Two parallel circular metal plates X and Y, each of diameter 18 cm, have a separation of 9.0 cm.

Question 16

Two parallel circular metal plates X and Y, each of diameter 18 cm, have a separation of 9.0 cm.

A potential difference of 9.0 V is applied between them.

Point P is 6.0 cm from the surface of plate X and 3.0 cm from the surface of plate Y.

What is the electric field strength at P?

A 50 N C^-1                         B 100 N C^-1                      C 150 N C^-1                      D 300 N C^-1

Reference: Past Exam Paper – November 2016 Paper 12 Q30

Solution:

Answer: B.

Electric field strength E = V / d

V is the p.d. between the plates (= 9.0 V) and d is the separation of the plates (= 9.0 cm). Note that d is NOT the distance of a point in the electric field from one plate BUT d is the separation of the plates and here, this is constant (and = 9.0 cm).

E = V / d = 9.0 / 0.09 = 100 N C^-1

 
Since the plates are parallel, the electric field strength is uniform between them. The field strength depends on the separation of the plates, not on the position of a point between the plates.