9/27/2005 Charge and Current.doc 1/3 Jim Stiles The Univ. of Kansas Dept. of EECS Charge and Current Say we have a conductor (e.g., wire) with I =1 Ampere of current flowing through it. Q: What does this mean, physically ? A: Current I simply describes the rate at which net charge passes through the wire cross-sectional surface S. For example, if a net charge Q∆ moves across surface S in some small amount of time t∆ , we find that: 0tQdQIlimtdt∆→∆==∆ Thus, we find that 1 Amp means +1.0 Coulomb of net charge passes by a location on the wire each second, with the net charge in this case flowing from left to right. 1 Amp wire S9/27/2005 Charge and Current.doc 2/3 Jim Stiles The Univ. of Kansas Dept. of EECS Q: The current is positive, does this mean that the current is made up of positive charge? A: No! Current generally consists of both positively and negatively charged particles. Remember, current is the net change in charge with respect to time. For example, say positive charges are moving from left to right through the wire: The current due to these charges is positive, as the total net charge on the right side of the surface is increasing with time. That was pretty obvious, but here’s the tricky part: say negative charges are moving from right to left through the wire (the opposite direction of that above). S wire + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + I S wire - - - - - - - - - -- - - - - - - - - -- - - - - - - - - -- - - - - - - - - -I9/27/2005 Charge and Current.doc 3/3 Jim Stiles The Univ. of Kansas Dept. of EECS Note in this case, the total charge on the right side of S is again increasing ! * With the first case, the net charge was increasing because positive charges were entering the right side. For this case, the net charge on the right side is also increasing, but because negative charge is leaving the right side ! * For reasons we shall learn about later, if positive charge moves one direction, then negative charge will generally move in the opposite direction. Therefore, total current is composed of charges moving in both directions: II I+−=+ * Generally speaking, it does not matter (in fact we generally cannot tell) whether the particles that form a specific current are negative or positive—all that matters is the net change in charge across a
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