about voltage

Discussion in 'General Electronics Chat' started by nikhilthunderlion, Nov 6, 2007.

  1. nikhilthunderlion

    Thread Starter Member

    Oct 1, 2007
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    suppose a circuit contains 5 volt battery and a 6 ohm resistor.Now upto the point where the electrons enter the resistor's they have a potential of 5 volt (i.e they have some potential energy)but when they leave the resistor whole of the voltage has already been droped across the resistor ,now they don't posses potential energy then how they can reach the battery's negative terminal as they dont have potential energy.
     
  2. Dragon

    Active Member

    Sep 25, 2007
    42
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    Imagine dropping a ball from a certain height. It will continue to fall until it hits the ground. It never 'loses' its potential(gravity) by going through a particular distance. Its quite analogous to your query.
     
  3. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    As the current circulates around the closed circuit the voltage drop in each part of the circuit is proportional to the resistance of that part so it would look something like this:

    R of each connecting wire: 0.01
    R of resistor: 6 ohm
    Total resistance: 6.02 ohm
    Current: 5 / 6.02 Amps
    Voltage drop in resistor: 5 * 6 / 6.02 V
    Voltage drop in each wire: 5 * 0.01 / 6.02 V

    So there IS voltage drop in the wires and it is proportional to the resistance (Ohm's law) but it is very small compared to the drop in the resistor and so it can be ignored.
     
  4. brianl

    Member

    Nov 3, 2007
    11
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    I think I see where you are coming at this from, you are imagining a single electron entering the resistor with potential to move it through the resistor and coming to a rest at the other end.
    Where in fact you maybe should be looking at it as electrons aligned end to end all the way around the circuit with no spaces between them, as one enters the resistor it pushes another one out and pushes the next one and so on until one enters the – terminal. This push is instantaneous throughout the circuit (ie all electrons move at the same time) from the + terminal through the resistor to the – terminal through the power source and out the + again, this is also why the current measured in a circuit is always the same.
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    GS3 is correct, you are ignoring the usually ignored resistance of any wire or connections (even if they are very very very short).

    Also that the voltage potential exists between the two battery posts, not just one battery post and the resistor.
     
  6. brianl

    Member

    Nov 3, 2007
    11
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    still learning, but now a bit confused, or maybe just reading it wrong.... does the above response mean that a circuit has to have potential between different points in a circuit in order to allow current to flow? that cant be the case...can it? I had assumed that current was equal throughout the circuit and determined with respect to the load and potential but not determined in entirety by the load, e.g if you had a perfect (theoretical) power source and zero resistance short across the power source, current would still flow? or if the source was perfect the wires were zero resistance and a load connected the electrons would still return and run around the circuit.
    apologies if i have interpreted the replies incorrectly, and if I haven't...its back to the beginning for me again :(
     
  7. cheddy

    Active Member

    Oct 19, 2007
    87
    0
    Hey guys,

    :confused:

    I'm (only slightly) disappointed that none of you noticed this.

    Well they don't reach the battery negative terminal because electrons always flow towards the positive terminal.

    :D
     
  8. cheddy

    Active Member

    Oct 19, 2007
    87
    0
    Yes

    Current is equal throughout the circuit, yes, that is true.

    You've forgotten that voltage current and resistance are interdependent and a tiny voltage can create a huge current if the resistance is low enough.

    If a piece of wire has .01ohm resistance then it only needs a very small potential difference between it's points in order to draw the same current as a high ohm resistor which has a higher potential difference across it.

    GS3 already explained this.
     
  9. brianl

    Member

    Nov 3, 2007
    11
    0
    Originally Posted by brianl
    does the above response mean that a circuit has to have potential between different points in a circuit in order to allow current to flow?

    yes I see when you extract my original statement like that it looks incorrect:)
    what I ment to say was potential between multiple different points within the circuit.
    as the OP's question was (leaving aside conventional vs electron flow) after the electron gets to the 0V side of the resistor, what drives it from there to the neg terminal of the power source.
    I suggested that all electrons move in unison, others suggest the potential between the 0V side of the resistor and the - terminal of the power source due to the resistance of the wire.

    my question above was does a perfect potential source require a load to enable the flow of electrons? (ie a short across the perfect power source with a 0 ohm piece of wire) and do differing potentials have to exist in a circuit to give potential in diffenent parts of the circuit to continue to exert a force on the electrons, which is causing me the confusion.
     
  10. cheddy

    Active Member

    Oct 19, 2007
    87
    0
    The answer is yes again. There is a different potential difference between any two points in a circuit. When resistances are very small such as in a copper wire the potential difference between few feet is very small and most often ignored.


    There isn't a 0V side to the resistor in reality, it's closer to .0001V and the difference is between the leaving side of the resistor being +.0001V, across the wire to the negative terminal at 0V is what drives the current. Additionally because the resistance of the wire is so small that tiny voltage will draw the same current through the wire as the current through the resistor.

    No load is necessary but in your ideal world that doesn't exist if there is a 0ohm piece of wire between a potential source you would have infinite current because by ohms law you would be dividing voltage by zero.

    Yes there are differing potentials in any two points in a circuit.
     
  11. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    (edit:simultaneous posting. similar explanation)

    if a electron reaches a point of zero potential difference it wont go any further.
    but that also means that there is no field or potential in to to go further.
    consider a electrochemical cell.
    if at all electron does reach this point it means there is no diff in e- concentration (in this case the concentration or lets say charge diff is what causes potential)
    now again consider what will happen after that. the reactions continue(which again involve movement of e-) and the difference is again created and the electron then again start flowing. this is a cont process and that is how a current flows in a cell continuously.
    similar thing can be explained in case of generators. in this case it is the field or the potential created by it that is causing the electron flow.
    since a piece of wire does have some resistance there is always PD across it.
    this can be seen as the cause of electron flow across it.
    (this part might explain Mr. brianl's confusion)there is potential drop across wire due to flow of current and this current is determined
    by overall resistance of circuit and hence potential drop across the same component will be different for different overall resistance of circuit since the current is determined by resistance and this current determines the drop in potential to send it thru that component.we can not have same voltage drop for different current flows ohms' law will get us for that.
    they are all interdependent .
    current can not be different across different components this will violate conservation of mass.this also answers why a electron will complete the circuit since other electrons will continue to flow across points having PD which in turn will cause this electron to flow (due to difference in charge concentration).silly way of looking at it.just wanted to convey that
    all observations/interpretations lead to one another.

    (seemingly Mr thingmaker made the post at the same time)
     
  12. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Both are true. Current flow is the same in all parts of a series circuit. All room temperature copper wires have some small finite resistance.
    Yes. Current is equal to EMF divided by resistance. If resistance approaches infinity, then current approaches zero.
    Ohm's law says "yes." Current in any part of the circuit will be equal to voltage across that part of the circuit divided by resistance of that part of the circuit. This is true whether we look at the entire circuit, or just one solder joint.
     
  13. brianl

    Member

    Nov 3, 2007
    11
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    Lads this site is fantastic!

    am I right in thinking, that voltage is not potential energy in itself but a measurement of differing potential energy between two points. or come to think of it the level of energy that would exist between these two points if they were connected, so you could say that a voltage drop is energy that is "kinda" used in the space where the drop occures.
     
  14. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    Yes.

    Check our Basic Concepts of Electricity Chapter: http://www.allaboutcircuits.com/vol_1/chpt_1/4.html

    Dave (3000)
     
  15. brianl

    Member

    Nov 3, 2007
    11
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    sorry Dave, I edited as you were posting are my additions still correct? nice to see you got Dave 3000:)

    so just to jump a bit forward when I was looking at caps I had difficulty understanding how potential existed between the seperated plates, so I now assume that an electric field exists between the plates and because of this field there is now potential so i would go further to think that voltage is created as a response to the attraction or repulsion of these fields. what i think i am saying is that voltage does not really exist as such but is created by the energy difference that is used or potentially to be used in moving from one point to another.
    feel free to shout if I have lost it alltogether.
     
  16. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    yes u are correct.
    this should help you in learning the finer details, also try to look up about electric field it will
    be helpful in learning about electric potential.
    also do search for definition of volts etc, they mostly are the same.


    edit: c:)ngratulations on 3:D:p:cool: Mr Dave.
     
  17. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Electric potential has a similarity to kinetic potential. If you take a rock to the top of a hill, both you and the rock (due to the energy you expended getting there) have gained potential energy relative to the bottom of the hill. This potential will always exist and may never be realized until you either let go of the rock, or slip and fall down the hill (note Newton reported being hit on the head with an apple). Also note the two points are the top of the hill, and the bottom of the hill (per +/- battery terminals). (Actually, the lower potential point maybe the center of the earth, or the center of the Galaxy, but we don't need to take this so far.)

    In a battery or power supply, irrespective of which direction you believe electrons to flow, there are at least two points required to supply the potential energy, they are typically referred to as + and - on a battery.

    The energy exists within a charged battery, regardless of whether or not there is a load, and this is why it is named a potential -- it has the potential to supply power via EMF (electro-motive force). A perfect battery that was charged and never loaded would stay charged forever, as would a perfect capacitor. However, due to defects of an imperfect world, this does not happen (and perhaps that is the perfect thing).
     
  18. brianl

    Member

    Nov 3, 2007
    11
    0
    Thanks for the links, wickipedia have used it before its an excellent resource, would be fair to say that I probably wouldn’t have understood what is was saying about voltage etc in as much detail if I had read that before posting here.

    but anyway, this may be a bit off topic but when reading about electromagnetic fields I was trying to picture what was happening and I think I may have got it wrong somewhere! My understanding now is that the energy is totally contained in the electromagnetic fields, the electrons or whatever charge carriers, are exactly that just carriers of charge.
    But this is where I think I may be applying things incorrectly I read somewhere (probably while reading about inductors) that the electric field does not exist inside the conductor if this is true and the energy is carried in these fields am I to assume that the energy in a circuit is actually carried outside the conducting wire/PCB track etc. where is the mag field? @ 90 deg to the electric, inside the conductor? does the mag field have energy?


    And before I post my next question (which is a bit of a mess at the moment) are EMF and voltage different? Is EMF the supplier of energy to the circuit ie what puts energy into the fields and Voltage the representation of this energy to another point in the circuit only.
     
  19. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    IIRC the electric field is zero in a metal only in case of electrostatic equilibrium which is not the case with condutctor (its been a while since i ever touched this so cant speak much right now).
    again cant say but a piece of metal does carry flux well but then a piece of conductor carries current well too.its hard to comment since i do not remember much.
    consider this
    and this pdf
    do a search on magnetic field and more specifically magnetic filed inside conductor.
    also read about skin effect to as there is a cae for high frequency ac currents where charges mostly flow at the surface else in case of dc they pretty much flow thru the entire conductor.
    as for EMF and voltage the term emf is for some reason rarely used except for back emf etc.
    there isnt much of a difference between them. maybe just a case of terminologies since
    terminal voltages of battery are always lower than voltage.
     
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