Voltage drop over a resistor

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Why is the electrical potential greater before it passes through a resistor than after?
 

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  • #2
BvU
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Some energy is dissipated in the resistor: it resists.
 
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  • #3
phinds
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Why is the electrical potential greater before it passes through a resistor than after?
I think you are looking at things in a way that is not helpful. Electrical potential does not "pass through" a resistor, it EXISTS on one side of the resistor relative to the other and causes current to flow through the resistor. That is, electrical potential at one point is always RELATIVE to another point --- it doesn't "pass through" anything.
 
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  • #4
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I think you are looking at things in a way that is not helpful. Electrical potential does not "pass through" a resistor, it EXISTS on one side of the resistor relative to the other and causes current to flow through the resistor. That is, electrical potential at one point is always RELATIVE to another point --- it doesn't "pass through" anything.
My bad, that was worded poorly. I meant to ask why the electrical potential is greater at a point before the resistor than a point after the resistor.
 
  • #5
Dale
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My bad, that was worded poorly. I meant to ask why the electrical potential is greater at a point before the resistor than a point after the resistor.
If it were the other way around then the resistor would supply energy to the circuit.
 
  • #6
phinds
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My bad, that was worded poorly. I meant to ask why the electrical potential is greater at a point before the resistor than a point after the resistor.
Do you think that's always true? I can EASILY show you a circuit where the potential "before" a resistor is 10 volts and the potential "after" the resistor is 20 volts. I think your question is more like "how can there be an electrical potential that is different at one end of a resistor than at the other end". The answer is "because some power source creates it"
 
  • #7
sophiecentaur
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Why is the electrical potential greater before it passes through a resistor than after?
I could actually be less. It just depends on where your reference is. It's Potential Difference that counts.
 
  • #8
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If it were the other way around then the resistor would supply energy to the circuit.
why does it have to do either?

If you push a block with a constant velocity along a frictional surface, energy is being lost similarly to in the resistor. But the block doesn’t gain or lose any kinetic or potential energy as it’s pushed.

Is this situation not analogous to current through a resistor?
 
  • #9
sophiecentaur
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why does it have to do either?

If you push a block with a constant velocity along a frictional surface, energy is being lost similarly to in the resistor. But the block doesn’t gain or lose any kinetic or potential energy as it’s pushed.

Is this situation not analogous to current through a resistor?
That's not a suitable analogy. Potential in the moving block situation would require a sloping board (gravitational potential difference) and there is no KE involved with energy transfer by an electric current.
 
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  • #10
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That's not a suitable analogy. Potential in the moving block situation would require a sloping board (gravitational potential difference) and there is no KE involved with energy transfer by an electric current.
So is there no force on the charge carriers while going through a wire, but some force on charge carriers going through a resistor. And if so what is the cause of that force.
 
  • #11
sophiecentaur
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So is there no force on the charge carriers while going through a wire, but some force on charge carriers going through a resistor. And if so what is the cause of that force.
There is a force (proportional to charge times local field) but the electrons do not speed up appreciably because they are interacting with all the positive ions. The average drift speed is around 1mm per second so not a lot of KE there for electrons with such tiny mass.
This is why electric circuit calculations use the concept of Electrical Potential Difference which tells the work expended (Joules) in moving a unit charge (Coulomb).
Basically it just doesn't;t lend itself to a useful mechanical analogy.
Also, there is very little work expended in moving charges through metal wires (which is why we use them).
 
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  • #12
Dale
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why does it have to do either?
Because if it didn’t then it wouldn’t be a resistor. It isn’t that everything has to behave that way, but things that do are called resistors and things that do not are not.

If you push a block with a constant velocity along a frictional surface, energy is being lost similarly to in the resistor. But the block doesn’t gain or lose any kinetic or potential energy as it’s pushed.

Is this situation not analogous to current through a resistor?
I dislike analogies for circuits. Circuits are so easy to understand on their own terms that there is simply no point in using analogies. The analogies are inherently flawed and almost always more complicated than circuits.

In the case of resistors there are only two simple equations: ##v=ir## and ##p=iv##. The equations are so simple that no analogy is simpler.
 
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  • #13
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Do you think that's always true? I can EASILY show you a circuit where the potential "before" a resistor is 10 volts and the potential "after" the resistor is 20 volts. I think your question is more like "how can there be an electrical potential that is different at one end of a resistor than at the other end". The answer is "because some power source creates it"
Uhmmm ... I suppose we can apply any definition we like to “before” and “after”, but if the potential is higher on one side of a resistor than on the other I’m pretty sure I can tell you which direction the current flows.
 
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I dislike analogies for circuits. Circuits are so easy to understand on their own terms that there is simply no point in using analogies. The analogies are inherently flawed and almost always more complicated than circuits.
Oh, I don’t know. I think when students are just starting out making analogies can be very helpful. Analogy is probably the most effective way people grasp new concepts. In particular water circuits are pretty intuitively understood by people and help them get a grasp on what is meant by current, potential, and resistance. Do you have to do some description and caveats? Sure. Can the analogy be carried very far into the process? No. But I’ve always found that if they make that fundamental connection early on they never later ask about potential flowing through resistors or similar.
 
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phinds
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Uhmmm ... I suppose we can apply any definition we like to “before” and “after”, but if the potential is higher on one side of a resistor than on the other I’m pretty sure I can tell you which direction the current flows.
Of course you can, and what relevance does that have to this discussion?
 
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Of course you can, and what relevance does that have to this discussion?
Because without a generous and strange definition of before and after what you implied is just wrong
 
  • #17
phinds
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Because without a generous and strange definition of before and after what you implied is just wrong
I think you're confused about who started the "before" and "after" and what was the point of my post.
 
  • #18
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I think you're confused about who started the "before" and "after" and what was the point of my post.
You wrote:
Do you think that's always true? I can EASILY show you a circuit where the potential "before" a resistor is 10 volts and the potential "after" the resistor is 20 volts.

If you define before and after by the direction of current flow (and I don’t really see how they could mean anything else) the potential before a resistor is always higher than after by Ohm’s law. You can’t “easily show’ any such thing.
 
  • #19
Dale
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In particular water circuits are pretty intuitively understood by people and help them get a grasp on what is meant by current, potential, and resistance.
Hydraulics are enormously more complicated than circuits.
I’ve always found that if they make that fundamental connection early on they never later ask about potential flowing through resistors or similar.
My experience on this forum is not so positive. We get many people who are confused in spite of or even as a result of the hydraulic analogy. Given that the thing itself is so simple, I don’t see the point of an analogy.

In the specific case of the OP, the suggested analogy was a block sliding with a constant frictional force. But what about a switched circuit? Is that like the block stopping? And then when you switch it back on is there static friction? And even if there isn’t static friction what about the KE of the block itself? Etc...
 
  • #20
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My bad, that was worded poorly. I meant to ask why the electrical potential is greater at a point before the resistor than a point after the resistor.
Do you have some idea what 'electrical potential' means? I guess not. :smile: Well, I have an idea: 'High positive electrical potential' means: "There are many positive charges nearby, but not so many negative charges". This potential is absolute potential, and it's very easy to understand - which does not mean that it's not a good idea.
 
  • #21
sophiecentaur
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I think when students are just starting out making analogies can be very helpful.
But you need to realise that students (especially young ones) can be very literal and will not spot a metaphor. They are often not well informed enough to spot the difference between a very limited analogy and 'the real thing'. That involves great risks of drawing false conclusions by taking an analogy too far. The water analogy is a good example which is ok for current conservation but it takes students in the direction of the erroneous "Electrical Energy is Kinetic Energy".
Analogies should, imo, be self generated and internal to the student's thoughts. If a teacher actually proposes an analogy, the student can give it far too much weight.
 
  • #22
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The water analogy is a good example which is ok for current conservation but it takes students in the direction of the erroneous "Electrical Energy is Kinetic Energy"
This is a great point and shows one of the problems of complicated analogies. Even in hydraulics the energy is not just kinetic energy, but people do often make exactly this mistake. They think they understand hydraulics without actually understanding it, and that flawed understanding of complicated hydraulics leads to a flawed understanding of simple circuits!
 
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