electrostatics - What is the difference between the potential energy and the energy of a test charge due to the electric field?

We are taking the electrostatic course in physics class, and I was wondering about some things related to potential, potential energy, and electric field. Imagine two identical particles with opposite charges separated by a distance $d$. In the middle of the two charges, half of $d$, there a test charge is placed. The test charge starts to move to the right, assuming the negative is on the right and the positive is on the left. But if we were to calculate the potential at that point, it would be zero which also implies no energy. The particle, however, accelerates to the right to the negative charge with some energy. Where did that energy come from? You might respond to the question simply by saying, the energy came from the electric field because the motion is caused due to the electric field (or the force of attraction and repulsion contributed by each charge). Then, what do we mean by zero potential/potential energy? What energy are we talking about when we say potential energy or and the kinetic energy of the particle caused by the electric field? Isn't the potential energy defined by the same particles as the electric field or force?

Answer

Saying that the potential energy is zero at that point does not make much sense.

Only potential energy differences matter.

• Think of a ball on the floor. The gravitational potential energy is zero. But only because the floor is our reference. If there is a hole in the floor, then the ball will roll down there, because that position has lower gravitational potential.

The gravitational potential energy in the hole is negative. That does not make much sense in itself. It tells us nothing. Only the difference compared to some other point matters.

• The ball would not roll to some other point at the same floor because the potentials are the same. And a ball on a shelf would also not roll to another point on that shelf, even though both of those points have gravitational potential energy, because the difference is zero.

Similar for electric potentials. If you calculate a zero electric potential at the starting point of the charge, then it is because you use that same point as your reference. That alone tells you nothing. But the charge sees that the potential gets lower, if it moves closer to the source. So it starts moving. The potential becomes negative seen from the same reference. That doesn't say anything in itself. Only the difference is important.

Bottom line: Whenever there is a potential energy difference, there is a force trying to make the object move. Regardless of the actual potential energy values.