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

Current Flow Through Circuit

  1. Dec 13, 2014 #1
    I have a simple series circuit, with a 9V power supply, and a 1 ohm resistor. If I place to identical fuses in series on opposite sides of the resistor that are rated for <9A, which fuse will blow first?

    I am essentially asking if current flows identically through all points in a wire the instant the circuit is closed, or if there is an instant at "the speed of light (electrons)" where current flows only in the high electron side of the wire.

    Basically I'm thinking (if I revert to the water metaphor), there is a hose and I can either open the supply to with it empty hose that needs to have water passed through it, or I canopen it with an already filled hose which will instantly start passing water though the free end.

    Hopefully this isn't too vague..
     
  2. jcsd
  3. Dec 13, 2014 #2
    I have two "identical" ice creams. If I take them out of a fridge at same time, which one totally melts first?
     
  4. Dec 13, 2014 #3
    I too can answer questions with metaphors that have absolutely no evidence... provide any?
     
  5. Dec 13, 2014 #4
    If same current passes through both "identical" fuses, they should melt at same instant of time (in theory). However, in the real world you'll never have two identical fuses placed in identical enviroment.
     
  6. Dec 13, 2014 #5

    Stephen Tashi

    User Avatar
    Science Advisor

    That question is clearer than the question about the resistors! It's not clear how much you reality that you want to allow in the question about the resistors.

    My answer is: yes, the change in the electric field propagates through the wire at a finite speed. This finite speed is not necessarily the speed of light in a vacuum. Viewing the electrons as classical particles, they have a mass. They don't instantly accelerate when acted upon by a force, so it isn't the electrons themselves that move at the speed of light. There are still a lot of undefined aspects to the above question, so I'm sure other people might not agree.
     
  7. Dec 13, 2014 #6

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    The water analogy can be useful but one must be rigorous.

    Your "full" hose cannot transmit pressure instantaneously to its other end because it is elastic and will swell slightly under the higher pressure. So you must add water to the hose to start the pressure rise propagating down the hose to accelerate the water.
    But you'd be hard pressed to measure that delay with a stopwatch.
    That compliance in your garden hose is analogous to capacitance. Inertia of the water is analogous to inductance. Interestingly c is a function of capacitive and inductive related parameters permittivity εο and permeability μο .
    http://en.wikipedia.org/wiki/Vacuum_permittivity

    Here's a link that'll make you scratch your head:
    http://amasci.com/tesla/tmistk.html


    have fun.
     
  8. Dec 14, 2014 #7

    davenn

    User Avatar
    Science Advisor
    Gold Member

    electrons DONT travel at the speed of light, google electron drift :)

    Dave
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook