Q#16 (Past Exam Paper – November 2016 Paper 12 Q30)
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
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.
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