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

Confusion with concept of voltage

  1. Jan 24, 2012 #1
    If I say,

    V = I x R
    V = 1 x 5
    V = 5 volts

    Now, if I interchange the values

    V = 5 x 1
    V = 5 volts

    When current is 1 amp and resistance was 5 ohms, the voltage was 5 volts. When I interchange the values then it is still 5 volts. How can I think of what voltage is?
    Last edited: Jan 24, 2012
  2. jcsd
  3. Jan 24, 2012 #2


    User Avatar
    Science Advisor
    Gold Member

  4. Jan 24, 2012 #3


    User Avatar
    Gold Member

    When I multiply 1 times 5, I rarely get 10.
  5. Jan 24, 2012 #4
    I don't know what I was thinking. I really meant 5 volts. Edited the question
  6. Jan 24, 2012 #5


    User Avatar
    Gold Member

    I don't really get what your question is. Voltage is an electrical potential. When you apply it across a resistor, electrons flow and we call that current.

    What is your question?
  7. Jan 24, 2012 #6


    User Avatar
    Science Advisor
    Gold Member

    Yeah I don't get the question either. 5 amps flowing through 1 ohm will build a potential of 5 volts across the resistor the same as 1 amp flowing through 5 ohms. How is it any different than 5 groups of 4 makes a total of 20 the same as 4 groups of 5 makes a total of 20? If you can accept the one why not the other?
  8. Jan 24, 2012 #7
    How is the electrical potential is same in both cases? I know the ohms law says it is but in what terms can I think what exactly 'voltage' is? I don't understand the terms 'electrical potential' and 'voltage'.
  9. Jan 24, 2012 #8


    User Avatar
    Gold Member

    Have you ever heard of Wikipedia? You should learn to use it.

  10. Jan 24, 2012 #9
    People hate water analogies but I love them so sue me.

    You have a tube filled with water. The width of the tube represents your resistance. The bigger the tube, the less the resistance.

    Apply pressure(voltage) across that tube.

    If your resistance(smaller tube) was 5(no units, generalized). Amount of water flowing through, with that pressure applied would be 1.

    Make the resistance smaller i.e. tube bigger, so 1, the amount of water flowing would be 5.

    Don't confuse speed of the water with the amount flowing.


    *hides behind kevlar*

    have in mind this is ANALOGY! Don't think of electricity as flow of water, you will get murdered :D
    Last edited: Jan 24, 2012
  11. Jan 24, 2012 #10
    Can't we sometimes tell them the answer straight off without linking them to wiki or other sites. I sometimes feel like some people here are behaving as if the questioner needs to learn the thing from somewhere else and come to this forum for only some specific question. Is that whats this forum is for?
    Pardon me, but I_am_learning. :)
  12. Jan 24, 2012 #11
    I_am_learning - I sympathise with you. If you come to a physics forum and are directed to Wikipedia then you have not gained much. If you are recommended to 'Google it' then you have nor learned much. Imagine the vicious circle.... Google 'physics question'..... get directed to 'Physics forums' ..... get advised to 'Google it'
    I don't think much of Wikipedia from what I have seen, I prefer straightforward text books.
  13. Jan 24, 2012 #12
    On the subject of Voltage and potential difference I would point out the units.
    1Volt means 1Joule per Coulomb. It is a measure of energy supplied to charge passing through a potential difference.
  14. Jan 24, 2012 #13


    User Avatar
    Science Advisor
    Gold Member

    Well we have to start somewhere. Maybe the "question" could have been answered with one of Maxwell's equations.


    Which would take several pages to fully explain.
  15. Jan 24, 2012 #14


    User Avatar
    Science Advisor
    Homework Helper

    hi david! :smile:

    (instead of wikipedia, you could try the physicsforums library :wink:)
    'electrical potential' and 'voltage' are different names for the same thing

    electric potential equals potential energy per charge

    the whole thing works because electricity is a conservative force, so the energy you have depends only on where you are (in a circuit) …

    that's the electric potential (times your charge) at that point :smile:
  16. Jan 24, 2012 #15

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    when you are just beginning you can use the analogy of water in pipes.
    be aware there's other ways of thinking about it.

    in fact the British used to say "Pressure" to describe what we now call voltage
    and say "Valve" to describe what we call a vacuum tube.

    in simplest terms:
    electrons move.
    when they move past a point , an electric current is said to be passing that point.
    when approximately 6 X 10^18 per second pass a point, it is said that one ampere is passing that point.
    they do not rocket by at speed of light. they drift by, slowly, in a disorderly column like water molecules in a river. imagine how wide is a tiny wire compared to size of an electron - you get the drift...
    an electron is such a tiny amount of charge that we usually deal with them in larger quantities - 6E18 of them is called a coulomb, after a scientist.
    just like 6E23 water molecules is a mole. (where'd that name come from?)

    something must induce the electrons to move.
    that something is called "Electro-Motive Force", abbreviated EMF and E for short.
    that force can come from repulsion & attraction of like and unlike charges which is an electric field (ever feel the hair on your arm around static electricity on a dry winter day?);
    or from a moving magnetic field as in a generator
    or from a chemical reaction as in a battery.
    how hard it pushes on the electron is a measure of intensity of the field that's pushing;
    how much work it does on the electron is measured in energy.. ever hear of an electron volt? It's a mighty small unit so we deal instead in joules.
    one joule per coulomb is one volt of "pressure",

    so if one volt pushes one coulomb per second through something
    there's one joule per second going somewhere
    which is one watt.

    sounds confusing at first.
    but EE's have a big advantage over ME's - our units are all nicely defined, already metric and conversion constants included. we needn't worry about mass in slugs or velocity^2/2g in our flow equations.

    so spend some time getting straightened out in your mind the fundamental units of:
    current(coulombs/second aka amps),
    energy(joules, aka newton-meters),
    power(joules/sec, aka watts),
    and EMF(joules/coulomb, aka volts)

    lastly be aware that just like water in a river
    the individual water molecules move slowly
    but a sound wave can traverse the length of the river much faster.
    it's a different phenomenon.
    you'll get into that when you study AC.

    this technique helps us 'average bears' bootstrap ourself up toward more rigorous thinking.
    it's a start

    now i'm ducking for kevlar.
    Last edited: Jan 24, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook