Question

Four charges are located at the corners of a square as shown below. Calculate the magnitude and direction of the net force on charge -3Q and charge 4Q.

(sorry don't have picture, but it is of a square with corners -Q, 2Q, -3Q, and 4Q clockwise).

I don't understand how to start with the information I've been given, it seems like its not enough.

Answer 1

Here is how you get the force on -3Q:

Perform a vector addition to find the net force due to the three charges acting upon -3Q. There are attractive forces towards adjacent charges 2Q and 4Q (in perpendicular directions) and a repulsive force pointed away from -Q, along the diagonal. Let a be the length of the side of a square, unless they have given you a different symbol or a numerical value.

Use Coulomb's law for each of the three forces.
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For the net force on charge 4Q, repeat the process.