Electric potential is zero but field is not

If a positive test charge is halfway in between 2 oppositely charged point charges
then the electric potential is zero.

Then why if you release that charge does it accelerate toward the negative charge away from the
positive charge. What is electric potential measuring if not potential energy?

What does it mean if the field is non-zero but the potential is zero?
thanks!
adam

You have to remember what 'potential at a point' means. It is the work (energy) per coulomb needed to bring a test charge from 'infinity' (i.e a very long way from the charges) to the point.

Imagine +q on the right (at x = 1, y = 0 say) and -q on left (at x = -1, y = 0 say) Mid point = P = origin..

Now put your test charge at 'infinity' on the y-axis and bring it to point P. No work is needed as it will be equally attracted and repelled (zero resultant force). In fact on the y-axis, V = 0 because of this.

However there is still a field at point P. Moving along the line joining the charges involves changing the potential.

Remember Ex = (-)dV/dx. Ey = (-)dV/dy If the potential isn't changing along the y axis, Ey=0. This doesn't stop the potential changing along the x-axis. The field component in a given direction depends how much the potential charges in the that direction per unit distance.

It means the RATE OF CHANGE with distance of potential energy at that point is zero. So electric potential is just how quickly it varies with distance, not the actual value.