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

What is Voltage?

  1. Feb 20, 2009 #1
    I have seen lots of things with voltage written on them.

    So what is voltage, what does it do?

    Does a battery contain tons of electrons (or protons, i dont know) on the positive side, and then when i use a wire to connect the both sides, what would happen? All the charged particles flow from + side to -? Or the battery only provide an E-field which pushes the electrons (inside the wire) thru the circuit to the negative side?

    How is a battery USED UP ?

    Plz help because its driving me crazy.....

    thanks in advance
     
  2. jcsd
  3. Feb 21, 2009 #2
    In a piece of metal only the electrons can move. Electrons have a negative charge.
    Most nonrechargable batteries contain zinc, manganesedioxide and some electrolyte.
    In an alkaline battery for example the electrolyte is potassium hydroxide
    http://en.wikipedia.org/wiki/Alkaline_battery

    The chemical reaction looks like this
    At the - side of the battery: Zn + 2OH− → ZnO + H2O + 2e−
    At the + side: 2MnO2 + H2O + 2e− → Mn2O3 + 2OH−

    So the chemical reaction causes electrons to be given off at the - side.
    However this reaction can not go on forever. The more electrons accumulate at the - side the stronger the electrical field between the zinc and the electrolyte will be. That field will stop the reaction if it gets too strong. That only takes a tiny fraction of a second since not many electrons are required. But if you connect - and + with a wire the electrons can flow away and the field will never get strong enough to stop the reaction. So the reaction inside the battery only occurs when the battery is used.

    The higher the excess of electrons at the - side and the greater the lack of electrons at the + side the higher the voltage will be.
    Voltage is actually a measure for the amount of energy a certain amount of electrons will receive if they move from - to +.
    So in other words the voltage will tell you how big the charge imbalance between + and - can get.
    If you connect 2 batteries together you get twice the voltage.
    Let's say the - side of the first battery is grounded and the + side of the first battery is connected to the - side of the second. That means that the - side of the second battery will be forced to become positive. That in turn will make the electrolyte in the second battery more positive which means the posive side of the second battery can become more positive.
     
  4. Feb 21, 2009 #3
    First, I'd like to congratulate DrZoidberg on such a clear response. I think it explains the battery well. I'd just like to add to this a few points, just to clarify a little.

    Voltage is a measure of "potential". Essentially, if an electron travels from 10V to 0V, it will pick up 10eV of energy (10 eV = 1.602 attojoules, atto = 1/1,000,000,000,000,000,000.) This isn't very much energy at all, but there are lots of electrons, so it adds up. The force that the electrons will feel making them move round the circuit is the "slope" or rate of change of the potential.

    As DrZoidberg has said, the chemical reaction in the battery creates a buildup of electrons on the negative terminal of the battery. These electrons repel each other and so *want to move away* from the negative terminal. At the other side, electrons are being used up and so there is a lack of electrons (it creates positive "ions", basically atoms missing an electron) so the positive terminal *wants* electrons. Connecting them together via a wire allows the electrons to move.

    When all the chemicals have been converted in the reaction, there is no longer the ability to create a surplus (or lack) of electrons.

    So the voltage is created by chemical reactions and provides the electrons with energy as they move around the circuit. This energy can then be used up by, for instance, lightbulbs or buzzers or whatever.
     
  5. Feb 23, 2009 #4
    First of all, Thank DeShark and DrZoidberg for helping.

    I understood alot from reading your response.

    Just want to make sure, does the negative charge electron only move from negative side of a battery to the positive side? That means the current is from - to +?
    My book always shows the direction of current from + to -. Is it just a THEORETICAL thing?

    And one last thing, Do the negative electrons INSIDE the wire actually go THRU the battery when a circuit is completed?


    Thanks
     
  6. Feb 23, 2009 #5

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Direction of "current" flow is an arbitrary convention defined by the author of the book you happen to be reading. Electrons always flow from negative to positive. However it is perfectly valid to discuss a positive flow of current as if there were a flow from + to -. The results are the same as long as you are consistent.
     
  7. Feb 24, 2009 #6
    Integral is correct. The flow of current is really definable to be either from positive to negative or from negative to positive and all will be well as long as you are consistent. However, at least in my 4 years as an EE i have never used a book that defined the flow of current any other way than from positive to negative. So it's probably best to go with that :)

    I was told that the military teaches the opposite approach though... but really don't let it confuse you it's just convention.
     
  8. Feb 24, 2009 #7
    Can I assume the -ve and +ve side chemicals should be separated by insulated walls? If the water is pure, is there need to separate it?

    Also, there will be an electric current even if the + side is unconnected, as long as we create a path for the electrons to discharge, right?
     
  9. Feb 24, 2009 #8
    Did not understand the double voltage concept. There must be something incorrect in my understanding. Cannot figure out what.

    Zn1- Zn side of first battery
    Mn1 - Mn side of first battery
    ///ly, for the second battery.

    Let the connection be as follows:
    Zn1: Mn1 - Zn2: Mn2
    Zn1 is externally connected to Mn2 through a wire.

    Zn1 created 2e- and it reaches Mn2 through the wire.
    Mn2 takes the 2e- and creates 2OH-.
    Mn2 passes the 2OH- to Zn2 internally.
    Zn2 takes the 2OH- and creates 2e- and gives to Mn1.
    Mn1 takes 2e-, creates 2OH-, passes to Zn1 internally.

    How did this increase the voltage?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What is Voltage?
  1. What is voltage? (Replies: 32)

  2. What is voltage drop? (Replies: 29)

  3. What is Voltage (Replies: 42)

Loading...