In Fig. 25-2, the potential difference between the metal plates is 40 V. (a) Which plate is at the higher potential? (b) How much work must be done to carry a +3.0 C charge from B to A? From A to B? (c) How do we know that the electric field is in the direction indicated? (d) If the plate separation is 5.0 mm, what is the magnitude of E?