In my student book they separate these two and indicate that sparks need high voltage to occur while arcs need low voltage with normal or low pressure and electric arcs CAN come with heat and bright light. But when I search on the web many people said that arc is just a hotter version of spark. So what is the difference between them?
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According to wikipedia (and an old textbook I have read), an arc is maintained by "thermionic emission of electrons from the electrodes", that is, in practice it relies on energy already being dissipated from the current to heat the electrodes.
A spark can occur immediately when breakdown voltage for the air gap is exceeded and the gas is ionized enough to become relatively conductive; depending on the source parameters and overall geometry, the spark can then become an arc (the conductivity rising even more) or extinguish within milliseconds.
Once both electrodes are hot enough, they present a constant supply of charge carriers (ions and electrons) into surrounding air. Unlike the spark, the ionized conductive channel in the arc can then survive even short drops in power input, like those occuring each 10 ms in AC network or cheap transformer welding. It can also maintain its conductive properties even when extended to several centimetres or even metres - this no more depends on voltage, but rather on the availability of current to keep electrodes and arc's plasma hot.
Typically, an arc is extinguished similar to a flame, either by blow of fresh air, or by its own updraft that stretches it too long to keep hot enough. Both mechanisms are employed in HV power lines, where arcing is a major danger and must be prevented.
