Question

When an electrical appliance is switched off, sparking occurs. Why?

Answer 1

Before understanding why spark occurs we need to understand Dielectric strength and Dielectric breakdown.

$DielectricStrength$ :

Of an insulating material, the maximum electric field that a pure material can withstand under ideal conditions without breaking down is called Dielectric strength. It is intrinsic property of the bulk material.

$Dielectricbreakdown$:

When we apply an electric field across material more than dielectric strength then dielectric breakdown occurs.

Dielectric Strength of air is: $3.0x{10}^{6}V/mor30KV/cm$

So we need an electric field of $30KV/cm$ for dielectric breakdown of air molecules.

Let us assume a daily household switch.

We generally operate our appliances on $220V.$

so, $V/d=30KV/cm$

implies $d=220/30000=.0073cm\left(approx\right)$

When a gap less that the above distance occur between the terminals of a key when we switch off,a sudden dielectric breakdown of air molecules occur.

This results in conduction of air molecules for a short duration and during that period of time we see spark.