Electronics > Beginners
Dumb question about resistors in series/parallel
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TheN00b:
Hey folks. Just joined EEVBlog's forum, but I've been a big time viewer for 2 years. I'm somewhat new to the world of electronics (just picked up my first oscilloscope 2 months ago!) and I am barely a hobbyist; just a guy who likes sticking probes in places they probably shouldn't be to find out how things work!

With that out of the way, we're learning about circuits in high-school physics right now (so no doctoral dissertations please!). One thing I cannot understand, is resistors in series/parallel. When we find the amperage through a resistor, whether it be the 2nd or 3rd resistor in the series, we always use the source voltage divided by the resistance of the resistor. Maybe I don't understand how resistors work, but shouldn't we subtract the calculated voltage drop of the resistors before them from the source, and then use that for our voltage?

Let me know if I can provide any more information. Thanks in advance folks!
spitta:
The current through a series circuit will be the same through every resistor.

If you are taking the source voltage divided by a single resistor to find current in a SERIES circuit, you're doing it wrong. The current should be the source voltage divided by the sum of all the resistances.
This technique, however, would work to find currents for each branch of a PARALLEL circuit.

I recommend watching some youtube videos on series parallel circuits to get the ball rolling...
IanB:

--- Quote from: TheN00b on May 02, 2018, 12:56:58 am ---With that out of the way, we're learning about circuits in high-school physics right now (so no doctoral dissertations please!). One thing I cannot understand, is resistors in series/parallel. When we find the amperage through a resistor, whether it be the 2nd or 3rd resistor in the series, we always use the source voltage divided by the resistance of the resistor. Maybe I don't understand how resistors work, but shouldn't we subtract the calculated voltage drop of the resistors before them from the source, and then use that for our voltage?
--- End quote ---

Voltage is the potential difference between two points. So if you are trying to find the current through a resistor in a circuit, you need to take the voltage difference between each end of the resistor and divide that by the resistance. In this case the "source" voltage is the voltage across the resistor.

On the other hand, if you want the current through a network of resistors, you need to take the voltage across each side of the network and divide that by the equivalent resistance of the network taken as a whole. Now for a simple case of three resistors in series the equivalent resistance is just the sum of the individual resistances, and the current through each resistance is the same. So you can find the current through a single resistor by finding the current through the network (in this one special case).
TheN00b:

--- Quote from: spitta on May 02, 2018, 02:16:54 am ---The current through a series circuit will be the same through every resistor.

If you are taking the source voltage divided by a single resistor to find current in a SERIES circuit, you're doing it wrong. The current should be the source voltage divided by the sum of all the resistances.
This technique, however, would work to find currents for each branch of a PARALLEL circuit.

I recommend watching some youtube videos on series parallel circuits to get the ball rolling...


--- End quote ---

Oh, so what I was explaining would work for parallel, because the voltage from the source stays the same when something is wired in parallel! :palm: But let's say I am asked to find current through one specific resistor in parallel(and that can be done using Ohm's law), but before that set of the parallel resistor, I had a resistor in series that dropped the voltage. For finding the current of the single resistor in parallel, using Ohm's law again, do I use the source voltage or the source voltage minus the Vdrop of the resistor that was in series before the resistor in parallel?


--- Quote from: IanB on May 02, 2018, 02:28:23 am ---
--- Quote from: TheN00b on May 02, 2018, 12:56:58 am ---With that out of the way, we're learning about circuits in high-school physics right now (so no doctoral dissertations please!). One thing I cannot understand, is resistors in series/parallel. When we find the amperage through a resistor, whether it be the 2nd or 3rd resistor in the series, we always use the source voltage divided by the resistance of the resistor. Maybe I don't understand how resistors work, but shouldn't we subtract the calculated voltage drop of the resistors before them from the source, and then use that for our voltage?
--- End quote ---

Voltage is the potential difference between two points. So if you are trying to find the current through a resistor in a circuit, you need to take the voltage difference between each end of the resistor and divide that by the resistance. In this case the "source" voltage is the voltage across the resistor.

On the other hand, if you want the current through a network of resistors, you need to take the voltage across each side of the network and divide that by the equivalent resistance of the network taken as a whole. Now for a simple case of three resistors in series the equivalent resistance is just the sum of the individual resistances, and the current through each resistance is the same. So you can find the current through a single resistor by finding the current through the network (in this one special case).

--- End quote ---

Is that why, when simplifying circuits, we simplify the parallel elements into a single resistance and then add all of the resistors together like it were a series to find a single equivalent resistance?
IanB:

--- Quote from: TheN00b on May 02, 2018, 02:57:12 am ---Is that why, when simplifying circuits, we simplify the parallel elements into a single resistance and then add all of the resistors together like it were a series to find a single equivalent resistance?

--- End quote ---

Yes, exactly.
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