idea for the parts of this question. Can any1 give me any starters tips or just point me in the right direction for thinking?

 

also, i have another question:

 

Two identical point charges in free space are connected by a string 6.6cm long. The tension in the string is 0.21N. Find the tension in the string if +1.0microC of charge is transferred from one point charge to the other.

 

what im not sure is what they mean by the charge being "transferred from one point charge to another"...can any1 explain?

 

thanx

[gnpatterson]

can you add vectors?

 

if so just calculate the field due to each charge and add them up

 

one situation will totally cancel out and the other will add up to give terms that "cancel" in mathematical terms to a very simple expression in q and a

 

it is possible to think of the problem in geometric terms but that makes peoples heads hurt.

[Dave]

As for the other question; it simply means that one particle will gain 1.0uC of charge, and the other will lose 1.0uC charge. I think that you're going to have to set up some simultaneous equations, but they're rather simple to solve.

[gnpatterson]

i dont think you need simultaneous equation for second as it gives you the fact that the first case charges are identical