Electrostatics by 93WVi17

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									 Ch 21 Electrostatics

The study of the properties of
      stationary charges
             Properties of Charge
• Fundamental Property of Matter
• Two types of charge
     positive
     negative
• Like charges repel
• Unlike charges attract
• Charge is quantized (distinct fundamental units)
     e = 1.602  10-19 C
     Coulomb (C) is the SI unit of charge
• Law of Conservation of Charge: The total charge
  of any isolated system is a constant
          Charge Conservation
238
 92
      U  Th  He
               234
                90
                                     4
                                     2
There are 92 protons on either side of this
nuclear reaction equation.
Charge Conservation Particle
Physics


   e e            


The decay of a neutral photon
into and electron and a
positron (positively charge
electron) in GREEN !!!
Method of generating static charge
• Process of
  inducing a
  positive charge
  on a conducting
  sphere.
• Contact or
  induction on
  isolated
  (insulated)
  conductors
           Material Behaviors
• Insulator - large effort required to move
  charges through material
   – Examples: wood, plastic, rubber, glass, dry air,
     vacuum
• Conductor - very little effort required to move
  charges through material
   – Examples: most metals, ionic solutions, plasmas
• Semi-conductor - intermediate effort required
  to move charges through material
   – Examples: germanium, silicon
             Charge and Force




Insulators        Conductor and Inuslator
  Coulomb’s Law (Force Law
      between charges)
• Charles Auguste Coulomb - 1785
• Force between two point charges is
  proportional to each charge
• Force between two point charges is
  inversely proportional to the distance
  separating them
• Force between charges obey the Law of
  Superposition (vector addition)
   Coulomb’s Law (point Charges)
          q1 q2
     F k     2
                ˆ
                r
            r
               9        2    2
k  8.99  10 N  m / C
                                   like charges
               1
          k
             4  0
 0  8.85  10 12 C2 / N  m 2    unlike charges
                   Example 1
A positive 1.0 mC charge is located at the origin and a
-0.3 mC charge is located at x = 2.0 cm. What is the
force on the negative charge?


           q1 q2
      F k     2                    1.0 mC        -0.3 mC
             r                      0.0 cm        2.0 cm
                                   6               6
             N  m 1.0 10 C 0.3 10 C
                        2
F  8.99 10   9

                           0.02m 
                2                   2
              C
                                
     F  6.74N                            
                                F  6.74Ni
                        Example 2
A positive 0.1 mC charge is located at the origin,
 a +0.2 mC charge is located at (0.0 cm, 1.5 cm),
 and a -0.2 mC charge is located at (1.0 cm, 0.0
 cm). What is the force on the negative charge?
 3   0.2 mC
     (0.0 cm, 1.5 cm)
                                           q1 q2
                                      F k     2
                                                 ˆ
                                                 r
     1     a       2
                                             r
     0.1 mC        -0.2 mC
     (0 cm. 0cm)   (1.0 cm, 0.0 cm)
Determine the force between the negative charge
and each positive charge.
3   0.2 mC
    (0.0 cm, 1.5 cm)
                                                     6          6
                                       N  m 2 0.110 C 0.2 10 C ˆ
                       F21  8.99 109                              i
                                                      0.01m 
                                          2                    2
                                        C

    1             2                       
                                                     
                                          F21  180Ni
                                                  .
    0.1 mC        -0.2 mC
    (0 cm. 0cm)   (1.0 cm, 0.0 cm)

The force between charges 1 and 2.
                  q1 q2
        F21  k             ˆ
                            i
                        2
                      r21
         3   0.2 mC
             (0.0 cm, 1.5 cm)




             1     a           2
             0.1 mC            -0.2 mC
             (0 cm. 0cm)       (1.0 cm, 0.0 cm)

      The force between charges 3 and 2.
                 9 N m
                        0.2 106 C 0.2 10 6 C
                           2
F23    8.99 10
                          0.01m    0.015m 
                                  2             2
                    C2
                   
                   F23  111N
                          .
                 3   0.2 mC
                     (0.0 cm, 1.5 cm)




                     1       a     2
                     0.1 mC        -0.2 mC
                     (0 cm. 0cm)   (1.0 cm, 0.0 cm)
              Determine the components of each force.
               
                                
               F23   F23 cos ai  F23 sin a
                                             j
                        0.01m                        0.015m
F23  111N
        .                                .
                                        i  111N               
                                                               j
               0.01m2  0.015m2   0.01m2  0.015m2
                   
                                    
                  F23  0.62Ni  0.92Nj
       3    0.2 mC
            (0.0 cm, 1.5 cm)




            1     a       2
           0.1 mC        -0.2 mC
           (0 cm. 0cm)   (1.0 cm, 0.0 cm)
Determine the sum of the components of the forces.
    
                              
    F  1.80Ni  0.62Ni  0.92Nj
           Determine the resultant force
           F   2.42 N i  0.92 N
                                 j
           
           F  2.59 N @ 159 o from +x axis
Sample prob 21-3

          a) What is force between
             spheres


          b) Ground sphere A, then
             what is the force ?
                  Field
Region of space influenced by some physical
 phenomena; e.g., temperature, gravity,
 electric, magnetic behaviors
              Electric Field
• Region of space where another charge would
  be influenced by a charge or distribution of
  charges
• Units of electric field is Newtons/Coulomb
  (N/C)
• Electric Field points away from positive charge
  and toward negative charge
                            
                          F
                 E  lim
                     q0 0 q
                            0
          Field of a point charge
           
         F
E  lim
    q0 0 q
           0
              1    qq0
                         ˆ
                         r
          4 0 r    2
E  lim
    q0 0        q0
            1 q
E  lim                ˆ
                       r
    q0  0 4      2
                0 r
      1 q
E              rˆ
    4 0 r   2
 Effect of a finite test
charge on a conductor

								
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