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Q & A: voltage in a short-circuit

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Most recent answer: 12/05/2011
Q:
Deat AskTheVan team! I really need advice on a subject that I need to understand properly before teaching anyone else. I have a problem with voltage drop to zero while there is a short circuit. This seems odd, because for one thing: voltage can be defined as work per unit charge that is done by the electric field when moving it from one point to another. Clearly the work is not zero, otherwise there would be no current. Also: potentsial difference corresponds to energy conversion, when there is none, there is no energy to convert. But clearly that is not the case because it is a well known fact, that wires get hot and batteries may start to boil and the fuses go in case of a short circuit. So intuitively I find something wrong with the idea, that voltage becomes zero. Another factor: Ohm's law (though it does not hold in really well in practice) states: V= IR. It is well known, that the current is high during short circuit; but the only way I can manipulate I to be high is to make R near zero (and it does happen in reality), and not change V too much, cerainly not lowering it towards zero. One idea that came across to me, is that when current is really high, the conductor becomes closer to its equilibrium state: becoming cloes to equipotential. BUT: in case of a conductor being equipotential, there should not be any field inside it. Clearly, since the wires ARE NOT totally resitance free, there MUST be electric field in there, otherwise there would be no current. Suppose it is low, so how does the energy get converted then? The charges must have some energy? Does it manifest itself in another form than potential energy? What is exaclty going on during short circuit and how to make those equations and assumptions right? Kind regards!
- Ott (age 25)
A:
Your question does a good job of pointing out that the language we use is often approximate or even inconsistent. In a short-circuit, the voltage across the short doesn't really drop to zero. It does drop well below the value it's intended to have. The current goes up a lot, but not to infinity. There's typically some resistance on the way to the short circuit that limits the current no matter how low the resistance in the short gets.

Mike W.

(published on 12/05/2011)

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