# Homework Help: Point charges, midpoint zero or non zero?

1. Aug 17, 2011

### rcmango

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

Point charges +q and -q are placed at +50 cm and -50 cm on the x-axis, respectively. Is the electric field at the origin (halfway between the charges) zero or nonzero? If nonzero, which direction does it point?

2. Relevant equations

3. The attempt at a solution

Well I know the charges will be attracting each other because they are opposite in polarity. I'm not sure if the electric field will be zero or non zero. I do know that if the charges were the same polarity, then the midpoint in the middle would both be zero. Please help me figure this one out completely.

2. Aug 17, 2011

### Staff: Mentor

Use the superposition principle. Google "electric field" and "superposition".

3. Aug 17, 2011

### kuruman

Place a positive test charge at the origin. If the net Coulomb force on the test charge is zero, the electric field is zero; if the net Coulomb force is not zero, then the electric field is not zero and point in the same direction as the net Coulomb force.

4. Aug 17, 2011

### HallsofIvy

Looking at the +q charge at +50 cm only, a negative charge would feel a force toward it, in the positive direction (and a positive charge in the negative direction). Looking at the -q charge at -50 cm only, a negative charge would feel a force away, in the positive direction (and a postive charge in the positive direction). The two forces add, they do not cancel.