So, if I add an extra resistor in series...

[pineappletree]

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So this is from BR chapter 9, problem 37

Generally, this is the problem:
There are several resistors in series (R1, R2, R3), and the question asks what will happen to the current passing through R1 if I add an extra resistor...
I thought: oh, it'd be perfectly logical that the current passing through R1 decreases because of the Junction Rule (I in equals I out).


But, wtf. The Current remains the SAME?
and I thought about it, and it does seem to make sense. =)
V=IR


But what I don't understand is How can resistance remain the same and the current remain the same, but you come out with a lower voltage??
For example, if R1 has the same ohm value, and the current going through it is the same... How can voltage of the sum of the resistors = one extra resistor be the same???


Is it just that we NEVER figure voltage for individual resistors, and only for the system as a whole???


Also, it would probably help if someone teaches me how to sum voltages in a system parallel/series.

 

[FrkyBgStok]

Because the effective resistance for that resistor is different, and so is the voltage drop across that resistor. Current remains constant. That resistor will use less of the available voltage. Kind of like having one bulb will be bright, but 100 they will all be equally dim.

 

[fizzgig]

just out of curiosity what is the whole problem? if you have a constant voltage source with R1 and R2 which yields Req and that yields I..

if you add a resistor in series the Req goes up, so I should go down...

what am i missing?

 

[lorenzomicron]

from ur problem, it seems I should go down. if we have 2 R in series with a V, then i=V/2R; if we have 3 R, then i=V/3R. indeed, the current going through all the resistors is the same; but this is if the source is a voltage source. the only way the current would be constant is for a current source.

what exactly is the problem?

also, in this case, the voltage across each resistor in the 3 R is lower than that of the 2 R because the voltage V is divided into 3 whereas in the first case it's divided into 2.

 

[Rayhaan]

This is one of the areas where you can't just use the equations and expect to get the answers. Think of circuits as a combined system. What is current really? Well it comes from flow through the circuit, and this is a circuit with resistors in series. That means there is one path only, the current can ONLY go through one path. This is a good analogy: Current follows the path of least resistance, much like a river with branches coming out from it. Well there's only 1 path, so all the current must go that way. In terms of voltage drops, think of it in terms of hydroelectric dams to fit our river analogy (they generate power by dropping water from heights = higher means more potential energy). Much like resistors then, bigger = bigger voltage drop.