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: I don't understand voltage

  1. Mar 10, 2005 #1
    I am trying to understand what exactly voltage is. I have been told that it is like the "pressure" of the electrons as they flow into the circuit.

    It makes sense to me that in a series circuit that voltage changes as it goes through a resistor. A resistor resists the flow of electrons, so it makes sense that voltage or "pressure" would be lower after passing through a resistor. The electrons have slowed down as they pass through the resistor so they wouldn't have as much pressure afterward.

    Now, my whole understanding of voltage falls apart when I look at parallel circiuts. How can electrons flow through resistors in a parallel circuit and not have a decrease in voltage? The resistors still slow down the electrons so why isn't there a change in voltage "pressure"? It seems like the same process is happening as in a series circuit, but there are 2 or more paths that the electrons can take.

    Please help me before my brain explodes! :confused:
  2. jcsd
  3. Mar 10, 2005 #2


    User Avatar
    Science Advisor

    In a parallel circuit, there is STILL a voltage drop, it just occurs on EACH of the separate parallel paths and is the SAME for each parallel path.

    With series resistors, voltage drops in stages when the current flows thru each separate resistor. In general, the voltage drop will be different across each separate resistor. However, with a series circuit, the current is the same in each resistor.

    For a parallel circuit, the same voltage is simultaneously presented to each parallel path. For a parallel circuit, voltage drop is the SAME for each separate parallel path. However, in general, the current thru each separate path will be different.

    So here's a summary:
    SERIES RESISTORS ---> Current the same thru each resistor; voltage drop (generally) different across each separate resistor.
    PARALLEL RESISTORS ---> Voltage drop the same across each separate parallel path; current (generally) different in each separate path.

  4. Mar 11, 2005 #3

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

    It appears that you do understand voltage. Add to the "pressure" analogy the fact that potential is the "energy per unit of charge" (a volt is a "joule per coulomb"). So at a high potential each charge has more energy.

    It is NOT good to think of this energy in the form of speed, since electrons are not "slowed down" by the resistor. Think of them as being more tightly packed together before the resistor and less tighly packed after the resistor.
  5. Mar 11, 2005 #4


    User Avatar
    Science Advisor

    Think of voltage as "height" (it's really a potential energy but "height" gives the same idea since potential energy is mgh). If you have water coming through a pipe down from a height, the speed of the water, and, so, the amount of water flowing through the pipe ("current") will depend upon the height. "Resistance" is the friction in the pipe that slows down the water. If you have several pipes (parallel paths) then you will have more water through those pipes (higher current) but the same "height" and so the same voltage.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook