What circumstances could cause a spark?

Answer 1

• electrical discharge
• striking "flint" on "steel"
• friction in certain circumstances

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The more common type of spark, an electrical spark, occurs any time you have a combination of a large enough difference in voltage potential between two objects and a short enough distance between them. Clouds are considered objects in this case, because lightning is a spark. Many sparks, including lightning, are caused by the transfer of electrons from one object to another as they rub against each other. Another example of that is when you've been walking around, especially when the air is dry, and you reach for the handle of one of your supermarket's freezers. Also, any electrical switch or relay can produce a spark, although they are usually not seen. That is because when a switch or relay is closed (turned on), two pieces of metal are brought into contact with one another to complete a circuit. When that happens, there is a tiny fraction of a second when they are not yet touching but are so close that the electricity jumps the gap between them.

Another type of spark is when two objects rub against each other causing tiny bits of one of the objects that are glowing hot due to the friction of the rubbing fly off. Examples are when the jagged wheel of a cigarette lighter rubs against the lighter's flintstone, or when a car muffler is being dragged along the pavement.