Lecture 07

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Lecture 07 Powered By Docstoc
					PHY-2049


Current & Circuits
February ‘08
A closed circuit
Power in DC Circuit

In time t, a charge Q is pushed through
the resistor by the battery. The amount of work
done by the battery is :
W  VQ
Power :
W        Q
     V        VI
 t       t
Power  P  IV  I IR  I 2 R
                 E2
P  I 2 R  IV 
                 R
       The figure below gives the electrical potential V(x) along a copper wire carrying a
       uniform current, from a point at higher potential (x=0m) to a point at a lower
       potential (x=3m). The wire has a radius of   2.45 mm.    What is the current in the
       wire?


                                         What does the graph tell us??

                                                    *The length of the wire is 3 meters.
                                                    *The potential difference across the
                                                     wire is 12 m volts.
                                                    *The wire is uniform.


                                                 Let’s get rid of the mm radius and
                                                 convert it to area in square meters:
                                                 A=pr2 = 3.14159 x 2.452 x 10-6 m2
                                                 or
           copper                                A=1.9 x 10-5 m 2
12 volts                    0 volts
   Material is Copper so resistivity is (from table) = 1.69 x 10-8 ohm meters
     We have all we need….

                  8
     L 1.69 x10 ohm - m  3.0 m
R                     5
                                 2.67m
     A           1.9 x10
From Ohm's Law :
             6
  V  12 10 volts
i         3
                   4.49 ma
  R 2.67 10 ohms
Let’s add resistors …….
SERIES Resistors
               i                  i

  Series Combinations
                 R1               R2

                 V1               V2
                            V
         V1  iR1
         V2  iR2
         and
         V  V1  V2  iR  iR1  iR2
          R  R1  R2
          general :
          R( series )   Ri
                        i
The rod in the figure is made of two materials. The
figure is not drawn to scale. Each conductor has a
square cross section 3.00 mm on a side. The first
material has a resistivity of 4.00 × 10–3 Ω · m and is
25.0 cm long, while the second material has a
resistivity of 6.00 × 10–3 Ω · m and is 40.0 cm long.
What is the resistance between the ends of the rod?
Parallel Combination??

 R1, I1
              V  iR
                           V V V
              i  i1  i2     
                           R1 R2 R

 R2, I2
              so..
               1   1   1
                    
              R1 R2 R
              general
              1    1
                
   V          R  i Ri
                 In Fig. 28-39, find the
                 equivalent resistance
What’s This???   between points
                 (a) F and H and [2.5]
                 (b) F and G. [3.13]
   (a) Find the equivalent resistance
    between points a and b in Figure P28.6.
    (b) A potential difference of 34.0 V is
    applied between points a and b. Calculate
    the current in each resistor.
     Power Source in a Circuit




The ideal battery does work on charges moving
them (inside) from a lower potential to one that is
V higher.
            A REAL Power Source
            is NOT an ideal battery



Internal Resistance   V




  E or Emf is an idealized device that does an amount of
  work E to move a unit charge from one side to
  another.

        By the way …. this is called a circuit!
A Physical (Real) Battery


                     Emf
                  i
                     rR
Back to which is brighter?
Back to Potential


                 Change in potential as one circuits
                 this complete circuit is ZERO!




    Represents a charge in space
Consider a “circuit”.




 This trip around the circuit is the same as a path
 through space.

 THE CHANGE IN POTENTIAL FROM “a” AROUND
 THE CIRCUIT AND BACK TO “a” is ZERO!!
To remember
   In a real circuit, we can neglect the
    resistance of the wires compared to the
    resistors.
       We can therefore consider a wire in a circuit to
        be an equipotential – the change in
        potential over its length is slight
        compared to that in a resistor
   A resistor allows current to flow from a
    high potential to a lower potential.
   The energy needed to do this is supplied
    by the battery.
                          W  qV
NEW LAWS PASSED BY THIS SESSION
OF THE FLORIDUH LEGISLATURE.

   LOOP EQUATION
       The sum of the voltage drops (or rises)
        as one completely travels through a
        circuit loop is zero.
       Sometimes known as Kirchoff’s loop
        equation.
   NODE EQUATION
       The sum of the currents entering (or
        leaving) a node in a circuit is ZERO
TWO resistors again
                     i



          R1               R2
          V1               V2

                     V
V  iR  iR1  iR2
or
R  R1  R2
General for SERIES Resistors
R  Rj
      j
 A single “real” resistor can be modeled
 as follows:

                    R
     a                             b




 V




                               position

ADD ENOUGH RESISTORS, MAKING THEM SMALLER
AND YOU MODEL A CONTINUOUS VOLTAGE DROP.
     We start at a point in the circuit and travel
     around until we get back to where we
     started.

   If the potential rises … well it is a rise.
   If it falls it is a fall OR a negative rise.
   We can traverse the circuit adding each
    rise or drop in potential.
   The sum of all the rises around the loop
    is zero. A drop is a negative rise.
   The sum of all the drops around a circuit
    is zero. A rise is a negative drop.
   Your choice … rises or drops. But you
    must remain consistent.
Take a trip around this circuit.




 Consider voltage DROPS:
                  -E +ir +iR = 0
                         or
                     E=ir + iR
Circuit Reduction




                    i=E/Req
Multiple Batteries
Reduction




            Computes i
Another Reduction Example


                     1 1      1   50   1
                                  
                     R 20 30 600 12
                     R  12



               PARALLEL
   START by assuming a
DIRECTION for each Current




Let’s write the equations.
The Unthinkable ….
RC Circuit
                Initially, no current
                 through the circuit
                Close switch at (a)
                 and current begins to
                 flow until the
                 capacitor is fully
                 charged.
                If capacitor is charged
                 and switch is switched
                 to (b) discharge will
                 follow.
        Close the Switch




           I need to use E for E

Note RC = (Volts/Amp)(Coul/Volt)
       = Coul/(Coul/sec) = (1/sec)
        Really Close the Switch
                                     Loop Equation
                                                q
                                      E  iR   0
                                                C
                                               dq
                                     since i 
                                               dt
           I need to use E for E
                                       dq q
                                     R  E
Note RC = (Volts/Amp)(Coul/Volt)        dt C
       = Coul/(Coul/sec) = (1/sec)   or
                                     dq q   E
                                         
                                     dt RC R
    differential
This is a

equation.
   To solve we need what is called a
    particular solution as well as a
    general solution.
   We often do this by creative
    “guessing” and then matching the
    guess to reality.
   You may or may not have studied
    this topic … but you WILL!
General Solution
q  q p  Ke  at
Look at particular solution :
dq q       E
        
dt RC R
When the device is fully charged,dq/dt  0 and
q p  CE
When t  0, q  0 and from solution
0  CE  K
K  -CE
dq q          E
            and q  CE(1 - e -at )
 dt RC R
CE (ae at )  CE(1 - e -at )  E / R
for t  0
CEa  0  E/R
      E   1
a      
     RCE RC
Time Constant



         RC
Result q=CE(1-e-t/RC)
q=CE(1-e-t/RC) and i=(CE/RC) e-t/RC

  E t / RC
i e
  R
Discharging a Capacitor
 qinitial=CE BIG SURPRISE! (Q=CV)
              i
                   iR+q/C=0
                      dq q
                    R      0
                      dt C
                    solution
                   q  q0 e t / RC
                      dq    q0 t / RC
                   i        e
                      dt    RC
In Fig. (a), a R = 21, Ohm a resistor is connected to a battery. Figure (b) shows the increase
of thermal energy Eth in the resistor as a function of time t.
(a)What is the electric potential across the battery? (60)
(b) If the resistance is doubled, what is the POWER dissipated by the circuit? (39)
(c) Did you put your name on your paper? (1)

                                     Looking at the graph, we see that the
                                     resistor dissipates 0.5 mJ in one second.
                                     Therefore, the POWER =i2R=0.5 mW


             P 0.5 mW
        i2                2.38 10 5 amp 2
             R     21 Ω
        i  4.88 10 3 amp  4.88 ma      Voltage drop across the reisitor  iR or
                                            V  iR  4.8810-3 amp  21  102mV
If the resistance is doubled what is the
power dissipated by the circuit?



     R  42       V  102 mV
                      3
        V 102 10
     i               2.43ma
        R        42
     P  i R  0.248mJ
          2

				
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