What is meant by Electric arc?What is cracking?

An electic arc  is nothing but Ionisation of electrons through air (or any medium ).Industrially, electric arcs are 
used for welding, plasma cutting, for electrical discharge machining, as an arc lamp in movie projectors and 
followspots in stage lighting. 
Electric arc furnaces are used to produce steel and other substances. 

In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as 
kerogens or heavy hydrocarbons are broken down into simpler molecules (e.g. light hydrocarbons) by the 
breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products are strongly 
dependent on the temperature and presence of any catalysts. 
Cracking, also referred to as pyrolysis, is the breakdown of a large alkane into smaller, more useful alkanes 
and an alkene.Simply put,hydrocarbon cracking is the process of breaking long chain hydrocarbons into short 
ones.

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regards
Ramesh