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					                           II. Electric current
  1. Definition
                                                    Conventional        Electron
                                                    current             flow
                  Units: [ I ] = 1A = 1 C/s


Example: 1020 electrons passed through the electric conductor during 4 seconds.
Find the electric current through this conductor.




Example: The electric current of 0.5 A is flowing through the electric conductor.
a) What electric charge is passing through the conductor during each second?
b) What electric charge will pass through the conductor during 1 minute?

            a)

            b)
2. Ohm’s Law                                    Nonohmic device

   I                                      I



                V                                       V




3. Resistance

 Definition:        Units: [ R ] = 1Ω = 1 V/A      Ohm’s Law:
 4. Resistivity
                                                     Definition:
      I            A
          L


Example: What is the resistance of 1 m of nichrome wire of 2 mm diameter ?




Temperature dependence of resistivity




                                                                             T
Example: Two cylindrical resistors, R1 and R2, are made of identical material.
R2 has twice the length of R1 but half the radius of R1. They are connected to a
battery V as shown. Compare the currents flowing through R1 and through R2.

       A. I1 < I2               B. I1 = I2               C. I1 > I2

                                                                I1       I2
                                                V
5a. Resistors in series
Two resistors R1 and R2 are in series when they are connected
one after the other


I       R1                R2                              I     Req

        V1                V2
                 V                                              V
5b. Resistors in parallel
Two resistors R1 and R2 are in parallel when they are connected to the
same potential difference


         I1   R1                                            I    Req
  I                         I
         I2   R2                                                  V
Example: A 14-A current flows into a series combination of a 3.0-Ω and a 4.0-Ω
resistors. What is the voltage drop across the 4.0-Ω resistor?
    A) 38 V;     B) 42 V;           C) 56 V ;       D) 98 V.




Example: A 22-A current flows into a parallel combination of 4.0 Ω, 6.0 Ω, and
12 Ω resistors. What current flows through the 12-Ω resistor?
A) 18 A;       B) 11 A;           C) 7.3 A;       D) 3.7 A.
      Symbols for circuit elements

          Conductor (wire) with negligible resistance


           Resistors

- +
          Source of emf (for example, a battery)



           Switch

           Ground


           Voltmeter (should be connected in parallel)

           Ammeter (should be connected in series)
6a. Ammeter (used to measure current)
                                                                        G
           A                                         A        =
                                                                        r
                                             r is small shunt resistance or shunt
                                             G is galvanometer

• Ammeter should be connected in series
• Ammeter has very small (ideally zero) internal resistance,
  so it does not affect the current to be measured


6b. Voltmeter (used to measure voltage or potential difference)
                                                                            R
                    A                            V                  G
                                                          =
                             V
                    B                                             R is large

 •   Voltmeter should be connected in parallel
 •   Voltmeter has very large (ideally infinite) internal resistance,
     so that very little current is diverted through it.
6. Voltmeter and Ammeter
•   Voltmeter should be connected in parallel
•   Ammeter should be connected in series
•   Voltmeter has very large (ideally infinite) internal resistance
•   Ammeter has very small (ideally zero) internal resistance


                              Example: What connection is not correct?




                              A




                                                                      C
                                  B

				
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