In this circuit, the current only flows in the first loop:
There is a short circuit, so I think it's because of that, but I don't understand why it happens.
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2\$\begingroup\$ Why would current flow in the other loop? Where would it flow from & to? \$\endgroup\$– brhansFeb 8 at 18:40
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\$\begingroup\$ @brhans my confusion comes because of the wire at the bottom, why the current doesn't flows in that way to the loop at the right? \$\endgroup\$– MarcoPoloFeb 8 at 18:47
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\$\begingroup\$ A dogmatic solution would be in oking KCL: draw a closed surface around the first half of the circuit, so that only the bottom wire will go through it. Then apply KCL. What do you get? \$\endgroup\$– Sredni VashtarFeb 8 at 18:53
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2\$\begingroup\$ For current to flow, there has to be a complete loop between whatever is driving the current - a voltage or current source - and wherever the current is going, a complete circuit. The components on the right form a loop, but it does not form a loop with the current source. \$\endgroup\$– virFeb 8 at 18:55
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\$\begingroup\$ @vir your condition might be necessary but is not sufficient. For example add a wire at the top, parallel to that at the bottom, and both wires become parts of a loop with the generator. But will current flow in them? Likewise, the fact that 'R and L are shorted' has nothing to do with the lack of current in the connecting bottom wire. \$\endgroup\$– Sredni VashtarFeb 8 at 19:47
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2 Answers
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Think about the argument I put forward below: -
It's all about simplifying the circuit and asking a sensible question.
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The best analogy I can think of to explain why current won't flow around the right hand loop is water in pipes. If you pump water into one end of a pipe, it must come out somewhere. 1 litre per second going in must result in 1 litre per second coming out somewhere.
If the right hand loop were pipes full of water, and you tried to push in more water via the bottom-left inlet pipe, water would have to emerge somewhere else. Where would that be? In your system, you can raise the pressure at the inlet pipe, which would raise the pressure in the entire system, but no water would actually move.
This principle is the basis of Kirchhoff's Current Law (KCL), which for the simplest two terminal device, or wire, boils down to current going in one terminal must equal current emerging out of the other. For a junction where three paths are joined, if you have 0.5A flowing into the junction via one path, and 1.0A flowing into that junction from another path, then there absolutely must be a total of 1.5A leaving the junction by the third route.
You can apply this principle at any and all points in the right hand loop of your circuit, including that "inlet" pipe bottom left, and you must conclude that absolutely no current can flow in or out of that inlet without violating KCL.
Interestingly, it is possible for current to flow around that loop, as long as none leaves or enters via that inlet. This would not violate KCL, as you can see happening in the left-hand loop. However, you would would need a voltage difference in the loop somewhere to push that current. That is exactly what would happen if you moved a magnet near the inductor.
