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I think you're confusing yourself here... "ground" is not necessarily zero potential - you're free to make any level "ground". What is of interest is the change in potential, not the "absolute" potential. If you're talking about Kirchoff's loop rule, the sum of the changes in potential around a loop is zero (if you are dealing only with conservative fields). Saying that the change in potential around the loop is zero is equivalent to saying that energy is conserved...
then wat is the zero potential? is it the negative -ve terminal........there must be a zero potential located somewhere.
I think it is a little like asking what is the zero level of potential energy in mechanics... it is wherever you define it in the problem because it is potential energy differences there that matter. The absolute zero of electric potential is at any point infinitely far away from any charge, but in a circuit, we are talking about changes in electric potential, not absolute potential... so you can set the zero level anywhere
another question. why does the current remains constant but the voltage keeps on dropping around the circuit......voltage is directly proportional to current according to ohs law. why is it so?
Yes, but the proportionality constant is the resistance (V = IR). each time current passes through a resistor, work must be done (energy is lost - usually as heat in the resistor). The potential difference (voltage) is related to the work which can be done or energy available since U = qV = -W (put deltas in front of U and V). Does this help?
remember also that even with ohm's law, the V is really Delta V, the change in potential between two points
one more question. in superposition theory when a emf source is switched off then why current is taken as 'open circuit' and voltage is taken as a 'short circuit'? whats the logic behind that?
confusion is arising at the concept that voltage or pd produces current and conversely current produces pd.......is it made on the assumption that an emf source would allow another emf to pass through it but wld resist current. is it so that an emf source would allow another emf to pass through it but wld resist current?
I think you have some misconceptions about potential differences (emf's). When you maintain a potential difference in a conductor, you are creating an electric field in the conductor, which is capable of doing work on charges via the coulomb force causing the charges to move (drift velocity) through the conductor. An emf is not a thing like a charge... it doesn't pass through anything or allow another emf to pass through it. Possibly someone is giving you a shortcut way to think about about a circuit, but it's just a device, not physics. If you "switch off" a potential difference, (like by opening a switch in the circuit or removing the battery) then the charges will arrange themselves as they would in electrostatics - excess charge on the surface of the conductor, etc. Current would cease to flow, because there would be nothing to maintain an electric field (charges will have moved to cancel E inside the conductor). In that sense, in switching off the potential difference the current is like like an open circuit (weird wording) - a circuit without a path for current to follow. The potential difference you could think of as a short circuit (although I don't know why you would use that term) because it no longer "pumps" current through the circuit. I think the use of these terms is what is confusing you - think of the physics, not the shortcut
well thts wat is said in superposition theory........check out electrical engg and computer sc lecture videos.......6.002 circuit and electronics.......video lecture 3........superposition theory, thevenin and norton....thts wat is said by prof Anant of MIT
Here is the hyperlink check out video 3 http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-002-circuits-and-electronics-spring-2007/video-lectures/