Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Some questions of voltage, charge, and batteries. Thanks.

  1. Jan 22, 2013 #1
    Hello everyone,

    I know electronics pretty well. I'm pretty good with that, but my physics and chemistry understanding of what is going on is a little weak.

    I'm trying to do a good Youtube video on Ohm's law and I want to have a solid foundation to my understanding. Not just V = IR, but actually understanding what it means, so I'd appreciate your help

    Voltage is the measure of the amount of energy on a charge. I know you have to measure the voltage across two points. So is voltage technically the difference in the energy of a group of electrons to the amount of energy of a group of...protons? As in, there's lots of electrons at the negative terminal of a battery, and lots of protons at the positive terminal. So the voltage would be a difference in J/-C to J/+C. Is my understanding incorrect?

    So electrons leave the anode of a battery and eventually make their way to the cathode.The reason they move is because they want to get to a more positive place. As they move through the circuit the electrons loose energy in the elements they travel through because they encounter resistance. Usually the energy is expelled as heat. By the time the electrons are near the cathode of the battery, they don't have a very large difference in voltage in respect to the voltage on the cathode. Is that correct?

    Okay so when the particles are near the cathode, they have a low voltage, so what makes them move? How do all of the eletrons in a simple battery-resistor circuit all move at the same (net) pace even though some of them are more more energized than others? This is probably a fundamental problem I have understanding this.

    My biggest question is how are they "re-energized"? They find their happy place at the positive terminal, but then they are suddenly made unhappy again by a chemical reaction which somehow puts them at the anode of the battery again, and then they seek to become happy again so they go through the circuit to reach the more positive place?


    If you have anything to add, I would really really really really appreciate it? Did I mention I'd really appreciate it? Thanks.
     
  2. jcsd
  3. Jan 23, 2013 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    It is the potential difference for charges at point A relative to point B. If you move an electron from A to B and have a voltage of 5 V between those points, you get 5V*(electron charge) as energy. If you move a proton (can be done in liquids, for example), you get 5V*(proton charge) - which has the same magnitude but opposite sign.

    The actual difference in electron density is extremely small, and is related to both potential difference and geometry of the setup. Do not use this, it just leads to misconceptions.

    Right

    Particles do not have a voltage. And you have a voltage drop over the whole circuit (everywhere where you have resistance), the "distance" to the battery does not matter.

    They do not. Some electrons are bound to atoms and do not move at all, for example.

    They were attracted by some positive ion (or something similar) - now that they reached the positive side of the battery, they can recombine with that ion. The battery gets discharged a bit, and to use that atom again you have to separate ion and electron again with energy from somewhere else.
     
  4. Jan 23, 2013 #3
    IMO - it will be impossible to make a good video about Ohms Law if you get involved in electrical particles.
     
  5. Jan 23, 2013 #4
    Thanks a ton!!. By particles, I meant electrons.Can you substitute the word electron for charges? Are they the same?

    When we are talking about voltage, we are talking about ONE particle and the energy it takes to go from one point in a circuit to another point in a circuit? Or in the same manner, the work that that one particle will do if it moves from A to B. So would charge be constant at the charge of ONE electron (1.6E19)? Can that charge every change?

    Your posts were some of the most valuable posts I've ever gotten. Thanks.

    And in regards to the Ohm's law video. I'm not planning on explaining all of this, because that would be rather silly in the scheme of things, but I do plan on using the water analogy and I want to make sure my water analogy coincides with what actually happens in the real electrical world.
     
    Last edited: Jan 23, 2013
  6. Jan 24, 2013 #5

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Electrons are one type of charge which can flow.

    You can calculate the energy for a single particle, but you do not have to.
    In a classical analogy, 1m of height on earth corresponds to a potential of 10J/kg. You can multiply this with a mass to get a potential energy for a particle, or a human, or whatever.

    :smile:
     
  7. Jan 24, 2013 #6

    nsaspook

    User Avatar
    Science Advisor

    Skip the 'water analogy' because it doesn't coincide with what actually happens in the real electrical world.

    http://www.furryelephant.com/content/electricity/teaching-learning/electric-circuit-analogies/
    http://forum.allaboutcircuits.com/showpost.php?p=532600&postcount=17
     
  8. Jan 24, 2013 #7
    Which do you recommend then?
     
  9. Jan 24, 2013 #8
    Suddenly I have some confusion again. Sorry for being an idiot. I know no one likes an idiot. Haha.

    I've been reading about and from what I read, an electron itself does not actually gain energy and the electron itself does not actually do work. This does not match what you said and I'm not sure who is right here. What I read is that it is the energy of the electrical field produced by the energy source which "pushes" on the electrons. So is it right to say that there's practically no energy change on an electron itself, and, a voltage is the energy that the source must provide to cause a charge to move from point A to point B? Now, if a resistor produces heat energy, how is the energy source (the battery) actually doing work on the resistor if it's not by electrons themselves releasing energy? Before I was under the impression that as it moves through the circuit, it looses potential energy and that it has to be re-energized at the energy source to continue on its path. I guess that's wrong. (What's right?)

    I am also having trouble with the resistance thing. If you have 1ohm and 1V across it and you have 1kOhm and 1V across it, the charge obviously moves much more slowly through the 1kOhm. And if Work = Charge x Voltage, then if both stay constant the work has to be the same for both. I guess this is like mechanical physics were two people can do the same thing but if one takes longer than the other, he has less power. But just to make sure, is it not true that the same amount of work would be done in both instances? The real difference would be in power?...because the electrons are moving more quickly through the 1ohm resistor than the 1kOhm resistor, but the energy it takes to move that electron from A to B is the same (still not sure why this is true exactly). If the electron is getting from point A to B more quickly then the current is greater. and thus power is greater?

    Yuck this is making less and less sense the more I think about it.

    Thanks so much.
     
  10. Jan 25, 2013 #9
    Hello TL....

    It is better to think of the energy that exists in a system - Also there are generally speaking 2 types of energy - potential (think stored) and kinetic (things in motion). The Electrons themselves do not change - they are fundamental particle, and they are all the same.

    In electrical circuits we take advantage different types of systems that store potential energy ( Capacitors=Electric Fields, Inductors = Magnetic Fields, Batteries=Chemical energy ). Then our circuits manipulate the energy ( quite creatively I might add) to do work.

    I tried to write up an analogy - but quickly got to a full page. I think your issue comes from perception : more force = faster ( acceleration) - this is not the case in a conductor, more force (voltage/ potential) means more electrons on the move (more current - not faster current).

    So you are on the right track of power = work / time - but a single electron moving across 2 volts - always does the same amount of work - and you really can not make that specific electron move faster. In electrical circuits you have more electrons - so for your 2 workers, they are the same, the only way to get more work done is to use 2 workers. ( Electrons must be Unionized (;-) )
     
  11. Jan 25, 2013 #10

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Right

    The difference is power. The amount of work depends on how long you run that experiment.
     
  12. Jan 27, 2013 #11
    TheLaw,

    "A" charge implies one charge. One charge has no energy reference. Two or more charges do. It takes some energy to pack a number of charges into a space, and more energy to pack them into a smaller space, or more charges into the same space. So voltage is measures by energy per charges (plural). In other words, voltage is the energy density of the charge.

    Nope, protons are bound charges. They stay the same everywhere within the atoms. Only the charges like electrons and holes are mobile, and holes are only present in semiconductors. Voltage is the energy density difference.

    Or it could be the difference between either of those points to ground. Whatever you want to define it.

    The reason they move is because they want to go where the energy density (voltage) is lower.

    Voltage is proportional to resistance. Why does that matter about electrons near the cathode?

    Charge moves to where its energy density (voltage) is lowest. Current in a series circuit is equal throughout. The charge that has a higher energy density has a longer way to go than a charge with a lower energy density. Charge becomes lower in energy density because it loses its energy in the form of heat.

    Happiness is an emotion, and has nothing to do with circuit flow. Rephrase your question in more scientific terms, or read about electrochemistry.

    Electrons and holes are charge carriers.

    1.6E19 what? Electrons and holes have the same charge but different polarities.

    Moving charge carriers like electrons set up magnetic fields which can turn motors. No current, no motor rotation.

    What do you define as an energy charge of an electron?

    It is a quantum thing. The voltage and associated electric field forces the charge through the material where it collides with the ionic cores of the atoms and other charges, thereby releasing heat. The battery or voltage source supplies the electric field for this to happen.

    Yes, same work for both, but it takes longer to do the work with the 1k resistor.

    Are you aware what some very good text books say about Ohm's law? They say that the formula V=IR or V=IZ is NOT Ohm's law. It is the resistance or impedance formula. Ohm's law is a property of a material, not a method of calculating current, impedance, or voltage. Read what the physics books say about this.

    "We stress that the relationship V=IR is not a statement of Ohm's law. A conductor obeys Ohm's law only if its V--I curve is linear, that is, if R is independent of V and I. The relationship R = V/I remains as the general definition of the resistance of a conductor whether or not the conductor obeys Ohm's law. ..... Ohm's law is a specific property of certain materials and is not a general law of electromagnetism, for example like Gauss's law."
    The above snippet is from Physics, by Prof David Halliday, University of Pittsburgh & Prof Robert Resnick,Rensselaer Polytechnic Institute, 1967 , page 780.

    And the following.
    "Ohm's law states that for many materials (including most metals), the ratio of the current density and electric field is a constant, which is independent of the electric field producing the current.
    Materials that obey Ohm's law, and hence demonstrate this linear behavior are said to be ohmic. The electrical behavior of most materials is quite linear for very small changes in the current. Experimentally, one finds that not all materials have this property. Materials that do not obey Ohm's law are said to be nonohmic. Ohm's law is not a fundamental law of nature, but an emperical relationship valid only for certain materials."
    The above is from Physics for Scientists and Engineers, Raymond A Serway, James Madison University, Third edition, 1990, page 745.

    http://www.launc.tased.edu.au/online/sciences/PhysSci/done/electric/resistnc/Resistance.htm

    http://in.answers.yahoo.com/question/index?qid=20110619080620AA4ZqhY

    Ratch
     
  13. Jan 27, 2013 #12
    Voltage is the force that tends to accelerate an electron. If it does no work such as resistive losses/heating then every electron accelerated by one volt will have a kinetic energy of one electron-volt. This is the product of the charge on an electron times the voltage. For this example, however, the electron loses that energy to heating in the resistor.

    Current is the number of electrons. The higher the voltage the more electrons can be forced through a resistor. At one volt across a one ohm resistor one amps-worth of electrons can be forced through it. With a ten ohm resistor only 1/10th amp can be forced through it.

    The battery acts as an electron pump, giving the electrons kinetic energy - I.e. voltage. This is lost through the external resisters and the spent electrons return to battery to to boosted up again.
     
  14. Jan 27, 2013 #13

    nsaspook

    User Avatar
    Science Advisor

    The actual electron kinetic energy is tiny when compared to the actual energy flow from the battery to the resistor. The bulk of the energy is in the fields around the wire.

    http://amasci.com/elect/poynt/poynt.html
     
  15. Jan 27, 2013 #14
    pumila,

    Voltage is not a force. It does not accelerate electrons in a consuctor. As I said previously, voltage is the energy density of the charge.

    The drift velocity of electrons in a conductor is very small, as is the kinetic energy. Most of the energy of the electrons is in the electrostatic field that all charged particles have. It does not matter how long it takes to move a charged particle through an electric field. The energy to do that is the same. Therefore, kinetic energy is not a factor.

    Current is the number of charge carriers per unit of time.

     
  16. Jan 28, 2013 #15
    I stand corrected, I was over-simplifying to the point of error. Voltage is the field structure that creates the force that would accelerate electrons free to do so; this force accelerates electrons through the battery but because of the constriction caused by the circuit resistance they bunch up into a higher electron density at the negative battery terminal against the elasticity of their mutually-repulsive fields, essentially converting the kinetic energy to static field energy. This bunching is what prevents the electron kinetic energy building up on each pass through the battery. Since it is constricted the energy that would have been retained as kinetic is dissipated as heat in the resistor.

    Hence voltage may be determined either dynamically from electron velocity, where the kinetic energy is the accelerating voltage times electron charge, or statically from a raised (or lowered) electron density. Firing a stream of electrons at an isolated electrode will cause bunching of electrons on the electrode, and it will be charged to the voltage of the electrons in the beam; any further electrons will simply be deflected from the electrode.

    The activity in the resistor is somewhat affected by the ambient noise, but in principle the electron bunching dissipates along the resistor. At any point along the resistor this gives an electric field difference that accelerates the electron until it interacts with the lattice structure and dissipates that kinetic energy as heat. The velocity is never great because of the short free path of the electron, but the overall effect is that electric field energy in the bunching is converted to kinetic energy is converted to heat.
     
    Last edited: Jan 28, 2013
  17. Jan 28, 2013 #16
    OP, you should probably decide on the audience you are trying to create the video for. If you are making the video for EE's or physicists, then you are going to have to spend a few months learning all the quantum theory and calculus involved to explore the laws that govern electricity. Also, a little bit of solid state physics wouldn't hurt either. Trying to make your video with anything less than those understandings would likely not have the result you are looking for

    If you are making a basic ohms law video using the v=ir relationship and some analogy to water (which is a pretty good way to think about it for people new to the concept). Then a lot of what is running around in here is just over complicating your basic question.

    If you really want to know more about ohms law, I suggest spending some time on google, or take a look at some algebra based based physics books. They should be detailed enough in theory to make a good video, and should be easy enough to understand without a BS in EE.
     
  18. Jan 28, 2013 #17
    pumila,

    The kinetic energy of a electron in a conductor is not a factor. Electrons move through a conductor like marbles move through a filled hose. Put on marble in at one end, and another pops out at the other end almost immediately. The movement of electrons through a conductor is called the drift velocity, and is very, very slow. It might take hours or days for a particular electron to move from one end of a conductor to the other end depending on the length of the conductor, voltage, and diameter of the conductor. Also electrons do not "bunch up" in a resistor like they would if a space charge was involved.

    Ratch
     
  19. Jan 29, 2013 #18

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    To back up that statement, the mass of an electron is about 1/120,000 of the mass of a copper atom. The average velocity of an electron in a copper wire carrying current is about 1mm/s. Can you consider its Kinetic Energy as being at all relevant when the wire may be delivering 1kW of Power?
     
  20. Jan 29, 2013 #19

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I'm not sure this is right. A very regular complaint on this and other forums is that "we were taught the wrong thing at school" and the water analogy is a typical example. Is it really a good idea to talk (or at least to go as far as writing it down as notes or a presentation) of electrical current, using a seriously flawed water model?
    Ohm's Law is a sophisticated concept and, essentially abstract (except in its most basic form) and so is Resistance (which is explained as "what makes it hard for electrons to be pushed around a circuit" etc. etc.). Would anyone seriously try to introduce Advanced Maths to a bunch of 12 year old students and use some naff model to show how differentiation works? Of course not - because it is generally acknowledged to be a hard topic. Well, so is the relationship and meanings of Current, Volts and Resistance.
    People seem to demand an easy explanation for things. Very often, one just doesn't exist. What and how we are taught Science at school is down to Politicians who's lives are not based on Evidence or Logic.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook