Direction of current density

Discussion in 'General Electronics Chat' started by logearav, Oct 21, 2011.

  1. logearav

    Thread Starter Member

    Aug 19, 2011
    248
    0
    In the link given below, i found a paragraph, which i had given below the link

    http://electron9.phys.utk.edu/phys136d/modules/m6/current.htm

    The current density is a vector. It represents the amount of net charge that crosses a unit area perpendicular to the flow per second. If just as many negative as positive charges move across a unit area in the same direction per second, then the current density is zero. If <v-> = 0 and only positive charges are moving, then the direction of the current density is the direction of the velocity of the positive charges. In ordinary conductors the positive charges have zero average velocity and only the negative charges are moving. Then the direction of the current density is opposite to the direction of the velocity of the negative charges, as shown in the diagram on the right.
    <v-> ---> average velocity of negative charges.

    Revered members,
    Why direction of current density is opposite to the direction of the velocity of negative charges when <v+> = 0?
    When <v-> = 0, the direction of current density is along the velocity of positive charges, similarly the direction of current density should be along velocity of negative charges, when
    <v+> = 0
     
  2. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    The "direction of the current density is opposite to the direction of the negative charges" because the direction of the current density is NOT opposite to the direction of the positive charges.

    If you have to declare that current density corresponds to one charge or the other, then one charge must be the direction the density changes and the other charge must be the other direction.
     
  3. logearav

    Thread Starter Member

    Aug 19, 2011
    248
    0
    I cant understand sir
     
  4. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    This is all a matter of definitions. Try to remember that this is a man made construct.

    Where n is the number of particles and q is the amount (or quality) of charge per particle, p is the product of number of charges times quality of charge. That is the amount of charge that exists in a certain, arbitrary place.

    If this charge moves, there is velocity (v). j (current density) is the product of velocity times number times quality, or velocity times amount of charge.

    Now, declare that there are positively charged particles and negatively charged particles such that either can move.

    As soon as it is declared that the direction of the current density shall be called positive when the positive particles move (and the negative particles don't move) then it must follow that when the negative particles move (and the positive particles don't move), the direction of the current density must be called negative.

    The purpose of all this declaring is to establish an understanding of what a positive sign and a negative sign in an equasion mean. Now you can do math.

    It is also declared that the positive charge carriers (particles that have a positive charge) do not move in an ordinary conductor (wire). When you get done with doing equasions, the sign of the answer will tell you which way the negative charge carriers are moving.

    The negative charge carriers are commonly called, "electrons". An "ordinary conductor" can be a wire. When the electrons are flowing from left to right, the direction of current density is defined as being from right to left.

    It is all a bunch of definitions that are simply declared by humans for the purpose of understanding each other. When you do your math according to these definitions, another person will understand what you mean.
     
    logearav likes this.
  5. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Good answer #12.

    We could make a simplified analogy with a cash register. Money going in the register is positive and money going out is negative. Now imagine that your friend lets you borrow 10 dollars and you decide to put the $10 note into the cash register. When you put the money in, you might also decide to put a hand written note (called an "I owe you" or IOU http://en.wikipedia.org/wiki/IOU) into the register that says you owe your friend 10 dollars. This IOU is like negative charge because it has opposite sign. An IOU going into the cash register subtracts the amount from the total, unlike money which adds to the total. The end result of putting in both the $10 dollar note and the IOU note is zero. This is similar to what the article describes. If a positive charge and negative charge are going in the same direction, they cancel out and there is no current density at that point.

    A word of caution to the OP. Current density is different than current. Make sure you understand that current density is something that exists at a point and it is more complicated than a simple number because it has directional qualities. The article describes the current density as "the amount of net charge that crosses a unit area perpendicular to the flow per second", but this description might make one think that is is over a wide unit area. It is really a mathematical limit of the flow per unit area as the area gets smaller and smaller until it is a like a point (but not really exactly like a point because the tiny area still has a direction).
     
    logearav likes this.
Loading...