17%20Electric%20Charge%20

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17 Electric Charge Electric Filed
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Nature of Electric Charge
Two types of Charges Positive and negative
charges Like charges repel each other
(force?) Unlike charge attract each other (force?)
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Physical Basis of Electric Charge
All the atoms are electrically neutral (ions,
ionization oxidization) Same number of
protons electrons Different number of
neutrons Isotopes
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Conductors Insulators
If charges can move freely in a
materialsconductor If charges cant move in a
materialsinsulator Semiconductor, Limits, and
Heat transfer
Metals are good conductors What about water?
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Induction Polarization
The displacement of charge in an isolated
conductor when placed near by an electrically
charged body Separation of positive and negative
charges
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Polarization
Uncharged insulator
the polarized insulator
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Conservation Quantization of Charge
Conservation Total electric charges in any
closed system is a constant. Charge can be
transferred from on object to another. Quantizati
on minimum amount of charge e, the basic unit,
the charge of the electron or proton. Any Q
integer x e e 1.602x10-19 C or 1 C 6x1018
protons() or electrons(-)! mC, mC, nC
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Coulombs Law
k 8.99 109 N m2/C2 Action Reaction
force Units, Sign, and Direction Superposition
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Electric Field Electric Forces
Definition of Electric Field
Unit N/C Direction ?
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Calculating Electric Field
Principle of Superposition e.g., point charge and
spherical charge distribution
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Electric Field Lines Physical Meaning
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Electric Field LinesExamples
Beginning with positive charge and ending at
negative or infinity
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Electric Field LinesExamples

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Gausss Law Field Calculation
Electric Flux Example point charge
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Charges on Conductors
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Summary Charge, Conductors Insulators
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Summary Coulombs Law
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Summary Electric Field and Electric Forces
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Summary Electric Filed Lines
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Summary Gausss Law
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Summary Charges on Conductors
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Problem-Solving Strategy  Coulombs Law

• SET UP
• As always, consistent units are essential. With
  the value of k given earlier, distances must be
  in meters, charges in coulombs, and forces in
  newtons. If you are given distances in
  centimeters, inches, or furlongs, dont forget to
  convert! When a charge is given in microcoulombs,
  remember that 1 micro C 1 mC 10-6 C
• SOLVE
• When the forces acting on a charge are caused by
  two or more other charges, the total force on the
  charge is the vector sum of the individual
  forces. If youre not sure you remember how to do
  vector addition, you may want to review Sections
  1.7 and 1.8. Its often useful to use components
  in an x-y coordinate system. As always, its
  essential to distinguish between vectors, their
  magnitudes, and their components (using correct
  notation!) and to treat vectors properly as such.
• Some situations involve a continuous distribution
  of charge along a line or over a surface. In this
  book, well consider only situations for which
  the vector sum described in Step 2 can be
  evaluated by using vector addition and symmetry
  considerations. In other cases, methods of
  integral calculus would be needed.
• REFLECT
• 4. Try to think of particular cases where you
  can guess what the result should be, and compare
  your intuitive expectations with the results of
  your calculations.

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Problem-Solving Strategy Electric Field
Calculations

• SET UP
• Be sure to use a consistent set of units.
  Distances must be in meters, charges in coulombs.
  If you are given cm or nC, dont forget to
  convert.
• Usually, you will use components to compute
  vector sums. As we suggested for problems
  involving Coulombs law, it may be helpful to
  review Sections 1.7 and 1.8. Use proper vector
  notation distinguish carefully between scalars,
  vectors, and components of vectors. Indicate your
  coordinate axes clearly on your diagram, and be
  certain that the components are consistent with
  your choice of axes.
• SOLVE
• 3. In working out directions of vectors, be
  careful to distinguish between the source point S
  and the field point P. The field produced by a
  positive point charge always points in the
  direction from source point to field point the
  opposite is true for a negative point charge.
• REFLECT
• 4. If your result is a symbolic expression, check
  to see whether it depends on the variables in the
  way you expect. If it is numeric, estimate what
  you expect the result to be and check for
  consistency with the result of your calculations

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If you charge a metal ball on an insulating
stand/rod by induction, which of the following
happens?
A. The charge on the ball changes while the
charge on the rod stays the same. B. The
charge on the rod changes, while the charge on
the ball stays the same. C. The charge on both
the rod and the ball changes. D. The charge does
not change on either the rod or the ball.
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If you charge a metal ball on an insulating stand
by induction, which of the following happens?
A. The charge on the ball changes while the
charge on the rod stays the same.
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A small object with a charge of magnitude q
creates an electric field. At a point 1.0 m to
the north of the charge, the field has a value of
2.0 N/C directed south. At a point 0.5 m to the
west of the charge the field has a value of
A. 4.0 N/C directed east B. 4.0 N/C directed
west C. 8.0 N/C directed east D. 8.0 N/C directed
west
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A small object with a charge of magnitude q
creates an electric field. At a point 1.0 m to
the north of the charge, the field has a value of
2.0 N/C directed south. At a point 0.5 m to the
west of the charge the field has a value of
C. 8.0 N/C directed east
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Example 17.2 Gravity Electric Force
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Problem-Solving
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Homework
Ch17 1, 3, 5, 7, 14, 20, 31, 34, 40, 50.