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

Homework Help: Can someone help explain the volt to me?

  1. Mar 12, 2006 #1
    I was sick and missed a week of school. It was the week we started electricity in physics, so I'm kinda confused about exactly what all the vocabulary means.

    Here's how I understand it so far:

    Charge: Literally, the amount of electrons an object has to transfer.

    Coulomb: A measure of charge.

    Current: The strength with which electrons transfer from one object to another. I suppose that if you have 2 objects with the same charge, and you allowed 1 charge to flow through a gold wire, and one charge to flow through an iron wire, then the current through the gold wire would be greater, since gold can lose electrons easier. Is that accurate?

    Amp: A measure of current.

    Watt: Electrical work.

    OHM: A measure of resistors, showing how many volts travel through the resistor per amount of current used.

    Volt: I'm lost. In terms of electrons, I really don't know what a volt is. I know how to use volts in formulas to figure out questions, but I don't actually understand what it is. The term "Potential Difference" makes no sense to me. The formula V = W/q doesn't make any sense to me either. W is watts, or work, and q is coulombs, or charge. So that says Volts are just how much work is done per amount of electrons you can transfer. I can't understand in what circumstance a 1 coulomb of charge would do more or less work than in any other circumstance.

    Thanks for any help,
    Last edited: Mar 12, 2006
  2. jcsd
  3. Mar 12, 2006 #2
    Firstly I don't really like your definition of charge. My physics teacher explained it really vaguely as a property of some kinds of matter. Secondly the equation you have is wrong it should be U = Vq, U is electric potential energy, V is electric potential and q is charge. U should be measure in joules and is a measure of work in a way but is not a measure of power as it is used in that equation. I explained generally this same concept in this other post https://www.physicsforums.com/showthread.php?t=113867" so maybe that will haelp you to understand a little better, try to come up with analogiies between electricity and gravity, it really helps if you have a good understanding of the concepts of force and field.
    Last edited by a moderator: Apr 22, 2017
  4. Mar 12, 2006 #3


    User Avatar
    Gold Member

    For one, W = work, not power.

    Your definition of charge is also incorrect. Charge is an intrinsict (sp?) property of matter. Something can be positively charged or negatively charged. Something positively charged has an excess of protons and something negatively charged has an excess of electrons when we're dealing with many atoms. W=qV is Work = charge * potential. What that means is that a certain amount of work will be done moving a charge across a certain potential. So if you have, say, a 110V potential like in a common household appliance, it takes (110 V * elementary charge) joules to move that electron across the potential... or in the appliances case, that work will be moved into the system of the appliance be it the motor or circuits or whatever.

    I guess you can think of it like a pulley system. You build up a potential by moving a mass, m, up some height, h. So you have say, 100 feet of height. Well, you know that the potential energy stored up is 100 * m * g. Once you release the mass, it'll fall down and all the energy will be expelled into whatever is on the other side of that pulley (maybe a generator or even another mass, whatever the system may be the work goes into that system). In electricity, the potential difference, your voltage, is just like the height in the gravity system. Let's say we have a bunch of electrons. Now let's say the electrons are allowed to move freely. Well they want to rush towards the positive charges just like your 10kg mass wants to rush towards the ground. When they are allowed to rush towards the positive charge, it's going to expell it's energy just like the mass falling to the ground expells energy into kinetic energy. Now the q comes in just like the m comes in. If you have more mass, you have more energy, thus more work needed to move it to some height, h. If you have a larger charge, q, it will require more energy to move it across that potential, V.

    I think that's an accurate way of saying it haha.
    Last edited: Mar 12, 2006
  5. Mar 12, 2006 #4
    Before I could begin to understand that, I would need to understand what you mean by Electrical Potential Energy.

    In terms of V, that would be V=U/q, so in your equation, U would just be work, which is the exact equation I was taught for voltage, V=W/q.
    Last edited by a moderator: Apr 22, 2017
  6. Mar 12, 2006 #5

    So Electrical Power = Watts per second then?
  7. Mar 12, 2006 #6
    No. Power is joules per second, and that unit is called a watt.
  8. Mar 12, 2006 #7


    User Avatar
    Gold Member

    Watts = Joules/second. Electrical power = Joules/second or Watts.
  9. Mar 12, 2006 #8

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

    OK, now all you higher level physicists don't jump down my throat here, because this is a useful analogy for voltage (aka electric potential). I know it is flawed, but all analogies are:

    This will also help you with capacitance when it comes up:

    Think of two boxes, one small like a milk crate, one large like the size of an airconditioner. Take 100 tennis balls and pack them into each box.

    It will take some extra force to pack all those balls into each box, right? But each box now has 100 tennis balls in it. What's the difference? The pressure inside each box is different. If one ball was to escape from either box, think of the speed with which it would eject. Which box would it eject faster? (The one under higher pressure, right?).

    OK, each ball is a coulomb of charge, the pressure is electric potential (or voltage). The potential also tells you the amount of kenetic energy each coulomb of charge will have if it breaks free.

    The size of the box is the capacitance.
  10. Mar 12, 2006 #9


    User Avatar
    Homework Helper

    Current is electric charge through an area, analogous to fluid mass flow rate:
    how much charge per second is going past some point. think highway traffic.

    A tall hill on a roller-coaster implies fast car at the bottom, regardless of mass.
    Electric Potential decribes the "environment" aspect of PE ... useful when the "landscape" doesn't change (as when topographic map is useful).
    Voltage is the DIFFERENCE of Electric Potential at two places Delta V ...
    Difference of Gravitational Potential is (gh_bottom - gh_top) => Delta KE/m.
  11. Mar 12, 2006 #10
    What? Oh god. So were you just saying that W isn't what is used to represent watts? What letter means watts?
  12. Mar 12, 2006 #11
    W can mean two things, Work or Watts. It depends on the context.

    Watts, (W), has the units of Joules per second (J/s).

    Work, (W), has the units of Joules (J).

    Watts is really just work per unit time, it is the derivative of work.
  13. Mar 12, 2006 #12
    P for power maybe? In your equation W = Vq, W is work, I though we had already established this, and that is why my equation is probably better
    U = Vq where U is the electrical potential energy or work.
  14. Mar 12, 2006 #13


    User Avatar
    Gold Member

    that weird P is the symbol for Power which has a unit of W. It's like current has a symbol 'i' but has a unit of A for Amps. Don't get the symbol confused with the unit symbol.
  15. Mar 12, 2006 #14


    User Avatar
    Homework Helper

    Try not to confuse a UNIT [Watt] with a quantity (Power).
    I like to put Unit symbols inside [W] square brackets to identify them as such.

    d_leet unfortunately has equated a process quantity (Work)
    with a condition "state" quantity (Potential Energy). This cannot be...
    Work might be equal to the CHANGE of PE , so W = Delta(U) = q Delta(V).
    This reminds us that Work is computed as Force * Delta s,
    so that there is one "Delta" in each term.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook