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Old 25th March 2005, 05:38 PM   #1
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Default Basic Electronics Question

I'm a newbie trying to learn, so go easy on me.

I'm having a difficult time reconciling in my mind why current in a simple DC-series circuit with two resistors of differing values is not determined solely by the highest Ohm-value resistor. For example, if a simple DC circuit has a battery of 100V and a resistor of 20 Ohms, the current is 5.0A. If you then insert a 10 Ohm resistor in series (which by itself would yield a current of 10.0A) the current drops to 3.3A. It seems like, if I imagine the charge carriers being passed along the chain, that once they get through the 20 Ohm resistor that they should be able to maintain that "speed" because the charge carriers would be coming out of the 20 Ohm resistor at a slower rate then they would be going into the 10 Ohm resistor. So, the 10 Ohm resistor would always be waiting for the next charge carrier and would not bring the speed down any lower. The classic water and pipes analogy doesn't help as it would seem to reach the same conclusion.

I know I'm missing something obvious here.

Thanks for helping.
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Old 25th March 2005, 06:36 PM   #2
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The circuit you describe contains only a single resistor, 30 ohms.
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Old 25th March 2005, 07:14 PM   #3
Stocker is offline Stocker  United States
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That is to say, resistance in series is additive. The difference at each resistor will be the voltage across them, not the current through them.
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Old 25th March 2005, 08:11 PM   #4
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Thanks for you replies but I think I need to clarify my question a little. I understand that they are additive and that effectively there is just one resistor @ 30 Ohms.

But on an atomic level the electrons physically go through one resistor first, then a lead/wire connector, then the next resistor. Right? If so, and I picture atoms passing one electron at a time such that they can't pass one until the next atom in the chain is ready to accept (sort of like a firemen bucket brigade), then the atoms in the 10 ohm resistor must pass them faster than the ones in the 20 ohm resistor. Then, if the flow is such that the electrons go through the 20 ohm resistor first, isn't it's output slower than the 10 ohm's input?

One thought I just had is: "Is it because the voltage is lower after the 20 Ohm resistor and therefore the "pushing" pressure isn't as great on the 10 Ohm?"

I know Ohm's Law is a correct but I'm just struggling to understand at the lowest level. That's probably my mistake as I'm not that bright.

Anyway, if you can think of another way to explain it such that it gets through my thick head I would appreciate it.

Thanks
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Old 25th March 2005, 09:05 PM   #5
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Imagine a piece of resistance wire long enough to make a 30ohm resistor. It is easy with ohms law to accept that this will tak 3.3A from 100V.

Now cut the wire one third of the way along you then have a 10ohm and a 20ohm resistor, if you join the two cut ends together you then remake the 30ohms, but this time it is made of a 10+20 in series.

The wire when passing 3.3 amps will drop a given voltage for each unit length, and this will be regardless of the length used.

The 20ohm resistor will drop 66V at 3.3A and the 10ohm 33V, when added together these add up to the 100V.

OTH

Chris
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Old 28th March 2005, 12:59 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
I'm not a physicist but I'll try to help.
At the electron level, trillions of electrons are floating about in a random cloud. Apply a voltage and then the cloud of electrons move in unison (i.e. all at the same time) to dissipate the driving voltage. That random cloud moving is the intrinsic noise in a perfect resistor and the unison movement is the current that we negatively measure. The electrons bump into things along the way & that is the heat we can feel.
Someone come in and help me!
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Old 28th March 2005, 01:33 PM   #7
Mr Evil is offline Mr Evil  United Kingdom
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That sounds about right. It's the 'bumping into things' that really makes resistors resist, turning electrical energy into heat. The more resistors you put in series, the more things there are for electrons to bump into, causing them to lose more energy along the way, reducing the current for a given voltage.
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Old 28th March 2005, 01:36 PM   #8
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if you put a valve on a waterpipe, the valve has a resistance, water pressure before the valve is higher than after it.
If the valve is placed there for that purpose it is called a choke, pressure is choked.
waterparticles bump into eachother, loosing kinetic energy transformed to heat.
The water looses energy passing the choke, place several choke valves behind eachother, the water will have no pressure, no energy left.

A turbine engine has several rotor stages.
After passing each rotor the gasses passing through the turbine have lower pressure, lower energy.
In the ideal turbine engine residual energy from gasses exiting the turbine is zero.

Why has a thin thread more resistance, because the same electrons need to pass through a smaller passage. They interact and loose energy.
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Old 28th March 2005, 02:41 PM   #9
K-amps is offline K-amps  United States
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Default Re: Basic Electronics Question

Quote:
Originally posted by cfitzger
I'm a newbie trying to learn, so go easy on me.

I'm having a difficult time reconciling in my mind why current in a simple DC-series circuit with two resistors of differing values is not determined solely by the highest Ohm-value resistor. For example, if a simple DC circuit has a battery of 100V and a resistor ....

I know I'm missing something obvious here.

Thanks for helping.
The phenomenon you describe might make more sense in a paralell connection scenario. The lower resistance will pass more voltage.... i.e will dominate the circuit more. In a series connection its the opposite, the larger resistance determines more of the circuit's character/ output voltage.

See what you made me do Jacco? reply seriously... !
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Old 30th March 2005, 03:42 AM   #10
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Quote:
....then the atoms in the 10 ohm resistor must pass them faster than the ones in the 20 ohm resisto....
I think this is where the problem is. None are faster..or slower. They are all pretty much the speed of light with the exception of the ones that are stopped and 'burned up'. The 20 ohm resistor simply burns off twice as many as the 10 ohm..


hum, correct me if I am wrong.

Does anyone really know the way this works at the lowest level? Isn't it mostly theory?
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