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

Why do resistors in series recieve the same current

  1. Apr 12, 2008 #1
    Helllo guys,

    I was wandering why was this the case. If there were 3 resistors (Let's say 100ohms,200ohms,300ohms) in this seriec how can they recieve the same current if resistance is inversly proportional to current. Shouldn't the 300 ohms one recieve the least current. I thought resistors decrease the current what do they really do. Any help would be appreciated. Thanks!! :smile: :smile:
     
  2. jcsd
  3. Apr 12, 2008 #2

    Nabeshin

    User Avatar
    Science Advisor

    A simple explanation I can think of is an analogy. Current is somewhat like water in a river. If the water ran faster at one point and slower at another, the river would not be a stable system! When the wire branches, however, the current can distribute itself according to the resistances of the branches and determine their specific currents.

    Not sure if that helps or if it's what you wanted to hear, but that's how I see the issue.

    EDIT: Upon thinking of my analogy I realized a potential problem. Water does flow faster in some areas of a river than others. This is because the depth changes, but the volume of water flowing past a point remains constant.
     
    Last edited: Apr 12, 2008
  4. Apr 12, 2008 #3
    Good analogy. I like it:smile:. So what do the resisotors really do then. So they don't decrease the current.
     
  5. Apr 12, 2008 #4

    Nabeshin

    User Avatar
    Science Advisor

    I think resistors sole purpose is to resist current flow, thereby dissipating energy, causing a voltage drop.
     
  6. Apr 12, 2008 #5
    Well then with this formula I=V/R doesn't that mean current is inversly proportional to resistance. V=IR. Higher the resistance higher the voltage.
     
  7. Apr 12, 2008 #6

    Nabeshin

    User Avatar
    Science Advisor

    What you're saying is correct, I just think you might be interpreting the equations wrong. Yes, current is inversely proportional to resistance. That is, a resistor with higher resistance will receive less current than one with less resistance. Also, the higher the resistance the higher the voltage drop across a given resistor in a circuit.
     
  8. Apr 13, 2008 #7
    The problem here is that you're defining a series circuit of three resistors and then proceeding to analyze three circuits of one resistor each. The series circuit is really about cumulative voltage drops and not the division of current while a parallel circuit is about a division of current and not about cumulative voltage drops. The series circuit gives you one current and three voltages through and across each of the resistors while the parallel circuit gives you three currents and one voltage through and across each resistor. In the series case you have 600 ohms of resistance that the source sees and in the parallel case you have 54.5 ohms of equivalent resistance that the source sees. In the parallel case, of the three resistors, the 300 ohm resistor would see the lowest current. If you were using a 12 volt source the series circuit would draw 20 ma and the parallel circuit would draw 220 ma. As to what the resistor actually does, it dissipates power P=E(I) or P=I^2(R) according to the characteristic of Ohm's Law.

    Chris
     
  9. Apr 15, 2008 #8
    WOW!!:smile:. This is one of the best answers I have seen in this forum. Thanks a lot I really understand your explanation. Yeah you were right I have being thinking about the series current as three separate circuits. The power thing is where I'm bit unsure. Isn't power the rate of workdone. Why is the resistor affecting the rate of workdone rather than work(energy). Great answer anyways. Thanks again :smile: :smile: :smile: :smile: :smile:
     
    Last edited: Apr 15, 2008
  10. Apr 15, 2008 #9
    Looking at it again I should have said "Dissipates energy..." with dissipation indicating the time factor of power.

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

Have something to add?