Answer:
a. The potential difference between two parallel charged plates can be calculated using the formula V = Ed, where V is the potential difference, E is the electric field strength between the plates, and d is the distance between the plates. In this case, the distance between the plates is 4.0 cm or 0.04 m and the electric field strength is 750 V/m. Plugging these values into the formula gives V = 750 V/m * 0.04 m = 30 V. So, the potential difference between the two parallel charged plates is 30 V.
b. The work done to move a charge between two points in an electric field can be calculated using the formula W = qV, where W is the work done, q is the charge being moved, and V is the potential difference between the two points. In this case, the charge being moved is +7.0 mC or 0.007 C and the potential difference between the positive and negative plates is 30 V (calculated in part a). Plugging these values into the formula gives W = 0.007 C * 30 V = 0.21 J. So, it would take 0.21 joules of work to move a +7.0 mC charge from the positive plate to the negative plate.