1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Why do wires melt?

  1. Feb 27, 2008 #1
    Hi all,

    I have been set an experiment of relating to the current needed to melt a wire (predominantly dealing with R = ρL/A). This sounds like a really dumb question (because it is :redface:) but why does the wire melt when too much current is passed through?

    Also, what is the correlation between the current that would melt a wire and the wire's resistance? And what the heck is the difference between resistance and resistivity (let me guess, resistivity is a standard measure, whereas resistance is length-dependent??)

    Any help would be great. Many thanks you guys, and hope your day is going good.
  2. jcsd
  3. Feb 27, 2008 #2


    User Avatar

    The wire melts due to generated heat (given by P = I^2/R and the heat capacity of the wire) which of course comes from resistance.

    Resistivity is just a sort of "resistance density" if you will. In classical physics you have a lot of these "density" type quantities like pressure, density (who'd have thought), etc. It's resistance per unit length, and nothing more - ohms per metre, Ω/m, Ωm^-1, etc, etc. Resistance is length dependant as you say.

    A little bit of dimensional analysis goes a long way here.
  4. Feb 27, 2008 #3
    Note that the unit of resistivity is Ohm metre Ωm and not Ω/m.
  5. Feb 27, 2008 #4

    Not exactly. :smile:

    If the resistivity of a wire 2m long and having area of cross-section [tex]1 cm^2[/tex] is 5 Ωm, then the resistance is not 5*2 Ω. Resistance will be [tex] \frac{5*2}{0.0001}[/tex]Ω
    Last edited: Feb 27, 2008
  6. Feb 27, 2008 #5
    Due to resistance there is a drop in potential. This potential energy is converted into heat energy and if the T reaches the melting point of the wire, it melts!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook