Question

A metal parallel plate capacitor has ‘a’mm diameter and the distance between the plates is 1mm. The capacitor is placed in air. Force on each plate is 0.035N and the potential difference between the plates is 1kV. Find ‘a’.

(a) 10mm

(b) 100mm

(c) 1000m

(d) 1000cm

This question was posed to me during an online exam.

The query is from Force of Attraction Between Oppositely Charged Plates topic in division Capacitance and Capacitors of Basic Electrical Engineering

Answer 1

Right answer is (b) 100mm

To explain: From the given data:

A=pi*d^2/4=pi*a^2/4

Potential gradient = V/x = 10^6V/m

F=epsilon*A*(V/x)^2/2

Substituting the given values, we get d=100mm.