Electric Spark vs. Arc

No difference really, except maybe for some connotations about size and duration.

Electrons will jump an air gap whenever there is enough potential (voltage) for them to leave one conductor and move to the other.

25,000 volts is enough to jump 1" in dry air.

Photons (arc light) are generated when the electrons break free from their valance shell.

Physicists feel free to jump in here, potential or otherwise.

Actually the voltage required to spark across a 1" gap between round surfaces (such as spheres) is 75,000 volts/inch. This was often seen in movies about Frankenstein's Monster.

One of the few things I still remember from Physics Lab in college

Spark: sudden, one time - if more: sparks - Intermittent character, Arc is a more sustained form - re: arc welding, and more articulate: the carbon arc light, a continuous arc, that provided the light for movies.

So lightening is a spark?

Yes. A very BIG spark, but indeed a spark.

I would say BertHickman has it just about right.

A Spark is generally thought of as a momentary event. Yes, like lightening. Or the spark you get when you slide across the plastic car seat and touch the door. Or the spark you may get when you plug an appliance in that is turned on.

An Arc is a continuous event. Like welding, arc lamps - projection or old spot lights. And arc can also be a very bad thing - as Bert pointed out, once the arc is established - it is a low resistance with max current. I have had the "pleasure??" of seeing two wall outlets arc internally and blow all the brass out of them. Although this did not last more than a few seconds, it would still be called an arc, as the outlets vaporized the brass and created that low current arc that lasted long enough to clean the brass out of the receptacles.

In the case of the receptacles, metal is vaporized. In the case of the Jacob's Ladder, air is Ionized, heated - and rises - leading the arc up the wires.

Spark timing is not a good parameter to define spark. Using a high frecuency and voltage power supply (from an old tv set) I have mantained an spark for around ten seconds or more.

Just did a very unusual thing for a poster on the CR4 forums, I looked it up in the dictionary! A very short explanation of the differences between a spark and an arc according to THE MERRIAM-WEBSTER DICTIONARY is:

spark: a short bright flash of electricity between two points

arc: a sustained luminous discharge of electricity (as between two electrodes)

Maybe in our quest for extensive technically accurate answers we forget to go to the basics and to "Keep it simple, Stupid!"