Question

A metal parallel plate capacitor has 100mm diameter and the distance between the plates is ‘a’ mm. 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) 1m

(b) 1cm

(c) 10cm

(d) 1mm

The question was asked in an interview for internship.

This interesting question is from Force of Attraction Between Oppositely Charged Plates in portion Capacitance and Capacitors of Basic Electrical Engineering

Answer 1

Right option is (d) 1mm

The explanation is: From the given data:

A=pi*d^2/4=0.007854m^2

Potential gradient = V/x = 1000/a

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

Substituting the given values, we find a=1mm.