Question

Two plate are parallel and separated from d distance.A uniformly electric field E exist between the plate B to plate A. Find the potential difference between the plates. Thedirection of electric field is from plate A to B 

Answer 1

V = Ed
where V = Potential difference
          E = electric field intensity
          d = distance between plates
Since the direction of field is from higher potential to lower potential, A is at higher potential than B.

Answer 2

This context is from the parallel plate capacitors.

Potential difference is defined as the integral of electric field along the path between the two end points.

equivalently,  electric field is equal to the gradient of potential along the path.

When electric field E is uniform,  then Potential difference between A and B will be:
  
$E = \frac{d\ V}{d\ x} = \frac{V_A - V_B}{d}\\V_A - V_B = E . d$

As the direction of field is from A to B, A is at a higher potential than B.