# Currents in Circuits.

Discussion in 'General Electronics Discussion' started by IanPSwift, Sep 23, 2011.

1. ### IanPSwift

6
0
Sep 17, 2011
Quick theoretical question. Say there is a circuit (in this particular case AC, but I believe DC would apply as well) with N resistors placed in series and or parallel. Can anyone offer a theoretical (perhaps using a water pipe or other some such metaphor if you feel so compelled) explanation for why the current through the first resistor in the circuit is equal to the source voltage divided by the TOTAL resistance? (As opposed to using some form of a current divider). Please feel free to use KVL and KCL if this helps you explain it, as I have been working with both of them.

2. ### shrtrnd

3,827
520
Jan 15, 2010
I don't understand how you can't understand that.
I don't feel compelled to use it, but you brought-up the water pipe metaphor.
You've got a water pipe with water running throught it. Down the line you've got other
pipes branching off the line.
The water current through the first pipe is going to be the same as the
combination of all the other branches off of the first pipe.
Any restrictions (resistors) in the branches are the varying resistances to the overall
water flow (the current). So the water pressures in the branches (voltage) can change
in the branches (including the first resistance, the input pipe (or resistor) itself).
But the overall current of the pipework (circuit) will remain the same in total, (the voltage drops) are the different branches of the pipe system, restrictions in the branches.
If you further simplify it:
The amount of water going into the pipe system is going to be the exact same amount
of water coming out of the ends of the pipe system, no matter how many of those there
are. The pressure changes (voltage changes) in the different branches of the pipe
system are due to the restrictions (voltage changes) of the different branches. But
the amount of water (current) flowing through the pipe system, will remain constant.

3. ### IanPSwift

6
0
Sep 17, 2011
thank you, that was very helpful. It was one of those things where I could have seen it making sense either way and just wanted to ensure I understood the theory.

4. ### shrtrnd

3,827
520
Jan 15, 2010
I really oversimplified it. In circuit theory and practical application, one of your most valuable tools, is reliably knowing your current. It tells you which devices to use in your designs. As a repair tech, the biggest issue I find is circuit damage due to excessive
current through a device. To save money in parts, manufacturers want engineers to
design as close to acceptable as possible, in parts. That means saving money by
buying parts with marginal current handling capability.
I guess you may not be interested, but I wanted to rant on this pet peeve of mine.
If you're cutting it close in a design and a 1/4W resistor will probably work, consider
using the 1/2W, just so the device doesn't have to be repaired by me later when the
current through the resistor exceeds the expected value.

1,074
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Apr 8, 2011