Why do switches spark when opening a connection to a load?

[david90]

What causes a switch's contact to spark when you use it to open a connection to a load?

I can understand why the contact sparks when opening an inductive load (inductive reactance), but what if the load is purely resistive?

 

[KingNothing]

The same thing that causes a spark in any situation: a voltage difference, separated by air. When the electric field strength exceeds the dielectric field strength of air, you get a spark. This happens whenever you move two bodies with a potential difference close together.

The only difference is, sometimes the distance at which it happens is so tiny that you don't see or hear the spark.

 

[Jiggy-Ninja]

Also, there is no such thing as a purely inductive load. Even a straight wire has a very small amount of inductance to it.

 

[Aranion]

a closed switch initially has the same electrical potential in its contacts. the moment you separate these contacts (by opening the switch), there would exist a potential difference in between the contacts. whatever the load is, when this potential difference exist between the contacts and at a very small distance, it will ionize the air in between and current will flow in the form of a spark.

 

[sophiecentaur]

The Elecric field between the switch contacts (volts per metre) will always be high, even in a low voltage circuit, when the contacts are very close. So the air will break down and a small spark will form. You can make sparks with a 9V battery by brushing two wires together for thi reason. Any finite inductance will increase the volts across the gap when the circuit is broken so the spark will be bigger when a long wire, motor or transformer is in series. But basically, a spark is always there and it's just a matter of degree.