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How does voltage affect particles in current

  1. Feb 24, 2013 #1
    So, I have a problem visualizing effect of voltage on the current in a circuit. Here goes:

    -Intensity (I) of a current is a ratio of charge traveling through a cross section of a conductor, and time. We can visualize it as a flow of water through a pipe: It is all about quantity of particles.

    -Voltage (V) of a circuit is a difference in potentials between two poles. We can imagine this as a height of a chamber from which we can release water into pipes.

    So, power of a current is I times V, in other words - to preserve power we can "increase quantity of water and reduce the height of a chamber" or "increase height of a chamber and release smaller quantities of water".

    In this analogy, we can easily see that we either have less water moving faster or more water moving slower to preserve total power of a system. The higher chamber results in FASTER water!

    Now this is my problem. What property of electrons does voltage affect, to make their energy higher?
    Is it their speed?
    Is it the drift speed, or just a speed of any electron in particular?


    Additional question, very much related:

    Let's imagine a small charge (-) between two parallel plates of a capacitor. It is near (-) plate of a capacitor.

    Now, the charge has a certain Electrical potential energy (voltage times charge).
    If we release the charge and let it move towards (+) plate of a capacitor, its potential energy is reduced as it approaches (+) plate, and all of its potential energy at this point is converted to... to WHAT?

    There is not much talk about this in textbooks, and it would help me visualize electricity if I could correctly realize what property of a particle, besides it's charge, plays a part in its energy.
    Last edited: Feb 24, 2013
  2. jcsd
  3. Feb 24, 2013 #2


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    Don't consider the speed of electrons in circuits, this just leads to all sorts of misconceptions. The average speed of those electrons is really small (of the order of millimeters per second).

    It determines the potential difference for the electrons. In a resistor (where more voltage usually leads to more current), you can view this as a higher force on the electrons, but be careful with conclusions based on that view.

    In the flight, it gets converted to kinetic energy. At the plate, it is converted to heat, electromagnetic radiation, or other forms of energy you don't care about in circuits.
  4. Feb 24, 2013 #3

    Simon Bridge

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    Voltage changes the potential difference ... like any potential difference.
    Consider gravity - if the gravity is stronger, the gravitational potential difference between some height and the ground would be bigger. The affects the resulting motion of objects moving in the gravitational field.

    You should see this same effect in your example of a parallel plate capacitor. Change the voltage and the potential-difference between the plates changes (this is actually a tautology) - changing the way the electron between the plates accelerates..

    The parallel plate example works just like a mass falling in gravity - the electron "falls" in the electric field toward the positive plate trading potential energy for kinetic energy. When it hits the plate it becomes part of the plate like the mass becomes part of the ground when it lands. In both cases, what happens next depends on the situation on the ground/plate: it can be quite complicated.

    As mfb points out - the water-pipe model for electricity in circuits is only an analogy - like any analogy it can only be carried so far. It is commonly introduced at early levels so students have something to help visualize simple circuits ... it's a "lie to students". One that often annoys college professors who have to undo the misconceptions that result.

    The different (imperfect and incomplete) models that are used to describe stuff when you are starting out can be confusing. As you progress in your studies, the different models will get merged, and the less useful ones discarded.

    They won't all get merged though - because we don't have a grand unified theory (not yet). It is best to think of them as models rather than actuality - and understand that different situations will call for a different picture.
  5. Feb 24, 2013 #4
    Thank you for your responses!

    Yes! All right.

    Just help me wrap my mind around these three problems:

    1) Does higher voltage mean electrons in a CIRCUIT accelerate faster (thus increasing their kinetic energy)?

    if so,

    2) Does the overall (Brownian) speed of electrons increase (which would be weird, because that would mean that the temperature of electrons increases)

    or does the Drift speed of a current increase?

    and, finally, if so

    3) how does this property affect the Power of a current?
    I am not asking for formulae, but for a visual explanation... In other words, if a current moves "faster", why does it make it stronger.

    NOTE: 2) and 3) are based on assumptions that I made not knowing answers to 1) and 2) so ignore these questions if they make no sense :smile:
  6. Feb 24, 2013 #5


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    Again, acceleration and kinetic energy are not useful quantities of electrons in circuits.
    Between parallel plates: Sure.

    Drift speed in resistors increases if you increase the voltage.

    The electrons have more potential difference - similar to a different height for gravity.
  7. Feb 24, 2013 #6
    Dropping an object from a height starts a process of converting potential energy to kinetic energy.

    The bigger the drop, faster the ball will eventually go.

    THAT'S what I'm asking - what will the difference in potentials in a circuit convert to? What kind of energy?

    Because, by definition of P=I*V, ONE "High Voltage" electron has more power than ONE "Low Voltage" electron. Why? Because of what property?

    Another possibility is that it is not an electron that changes, but the nature of the flow. Either way, this is where I hit a wall so far.

    Thank you for your cooperation mfb and Simon, we're getting there!
  8. Feb 24, 2013 #7
    First think what energy exactly does pressurized water have. Then see below.

    Some electrons in a circuit are affected by an electromotive force. Those electrons may have potential energy, for example if the voltage source is a capacitor.

    An electron in a wire can use the energy of the electrons inside the voltage source.

    What i'm saying is that those electrons affected by an EMF may have some energy. Other electrons don't have energy.

    Water up in a water tower has potential energy. Water that is under pressure has no energy. If pressurized water does work, a pump or a water tower is doing the work.
    Last edited: Feb 24, 2013
  9. Feb 24, 2013 #8


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    In a resistor? Usually heat.
    Other options are light, mechanic energy, chemical energy and some more.

    Voltage IS this property.
  10. Feb 24, 2013 #9
    Voltage is a property of a system or part of a system. It isn't a property of an electron
  11. Feb 24, 2013 #10


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    Heat, motion, light, etc. Stick a light bulb in a circuit and you have light and heat generated. Stick a motor in there and you have motion.

    Single electrons are not described by such rules and are not within the scope of this thread. It is the circuit as a whole that is affected by voltage and it is the circuit as a whole that is within the scope of the normal EE equations such as Ohms law.

    You are correct, it is the current that changes when you increase or decrease voltage. Specifically, more charges per second will flow by a certain point in a circuit when you increase voltage.

    Realize that electrons in a conductor are NOT like electrons in free space. You can't push an electron and have it whirl around the circuit. It is subject to all kinds of electromagnetic forces within the circuit which rob it of whatever energy you gave it nearly instantly. It takes a sustained difference in potential, aka voltage, to make the charges flow in a net direction. (Even with applied voltage you still have some that flow the opposite way)
  12. Feb 24, 2013 #11

    Simon Bridge

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    Not to take anything away from Drakkith here, but my immediate responce was that power is not something an electron has in the sanse that it has charge and energy, it is a process: power is the rate of change of energy.

    An electron released a high voltage (read - high "potential difference") potential will change it's potential energy faster than one released in a low voltage potential.

    "Voltage" is a problematic term here - the electric potential has units of "volts" and so does the potential difference and fair enough. Either can be called "voltage", but the voltage of an electric circuit is usually the applied potential difference of a "voltage source" - i.e. a battery. It may refer to the change in potential across a component like a resistor, and even the "rating" of a load - like a light-bulb rated for 240V 10A. For this discussion we probably need to distinguish - or just drop the term "voltage" completely for the sake of clear communication.

    I suspect that saying what kind of voltage is being referred to will clear up a lot of the confusion.
  13. Feb 25, 2013 #12
    So, how would you describe a "visual" difference between a state of high voltage - low intensity circuit, and low voltage - high intensity circuit (with same power)?

    let's just describe 3 situations:

    1) voltage is zero. there is no flow of free electrons in the conductors.

    2) voltage is rather low, and the current is quite intense. So, the flow of electrons is there, and quite a lot of electrons are flowing in a unit of time. (that's what high intensity says, isn't it?)

    3) this same current gets transformed to high voltage, with no losses.
    Now, the number of particles that are flowing in a unit of time is smaller than before (that's what low intensity is, right?). Also, the flow of electrons is still here, voltage isn't zero. Actually, it's high. And we have less electrons taking part in a current. And we have the same power of current.

    So, there has to be some other difference in the behaviour of a circuits, besides number of charges bustling around the wires. Something that compensates for the drop in number of particles.

    And don't say "that IS voltage, this other property!", because that is like saying that "HEIGHT OF A WATERTOWER" is a property of water! That is the CAUSE, voltage is the CAUSE, and I'm after the EFFECT.

    Please think about this.
  14. Feb 25, 2013 #13

    Simon Bridge

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    Who are you asking?

    By "intensity" do you mean intensité de courant or "currant intensity" which is just "current" these days? Or do you mean the square energy per unit area?
    Don't know what you mean by "visual" either - you don't normally see electrons or electric currents directly and what you see indirectly depends on the method used.

    Taking "intensity"="currant"
    1. the current is zero - so there must be an open circuit in DC or no charge carriers.
    2. for a low potential difference high current in an electric circuit means that the resistance is low.
    3. higher applied potential difference, same current, then the resistance was also increased.
    For a given circuit, the voltage and the currant have a relationship depending on the exact makeup of the circuit. They cannot take on just any old values.
    In a simple DC circuit, they are related by the load resistance through Ohm's law.
    The resistance is the feature you are looking for.
    Like drakkith was saying before
    ... the load resistance is not the half of it.
    Last edited: Feb 25, 2013
  15. Feb 25, 2013 #14


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    There is no "visual" difference. Attempting to visualize it will fail because you cannot see what is happening in the circuit. If you need something to visualize I'd recommend the water pipe analogy, where current is water flow. It isn't correct, but it's the closest thing you're gonna get to visualization.

    There is no NET flow. Charges, electrons in this case, move all over the place thanks to thermal energy. This is one reason objects emit thermal radiation, they have moving charges which emit radiation upon acceleration. But they do not flow in any specific direction, as a whole, on average.
    Then the circuit has little resistance and little power.

    The same current gets transformed to high voltage? Then you have the same current as before, but with a higher voltage, meaning higher power. This means that the resistance has increased and you will dissipate more power in the circuit.

    There is. Voltage and resistance.

    You are getting stuck trying to understand and visualize what voltage and current are. You WILL stay confused if you keep trying to visualize it. You simply cannot. It isn't possible to visualize what happens in the circuit. You cannot boil this down to "what happens to a single electron" because standard circuits have trillions of electrons doing all sorts of things all at once that results in the observable effects we see.

    It takes energy to keep a voltage applied to a circuit. This energy is lost as current flows. Either in the form of heat, light, or motion.
  16. Feb 25, 2013 #15
    First, Simon:

    I was talking about the current, yes. "I" comes from "Intensity" and I tought that I would use the word Intensity for the current, and "Current" for adressing electricity. Sorry for confusing you, after reading your answers, I have realised that I messed up by using word "Current" inappropriately in question 2).

    Actually, my idea was that if we have I1 and V1, making together for P1=I1*V1, and if we transform this system, without losing power, to I2=0.5*I1 and U2= 2*U1, then P2 would be P1 of course.

    BUT, since I2= 0.5*I1, that means that dQ2=0.5dQ1, only half the charge would flow through some cross section of a wire.
    And power remained the same.
    This would mean that an average particle in a second case somehow contributes more to the total power of a system. Average particle is somehow stronger, don't mind me using the layman's terms. Do you see what I'm getting at?

    And Drakkith,

    Thank you for adressing the real problem, and that's my intention to describe phenomenons to myself from "the ground level up".

    However, this is really something that shook my world.

    It is impossible to know what electrons are doing? It's impossible to even describe it? Is this a certainty or your point of view, Drakkith?

    Could you maybe point me to and article explaining why I can't imagine what is happening in an electrical cord?

    That is pretty discouraging for me, I must say. How to passionately study physics, if there is no chance of understanding the principle? I tought that was the whole point of science...
    Last edited: Feb 25, 2013
  17. Feb 25, 2013 #16

    Simon Bridge

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    Each charge starts out at twice the potential energy, yes.
    So each has more to convert to something else.

    Notice I am being careful to talk about "charges" rather than electrons - this goes to the difficulty in talking about what happens on an individual electron basis in a material object like a wire. We understand these things in terms of emergent properties like current and charge instead. That way we can make sense of specific situations like electric circuits.

    Perhaps you can visualize the difficulty:
    It is difficult to study a crowd by watching just one person in it. Each person is moving according to the various forces of their individual circumstances. But from a distance, everyone is basically moving randomly. We know it isn't really random - ask anyone and you'll see they have reasons for what they do. But that's what it looks like.

    Now if something were to attract the crowds attention - say they are a crowd at a ferry terminal, and the ferry just arrived, then the crowd gets some directionality to it. Most people are still mostly milling about but there is a general motion towards the ferry. That's what an electric current would look like. If the crowd were football fans waiting outside the stadium and someone just opened the gates - then there would be a more definite directionality - this would be a higher voltage. But we are still daling with only 100s-1000s of people. That's still small enough to see the crowd and the individual dynamics at the same time.

    Now make the number of people very large - say 100-million people are in this crowd ... and the regeon you are looking at is a largish island like Zanzibar... so everyone has about 3 square meters? To stand back far enough to see the whole crowd, you can no longer see the individual people in order to see how their individual motion contributes to the whole. Complicated things can happen that you won't see because you are either too far away or because it's happening because of someplace you are too close in to see. Instead you have to deal with the crowd as a whole.

    In a cubic centimeter of copper, there are around 10^24 free electrons ... trying to figure what is happening in the copper in terms of individual electrons would be impossible. Imagine trying to understand a crowd of 10 000 000 000 000 000 000 000 000 people?
    However - in special situations, a lot of the randomness averages out. If we deal with all those electrons in terms of what they look like when we stand back, some sort of understanding is possible.

    The methods form part of solid state physics ... which electric circuits are a small part of.
    Here you can think of clouds of particles moving around under the influence of forces - but only if you concede that these cannot be the same as electrons in a vacuum.
    They usually have the wrong mass for example - and can be positively charged. We call the positive charges "holes" and the weird masses are "effective mass"s because we understand that these things are emergent properties of a bulk material ... just like we understand that each person in the crowd has their own reasons for doing stuff.

    You have read too much in to the statement. The principles can be understood - just not by visualizing the behavior of individual electrons in a wire.

    There are a whole bunch of handy visualizations that get used as teaching aides - but they are all inadequate in some important way.
    At some point you will have to discard them. Be prepared. Discarding bad ideas is part of what science is about after all.
    Last edited: Feb 25, 2013
  18. Feb 25, 2013 #17
    Great analogy and great effort, Simon.

    So here's a picture I have so far:

    average kinetic energy of charged particles in a wire has little to do with voltage and current. It has more to do with temperature, probably.

    But what happens is when voltage is introduced, charged particles have a tendency to direct their motion in a particular direction. So, voltage doesn't add energy to particles, it only arranges their behaviour to a certain degree, which results in a net flow of energy?
  19. Feb 25, 2013 #18

    Simon Bridge

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    We'll find out if it is a good analogy when people start criticizing it ;)

    Thinking statistically is the way to go.
    Applying a potential difference shifts the statistics.
    The result, averaged over a lot of electrons is the electric circuit math you have been learning in terms of currents and power and so on.
  20. Feb 25, 2013 #19


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    You can understand the principles. You just can't visualize them. You can only visualize the analogies or something else you make up in your mind that you associate with the principles.
  21. Feb 26, 2013 #20
    If only we adopted Steinmetz' suggestion of defining voltage by the electrostatic (he called it the dielectric) field around the conductor, then all of these misunderstandings would not exist.

    The concept of particles hurts our understanding of electricity, it is much easier defined in fields and waves.
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