Basic question about current in an electric circuit

  • #1
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Let's say we have a basic electric circuit consisting of a battery with some potential difference, and a loop of wire. The wire loop also has some segment with a higher resistance. Now since there is a potential difference, the free electrons will move to make the circuit equipotential. The battery will keep supplying the voltage and so the movement continues.

Now in the case where the circuit doesn't have the separate resistive element the current through any part of the wire would be the same, so one just defined this as the current strength of the circuit. My question is, if the current will be the same everywhere in the case where the circuit has elements with different resistance?

Can one say that at first the the current will be higher in the part with lower resistance, but this will cause an increase in the electric field in the higher resistance part of the circuit until the current is steady?
 

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  • #2
BvU
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My question is, if the current will be the same everywhere in the case where the circuit has elements with different resistance?
Yes.
Can one say that at first the the current will be higher in the part with lower resistance, but this will cause an increase in the electric field in the higher resistance part of the circuit until the current is steady?
Yes, but... For a steady state there can be no accumulation of charge. When a connection is made, this steady state establishes itself with a speed comparable to the speed of light.

You also want to consider that just a tiny excess of charge already causes a huge potential
 
  • #3
Dale
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My question is, if the current will be the same everywhere in the case where the circuit has elements with different resistance?
In a single loop circuit the current is the same in any location as it is at any other location. This is a direct result of Kirchoff’s current law.
 
  • #4
sophiecentaur
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so one just defined this as the current strength of the circuit.
To avoid the risk of coming to paradoxical conclusions. it is essential to realised that any 'real' source of electrical power has a finite 'internal' series resistance. A low resistance load will allow enough current to flow that the voltage drop across the internal resistance will reduce the available PD.
 
  • #5
BvU
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@Fibo112 , can you clarify how much of the answers makes sense to you ?
 
  • #6
anorlunda
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Your words might confuse you. In a series circuit, the current is the same in all elements, as others already told you. However, adding more resistance in the loop decreases the amount of current everywhere.

So be careful with your words.

Current the same in all elements? Yes.
or
Current the same before and after changing the resistance? No.
 
  • #7
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@Fibo112 , can you clarify how much of the answers makes sense to you ?

The answers make sense to me. Another thing I was wondering about is the following: When there is a changing magnetic flux through a surface, which has a conductor on its boundary, this will cause an EMF through this conductor. In some examples the forces along the wire which constitute this EMF are not spread uniformly through the wire but are only in one area. Can it be assumed here too that this EMF will induce a steady current throughout the wire?
 
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The answers make sense to me. Another thing I was wondering about is the following: When there is a changing magnetic flux through a surface, which has a conductor on its boundary, this will cause an EMF through this conductor. In some examples the forces along the wire which constitute this EMF are not spread uniformly through the wire but are only in one area. Can it be assumed here too that this EMF will induce a steady current throughout the wire?

I think the answer here must be yes too, since if it wasn't there would quickly be a buildup of charge somewhere which, I gather from previous answers, would not be sustainable for any period of time at all.
 
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Does the conductors shape ever matter, like with an air outlet. Wouldn't a very sharp angle cause some of the electrons to collide with the wire causing even more energy dissipation?
 
  • #10
phinds
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@Fibo112, forgetting for the moment about the details of the physics of the situation, in terms of circuits you should think of current in a closed loop as being exactly like a bicycle chain. It moves everywhere exactly the same. Adding resistance in the circuit is equivalent to making the bike chain harder to move (it requires more energy expenditure to move the same amount of current)
 
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  • #11
robphy
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Does the conductors shape ever matter, like with an air outlet. Wouldn't a very sharp angle cause some of the electrons to collide with the wire causing even more energy dissipation?

Possibly interesting:
https://www.simbucket.com/circuitelectronflow/
( http://www.simbucket.com/simulation/circuit-electron-flow/ )
based on
http://www.glowscript.org/#/user/Bruce_Sherwood/folder/MI4e/program/18-SurfaceCharge
https://www.compadre.org/portal/items/detail.cfm?ID=10026
https://matterandinteractions.org/wp-content/uploads/2016/07/circuit.pdf

(Here is an old thread from 2007..
https://www.physicsforums.com/threads/electron-flow-in-a-circuit.175197/,
specifically my post
https://www.physicsforums.com/threads/electron-flow-in-a-circuit.175197/#post-1366733
with now-broken links that could be updated by moderator.)
 
  • #12
sophiecentaur
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Does the conductors shape ever matter, like with an air outlet. Wouldn't a very sharp angle cause some of the electrons to collide with the wire causing even more energy dissipation?
This question contains a lot of 'assumptions'. Whilst electrons do move around in a metal conductor, you can't just assume it's anything at all like water flowing. For a start, the electrons move about randomly with an average drift speed of only a mm or so per second.

We are lucky that we can deal with circuits using some very simple rules (Ohm's Law etc.) without needing to know much more about the Physics of wires or other components. The Electric Fields within wires can be largely ignored without compromising the accuracy of most calculations. The deeper Physics is relevant inside some components, like semiconductor devices but, even then, in analysing circuits, it is seldom necessary to involve the Physics. My point is that you can validly learn all about circuits on a more or less separate path from the Physics without compromising on your 'understanding' of either. Bringing the two together is really for the 'more advanced student', once both have been understood fairly thoroughly.
(This is not a copout! :smile:)
 
  • #13
Dale
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In some examples the forces along the wire which constitute this EMF are not spread uniformly through the wire but are only in one area. Can it be assumed here too that this EMF will induce a steady current throughout the wire?
What does Kirchoff’s current law tell you?
 

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