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Definition of electric current

  1. Apr 13, 2010 #1
    I'm confused by the definition of electric current. It is commonly defined as the time rate of change of charge (I=dQ/dt). However, considering a wire with a steady flow of electrons (a constant, non-zero current), wouldn't the number of charges entering a certain section of the wire equal the number of charges exiting from the other side of the section, such that dQ/dt=0?
  2. jcsd
  3. Apr 13, 2010 #2
    Suppose a general watching the troops saw 10 soldiers per second marching past.

    This is and example of steady flow.

    So he tells them to speed up and now 100 soldiers per second march past.

    Again this is steady flow.

    Flow of charge(s) is just like this.
  4. Apr 13, 2010 #3
    Studiot is correct. Current is defined by the movement of charge. You are computing a difference which is more like the divergence of charge.
  5. Apr 13, 2010 #4


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    Just to expand a bit on the answers others have given, what you described in your post would be a current density (i.e. current per unit area or volume), as opposed to the current. Your analysis is correct in that the current density in your example is not changing, however the current is clearly non-zero, as you yourself stated in the example.
  6. Apr 14, 2010 #5
    see it is amount of charge passing through the given cross section in unit time . so if 10 C charge pases in i sec. then I= 10 A
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