Electric charge?

Question

Hi,

Consider the four charges in the diagram below, in which d = 4.8 cm and q2 = +2 nC. The net force on the 1 nC charge is zero. What is q1?

Here's the figure: http://i137.photobucket.com/albums/q208/infinitbelt/p25-49.gif


I am confused that the net force is 0. Does that make the charge 0?


Thanks!

Answer 1

The net force can be zero if the forces in all directions cancel out. When q1 is the proper positive charge it can push against the 1nC with a force exactly equal to the force that the two q2 (2nC) are pushing against the 1nC.

The force is directly proportional to electrical charge and inversely proportional to the square of the distance. The distance from a q2 to the 1nC is sqrt(3cm^2+2cm^2)=3.6055513cm. So the forces are:
q1/((4.8cm-2cm)^2)
q2/(3.6055513cm^2)

Since each q2 is out to the side of the 1nC only 2cm/3.6055513cm of the force is in the direction of q1, but there are two of them so there is a 2 times.

q1/((4.8cm-2cm)^2)
= (q2/(3.6055513cm^2))*2
*2cm/3.6055513cm

q1 = (2.8cm^2)
* (2nC/(3.6055513cm^2))*2
*2cm/3.6055513cm

q1=+1.338107523 nC

Answer 2

The forces are vectors so you will have to calculate the direction and magnitude of the force acting on 1 nC by the other two charges which are given in the problem the third unknown charge should have such a magnitude and sign that the force by it on 1 nC should be equal and oposite to the resultant force of the lower two charges on 1 nC. It is obvious that the charge has to be positive. Solve and then put your query if any.