Updated:  9/7/04
                  
           Chapter 18 Electric Currents

          A. Instruction to students:
                    Homework:
                          1) Read carefully all sections except Sections 18-5 and 18-9.
                          2) Do all examples in the text, except 18-4, -5, -6, -13 and -14.
                           
                    Following formula and constants from Chapters 18 will be given
                    during the exams. Practice how to use and apply them.  Note the units!
                                         I = Q/t
                                         P = I V
                    OHM's law is NOT given in the formula sheet.  You are NOT supposed
                    to memorize it, but to use it in Chapters 18 and 19, enough to remember it.
                                   
                                       
          B. Biomedical (and technological) application:
                     The nervous system and nerv condition, Section 18-10,   pp546-548.

 
          C. Lectures and study guideline
 
          Electric current (pp530-532)
          An electric current is a flow of charged particles, usually of electrons.  The electrons
          mobile in the wire get pushed out from one end of the wire, as more of them are pushed
          in from the other end, due to the elctric potential difference between the two ends.
          Question: In this description of a currect, which end is the positive side?
          The definition of a current:
                                  I = Q/t;  1 A(mpere) =  1 C/s.
          (Example 18-1and -2.)

          Ohm's law (pp532-534)
          Memorize
                                   V = IR, 1 V = 1 A Ohm 
          and practice its use, because most of the materials in Chapters 18 and 19 are applications of
          Ohm's law!  V = IR will not be given during the exams.      
          The resistance R is unique (constant) for a given setup, being independent of each of V and I
          and depending only on the ratio of the two.
          (Example 18-3)
         
           Electric power (pp538-541)
                                    P = energy transformed/time = QV/t = IV,
                                    1 W(att) = 1 J/s = 1 A V.
           Using Ohm's law, we also have
                                     P = R I^2 = V^2/R.
           (Example 18-7, -8, and -9.)
                           
           Alternating current or AC (pp541-544)
           In contrast to a direct current (DC), an AC periodically changes the direction of the flow  
           (such as 50 times in a second, 50 Hz..)   In the US, the standard AC voltage is 120 V.  
           This is an effective (rms) voltage, for which the preceding formula (V = IR and P = IV etc.)
           for a DC are directly applicable.  The peak voltage of an AC is the effective voltage times
           sqrt(2).   The same goes with an AC current.
           (Example 18-11 and -12.)                

            WARNING (again)
            We now have several new units to deal with, which are related to each other.  As noted
            above, you can find the relations among the units by examining how the quantities
            themselves are related to each other.  It is thus vital to be able to use and apply various
            formula, as noted in A-3) above.