Question

I have two parallel charged plates at a distance 10cm from each other with air in between. What should be the minimum potential difference between the plates to get charges flowing between them?
(Dielectric strength of air : $3\times {10}^{6}V{m}^{-1})$

• A. 1000V
• B. 2000V
• C. 3000V
• D. None of these

Answer 1

The correct option is C 3000V

We just have to find what potential difference will cause an electric field greater than the dielectric strength of air. When a higher field is applied, the molecules of the medium (here, air) ionize. The field is so strong that it "breaks" the molecule into positive and negative charges. We see this in the form of sparks. Lightning is an example of dielectric breakdown of air. These charges will flow in the electric field.
Since the field between plates of a capacitor is constant we can say that potential gradient will be linear and hence,
$E=-\frac{V}{d}$
We know E and d. Find V
$3\times {10}^6=-\frac{-V}{10\times {10}^{-4}}$
$V=-3000V$
The potential difference between the plates must be 3000V.