Voltage, Current, Resistance: Simple Illustration & Ohm's Law

In summary: one resistor and a battery connected in series, then you have a current source and the voltage is present because the current flows.
  • #1
Pogba
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So what's the smplest illustration for voltage ,current and resistance and what's ohms law ?
 
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  • #2
virginia-route-29-copper-circuit.png


The picture shows the simplest case when the material for the resistor R is ohmic. Ohmic means that the ratio V/I is constant.

:welcome:
 
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  • #4
Simon Bridge said:
you said you had "voltage confusion"

I think that is that moment of disorientation after coming into contact with mains voltage. (PS: don't try this at home)

BoB
 
  • #5
One problem that can be confusing is the formula on paper fails to point out that voltage has to be there first for current to flow. People sometimes make the mistake of assuming a voltage is present simply because IR says it should be there.
 
  • #6
CraigHB said:
One problem that can be confusing is the formula on paper fails to point out that voltage has to be there first for current to flow. People sometimes make the mistake of assuming a voltage is present simply because IR says it should be there.

Sorry, but I disagree. In circuit analysis, for Kirchoff's laws to apply, we say that voltages and currents apply simultaneously. There is no first/second.

It is also perfectly valid to say "voltage is present simply because IR says it should be there". If you prefer, think of a resistor connected to an ideal current source. The voltage appears because the current flows (and the current flows because the voltage appears). No first and second, but simultaneously.

If you really want to find out what happens first, then you need time domain solutions to Maxwell's equations, but then that it no longer circuit analysis. You will find that the fields propagate at near light speed.

This recent thread talked about the assumptions of circuit analysis in some detail.
https://www.physicsforums.com/threads/why-does-a-voltmeter-measure-a-voltage-across-inductor.880100
 
  • #7
anorlunda said:
It is also perfectly valid to say "voltage is present simply because IR says it should be there". If you prefer, think of a resistor connected to an ideal current source. The voltage appears because the current flows (and the current flows because the voltage appears). No first and second, but simultaneously.
....
If you really want to find out what happens first, then you need time domain solutions to Maxwell's equations, but then that it no longer circuit analysis. You will find that the fields propagate at near light speed.

Yes - it is, certainly, correct for the purpose of circuit analysis to assume that both - voltage and current - do exist at the same time and that - for example - a current is able to produce a voltage across a resistor (V=I*R).
However, from the physical point of view it is clear that no current can exist without driving voltage - hence, voltage first and current next. This basic rule applies, of course, also to circuits with "current sources" which, in reality, always are voltage sources eqipped with a very large (dynamic) source resistance.

Remember: Current is movement of electrical charges - and the force which can cause such a movement is provided by the electrical field created within the conducting material by the applied voltage.
 
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  • #8
anorlunda said:
If you really want to find out what happens first, then you need time domain solutions to Maxwell's equations, but then that it no longer circuit analysis. You will find that the fields propagate at near light speed.

With respect to time, yes that's the case. I was not trying to say voltage is first in instance, but that voltage is a prerequisite for current to flow. V=IR does not make that distinction. If you assume a current you also have to assume a voltage which may or may not be there. That can give a person trouble conceptually looking at what's happening in a real circuit.
 
  • #9
LvW said:
However, from the physical point of view it is clear that no current can exist without driving voltage - hence, voltage first and current next.
You'd agree that Maxwell's equations describe your physical point of view? If so, then voltage does not come before current or vice versa - there's no causal relationship between them.

@LvW, can you give an example of something you'd consider a voltage source? A battery perhaps, do you consider that a voltage source?
 
  • #10
LvW said:
However, from the physical point of view it is clear that no current can exist without driving voltage - hence, voltage first and current next. This basic rule applies, of course, also to circuits with "current sources" which, in reality, always are voltage sources eqipped with a very large (dynamic) source resistance.

The physical equations that describe the behavior are Maxwells Equations. To solve the time evolution is a study of fields, not circuits. With fields, you need the exact geometry to describe the sequence of events because fields are 3D. If you have just a resistor, and no geometry information, you can't solve the field equations, so the only tool you have left is circuit analysis.

You may be thinking of the Drude Model. Maxwells Equations are a much better foundation.

Edit: Also, consider that the OP was very much a beginner's question. We (including me) should not be injecting this esoteric stuff to this thread.
 
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  • #11
milesyoung said:
You'd agree that Maxwell's equations describe your physical point of view? If so, then voltage does not come before current or vice versa - there's no causal relationship between them.
@LvW, can you give an example of something you'd consider a voltage source? A battery perhaps, do you consider that a voltage source?
OK - I must admit that I didn`t express myself clear enough. It was not correct to say "voltage first and current next" because this involves a time sequence.
It was my only intention to answer the question: Which force causes the charges to move within the conductive material. Is it not true that we need an electric field as a precondition? And - yes - I would consider a battery as a (of course non-ideal) voltage source.
 
  • #12
LvW said:
... I would consider a battery as a (of course non-ideal) voltage source.
Okay, let's use this as a starting point. In the following, in the questions I might pose, I'm not trying to trick you or anything, I genuinely just want to present you an alternative viewpoint. Your line of reasoning is shared by many.

Let's assume non-ideal components, no hocus pocus. How do you know a battery is a voltage source? I just mean in practical terms - is it something you can measure?
 
  • #13
milesyoung - I am not sure if it makes much sense to discuss here the properties of a battery.
On the other hand - it would be interesting to learn more about your "alternative viewpoint".
Perhaps you consider the moving/separation of charges within the device - prior to using it as a usable voltage source - also as a kind of current ?
In this case - my answer would be: Obviously, the OP was asking for some clarification regarding the terms "voltage", "current" and "resistor" - as they appear as electrical quantities in electrical circuits. And for such circuits the basic rule applies: Each current needs a driving voltage.
Do you see my point? I think - in particular for beginners - it is important to know that relations like R=V/I may be used also in other forms (like I=V/R or V=I*R) - however, it is not always (per se) allowed to automatically derive from such forms any information about cause and effect.
(Have you seen a similar discussion about the relation Ic=B*Ib?)
 
  • #14
LvW said:
milesyoung - I am not sure if it makes much sense to discuss here the properties of a battery.
I'm not interested either in a discussion on the specifics of batteries, but it's one of the simplest examples I could think of of something you associate with a voltage source.

I'm not even talking about circuit theory, I just want to pose you the basic question: what is it about the behavior of a battery that makes you sure it's a voltage source? To put it another way, if I gave you a 2-terminal black box with a battery inside and asked you to probe the terminals, what would make you conclude that it's a voltage source?
 
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  • #15
milesyoung said:
... I gave you a 2-terminal black box with a battery inside and asked you to probe the terminals, what would make you conclude that it's a voltage source?

Any electrostatic measurement device would react upon the potential difference, would it not?
 
  • #16
LvW said:
Any electrostatic measurement device would react upon the potential difference, would it not?
Sure, but that would also be the case for a nonideal current source.

What if I, in my EE studies, had a very difficult teacher. He/she had a preference for current sources and would almost never show a voltage source - practically everything would be modeled as current sources. I'd later come to this board, engage in this same discussion with you, but I'd be adamant about there being a current source in the box, not a voltage source. Who of us is right?
 
  • #17
milesyoung said:
Sure, but that would also be the case for a nonideal current source.

Yes - because each non-ideal current source is (in reality) a non-ideal voltage source.
Remember: Which effect (which force) causes the charges to move (forming the quantity we call "current") ?
 
  • #18
LvW said:
Yes - because each non-ideal current source is (in reality) a non-ideal voltage source.
Why is my black box not part of your reality? :cry:

LvW said:
Remember: Which effect (which force) causes the charges to move (forming the quantity we call "current") ?
The force can be either electric or magnetic, depending on your choice of reference frame. I assume you're analyzing this from the rest frame of the conductor, but that frame is no more fundamental than any other.

https://en.wikipedia.org/wiki/Classical_electromagnetism_and_special_relativity

I can try to find an example that shows the effect for a charged particle within a conductor that's not in electrostatic equilibrium, but even the simplest of cases tend to become very complicated very fast, and I'm afraid I never studied special relativity much beyond the simple cases I saw in classical EM.
 
  • #19
Doesn't the discussion come down to can we have a circulating current with no voltage? Is there not current in a copper pipe when we drop a magnet through it? In the real world there is a finite resistance in the copper. What about a super conducting pipe?
 
  • #20
The core of our discussion is: What is "electrical current" and where does it come from - correct?
More than that, I think, here we speak about classical electric/electronic circuits and applications (and not about "exotic" arrangements based on superconducting material).
In this context: Is there any real technical device which we call "current source" (labor slang), that is not energized by a voltage?
And - we shouldn`t forget that the start of the discussion was the question: Is it correct to say "a current produces a voltage according V=I*R" ?
 
  • #21
The OP has not shown himself since the top of the thread. Perhaps Pogba could respond and tell us
1. What he/she already knows about the topic
2. His/her views on what has been written already.
 
  • #22
Where are you going to draw the line? Sure most folks can't come up with a super conductor in their garage or basement. Most people can come up with a voltage source by popping the hood of their car and messing with the battery. Very little knowledge of electronics is required for that. Slightly more knowledge is required to come up with a current source. Is a current source considered exotic by those who are only able to hook a voltmeter and simple loads to a voltage source?
 
  • #23
Averagesupernova said:
Is a current source considered exotic by those who are only able to hook a voltmeter and simple loads to a voltage source?
I think, this can be answered only by means of an example for a technical realization of that what we call "current source".
Where is such an example?
 
  • #24
sophiecentaur said:
The OP has not shown himself since the top of the thread. Perhaps Pogba could respond and tell us
1. What he/she already knows about the topic
2. His/her views on what has been written already.

Yes - for my opinion, it is a very unfortunate situation (which happens rather often) that an OP never tells us (forum members who gave answers) if he is satisfied or not.
 
  • #25
LvW said:
I think, this can be answered only by means of an example for a technical realization of that what we call "current source".
Where is such an example?
There is no such example in the real world. This is why I brought up super conductors since they are as close to ideal as we can get in the real world. No voltage, yet we have circulating currents. Maybe this drags the discussion in a different direction. Just thought I would throw it out there.
 
  • #26
Re, A Current Source:
Averagesupernova said:
There is no such example in the real world.
We are talking 'real world' and not mathematics.
There is a pretty good example, certainly in Engineering Terms, of a current source and that is the electron beam in a CRT. With tens of keV available, currents in the order of 1mA will be 'whatever they want to be', when the Anode of a EHV electron tube is connected in circuit. It may not be in our general experience and whilst the nice constant voltage from a car battery is very familiar as a Voltage Source, a beam of charged particles is just as insistent in doing what it was designed to do.
Brute force High impedances may be a bit more trouble to produce than brute force Voltage sources but you can find them all over the place. (Solar wind?)
[Edit: Perhaps I should have included a note on Health and Safety at this point. But that also applies to the use of car batteries by the uninitiated]
 
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  • #27
Averagesupernova said:
Doesn't the discussion come down to can we have a circulating current with no voltage? Is there not current in a copper pipe when we drop a magnet through it? In the real world there is a finite resistance in the copper. What about a super conducting pipe?

Superconductors are real world. They can circulate current indefinitely with identically zero voltage drop.

The debate point in this thread is worth argument.

On one hand, we have circuit analysis in which V=IR happens simultaneously. There is no voltage first. Simultaneity is necessary for Kirchoffs laws (KVL, KCL) to work.

On the other hand we have fields described by Maxwells Equations. That is a 3D problem.

Still deeper, we have quantum electrodynamics (QED). QED is the only completely accurate way to describe voltages and currents. Even Maxwells equations need some simplifications before they can be derived from QED.

In between circuits/fields is a quagmire. Unfortunately, those yearning for a more physical explanation sometimes imagine electrons as little billiard balls, or as capsules of energy, and wind up in the middle of the quagmire. I argue "Stay away from the quagmire. Go all the way to Maxwells Equations as the next step deeper."

Even the history of the famous Drude model of electrical conduction shows quagmire.
https://en.wikipedia.org/wiki/Drude_model#Accuracy_of_the_model said:
Historically, the Drude formula was first derived in an incorrect way, namely by assuming that the charge carriers form an ideal gas. It is now known that they follow Fermi–Dirac distribution and have appreciable interactions, but amazingly, the result turns out to be the same as the Drude model because, as Lev Landauderived in 1957, a gas of interacting particles can be described by a system of almost non-interacting 'quasiparticles' that, in the case of electrons in a metal, can be well modeled by the Drude equation.

This simple classical Drude model provides a very good explanation of DC and AC conductivity in metals, the Hall effect, and thermal conductivity (due to electrons) in metals near room temperature. The model also explains the Wiedemann–Franz law of 1853. However, it greatly overestimates the electronic heat capacities of metals. In reality, metals and insulators have roughly the same heat capacity at room temperature. The model can be applied to positive (hole) charge carriers, as demonstrated by the Hall effect.

One note of trivia surrounding the theory is that in his original paper Drude made a conceptual error, estimating electrical conductivity to in fact be only half of what it classically should have been.
 
  • #28
I really don't wish to be involved in this any deeper than that circulating currents exist in superconductors with zero voltage. Anything beyond this is out of my area of expertise by several light-years. I tend to agree with anorlunda in that just accept that current and voltage are inseparable in simple circuit analysis and choose which is what based on convenience. I will blindly accept what has been said here about Maxwell's work and QM as it does not really apply to me at this point in my life.
 
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  • #29
LvW said:
In this context: Is there any real technical device which we call "current source" (labor slang), that is not energized by a voltage?

I don't see how that could be possible. Current being "movement" of charge, it's always going to take some force to cause movement. That's just classic physics. In the case of a super-conductor a constant application of voltage is not required since resistance is essentially zero, but it still takes an initial application of voltage to induce a current.

From what I understand there is still some mystery around the workings of current flow. For example in a battery it's positively charged particles that provide movement of charge and in a wire it's negatively charged particles. In some cases it can be both.
 
  • #30
I do wonder whether the merit of this question justifies 30 posts. It's a bit like 'In a fight, would a bear beat a lion?'
I would point out that the definition of Voltage involves Energy (the King of Physical quantities, if ever there was one) and also charge so ,if you really want a pecking order, Voltage would probably come out on top. But who cares? It's relationships that count in Science and you need more than one variable to have a relationship.
Whilst it's been pointed out that current can flow without a voltage, it is worth noting that such current doesn't involve any Energy Transfer - so the phenomenon of superconductivity is, despite its usefulness, a bit of a red herring in this discussion.
 
  • #31
An example of a current source is an energized inductor. If the terminating impedance suddenly changes, the current will be maintained but the voltage will abruptly change. Of course the current is sustained for only a finite time, as the inductor gives up energy. In switching power converters, LED drivers, & motor drivers, this principle is made use of. I will elaborate if need be. The capacitor is an example of a device that emulates a voltage source. If the energized capacitor discharges into a resistance, then that resistance changes, the voltage will be sustained while the current abruptly changes to a new value. Of course this is for a finite time as well.

A photodiode works very well as a constant current source. Light incident upon the device produces current. Usually, a PD is placed across the inputs of an op amp, with a resistor as a feedback element. Light on the PD generates current, which must pass through the feedback resistor. The non-inverting op amp input is usually at ground, so that the output is simply I*Rfdbk.

Another example is a generator, such as the alternator found in a car. Since most batteries operate much better in constant voltage mode, the regulator adjusts the alternator field current to maintain constant terminal voltage. But said alternator could just as well have its current regulated to a constant value. As loading changes, field current could be adjusted for steady current, & the voltage varies with load resistance.

Constant voltage vs. constant current regulation are options we have when building power sources like batteries or generators. Batteries seldom are built for current source operation, losses are too great. But nuclear battery cells, not commercially available yet (maybe never), are known to work best as current sources. Generators are easily regulated for current or voltage.

I can elaborate if needed. BR.

Claude :-)
 
  • #32
cabraham said:
A photodiode works very well as a constant current source. Light incident upon the device produces current.
Hi Claude - is there really no E-field involved? Which force then allows the movement of charges which were set free by photons?
 
  • #33
LvW - Yes there is no E field involved. I am at work. When I get home tonight I will scan my text (Sze - semiconductor physics). A charge can move under influence of an E or B field, as well as photonic interaction. E/M waves through space or through solid material such as conduction is both wave-like & particle like, neither model is complete. When photons strike a conductor, electrons are elevated from valence to conduction band, & currnet is generated. This is the photoelectric effect for which Einstein received a Nobel prize. Photons have Energy per Planck's Law E = hf. To raise an electron from valence to conduction requires an increase in energy which the photon provides.

An LED works in the opposite manner. Electrons in conduction drop to valence due to recombination. Valence being a lower energy state (more negative) than conduction means that the difference in energy must be accounted for. In this case photons are emitted, E = hf. All the energy is accounted for.

I hope this helps, I will elaborate if needed. Best regards.

Claude
 
  • #34
Reference: Sze & Ng, "Physics Of Semiconductor Devices", Wiley Interscience, 2007, 3rd Edition

Claude
 

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1. What is voltage?

Voltage, also known as electric potential difference, is the measure of the potential energy difference between two points in an electric circuit. It is measured in volts (V) and is represented by the symbol V.

2. What is current?

Current is the flow of electric charge through a conductor. It is measured in amperes (A) and is represented by the symbol I. Current can be either direct (DC) or alternating (AC) depending on the type of electric circuit.

3. What is resistance?

Resistance is the measure of opposition to the flow of electric current in a circuit. It is measured in ohms (Ω) and is represented by the symbol R. Resistance is affected by factors such as the type of material, length, and cross-sectional area of the conductor.

4. How are voltage, current, and resistance related?

According to Ohm's Law, voltage is directly proportional to current and resistance. This means that as voltage increases, current increases as well, while resistance decreases. Similarly, as voltage decreases, current decreases and resistance increases.

5. How can Ohm's Law be applied in real-life scenarios?

Ohm's Law is widely used in various fields such as engineering, physics, and electronics. It is used to calculate the voltage, current, or resistance in a circuit, and can also be used to troubleshoot problems in electrical circuits. It is an essential concept in understanding the behavior of electricity in everyday life.

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