SI System of units

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SI System of units
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METRIC PREFIXES

• To memorize

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METRIC PREFIX

• MEMORIZE THIS LIST

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Electric Charges

• 2 kinds of charge and
• Like charges repel
• Unlike charges attract

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Van de Graaf Generator

• Here the hairs are
• positive and repel

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Ben Franklins theory of charging

• Ben Franklin had a theory in 1700, later proved
  to be wrong, that all matter is neutral because
  it has a certain quantity of electrical fluid
  which is invisible. If this body acquires an
  excess of this fluid, it becomes positively
  charged. If it loses fluid, it becomes
  negatively charged due to the deficit.

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Example of Ben Franklins theory

• In this diagram, a rubber rod rubs a piece of
  fur, and electric fluid flows from rubber to fur,
  thus making fur () and rubber (-)

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Modern Atomic Theory

• In modern atomic theory (1900), matter consists
  of 2 electrically charged particles
• (1) electrons are negative -e
• (2) protons are positive e
• (3) charge of proton/electron e
• e 1.6 X 10-19 C
• where C Coulombs

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Example of atomic theory

• In the atomic theory, when the rubber rod rubs
  the fur, the electrons are transferred from fur
  to rubber, and since electrons are negative, this
  makes the rubber negative.
• Electrons are 2000 times less massive than
  protons so they always move instead of protons.

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Neutral matter

• According to atomic theory, neutral matter has a
  net charge of zero because there are as many
  protons present in its atoms as there are
  electrons. For example, a sodium atom has 11
  protons and 11 electrons so
• Net charge 11-11 0.
• Ions are atoms that have electrons added or
  removed. For example Na1 ion has 11 protons and
  10 electrons so
• Net charge 11-10 1

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Polarization of Neutral Matter

• Neutral matter has an overall charge of zero, but
  one side of the object can be forced to have
  positive charge while the other would have
  negative. An example

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Conductors vs. Insulators

• Conductors are metallic substances in which the
  conduction band electrons are free to move all
  over the metal . These electrons are not tightly
  bound to atoms.
• Insulators are substances that are non-metals
  like wood in which electrons are tightly bound to
  atoms and not free to move throughout the
  substance.
• Pure water is a poor conductor but it conducts a
  lot better when ionic impurities such as salts
  are dissolved in it.

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Charging by Friction
HERE ELECTRONS FLOW FROM GLASS TO SILK SO THE
SILK IS CHARGED NEGATIVE WHILE THE GLASS ROD
BECOMES POSITIVE
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Charging by conduction

• In charging by conduction, a rod touches a metal
  directly, thus transferring its charge to the
  metal.
• For example, if a positive rod touches a metal
  sphere, the metal sphere will also become
  positive.

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Charging by induction

• To charge by induction, a charged rod is brought
  close to a metal object. The metal object is
  then grounded and the excess charges flow to
  ground. After the ground is broken, the metal
  has the opposite charge to the rod. For example
• PHYSICS CLASSROOM ANIMATION OF INDUCTION

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Coulombs Law
http//www.pbcc.edu/faculty/sundquij/phy2049/PHY20
49-Physlet-Database.htm
F magnitude (absolute value) of force in
Newtons (N) k 9 X 109 qs in Coulombs (C) r
distance in meters (m)
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Net force on one dimensional configuration of
charges
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Net force on 2-dimensional configuration of
charges
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Procedure for solving configuration problems

• (1) draw all forces acting on the correct charge
  only the forces on that charge and no other
  charge
• (2) calc. the absolute value (magnitude) of each
  force from Coulombs Law
• (3) determine the angle from 0-360 for each
  force vector
• (4) calc. x and y components of each vector
• F F (cosq i sin q j )
• (5) add all x and y components to get x and y
  components of net (resultant) force vector
• (6) calc. the magnitude of resultant vector using
  Pythagorean thm. and angle using arctan (Ry/Rx)

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Definition of Electric Field Vector
Unit N/C V/m
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Two cases for force vs. electric field

• If the charge is positive, the electric field and
  force vectors point in the same direction.
• If the charge is negative, the electric field and
  force vectors point in opposite directions.

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Kinematics of Charged Particles moving in
Electric Fields

• Use Chapter 2 eqns. like v v0 at
• Where a F/m and F qE
• SEE ACTIVE FIG. 23.26 at pse6.com

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Electric Field lines for Point Charges
SEE ACTIVE Fig. 23.13 at pse6.com
Notice that the field lines are densest (closest
together) near the charges and this represents
the places where the electric field strength is
greatest. Further from the charges, the lines
are less dense, and weaker.
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Electric Field lines for Dipole
Notice that the field lines are
stretched between the unlike charges, like
rubber bands, and this explains why unlike
charges attract each other.
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Electric Field lines for like charges
Notice that the lines are repelled between the
like charges, thus explaining why like charges
repel
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Electric Field lines for Parallel Plate Capacitor
The field lines are straight and uniform in
intensity, and travel from positive to
negative charges.
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Electric Field Strength of Point Charge
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Electric Field for Configuration of Charges

• Do this problem exactly like Coulombs Law
  problem using formula on previous slide. At
  point P place a positive 1 Coulomb charge and
  calc. the net force on it.

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Electric Field for a symmetric arrangement of
charges

• For example, in this case, the y components of
  the two electric field vectors cancel each other
  , so the net electric field is just E 2E1x

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Electric Fields of Continuous Charge Distributions
l represents linear charge density. l q / L
units C/m For linear geometry dq l
dx For circular geometry dq lRdq
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Example 23.7 Linear geometry
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Electric Field at center of semicircular ring of
charge
By symmetry, Ex 0 (or try the integral and
itll equal zero)
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Electric Field along axis of ring of charge
r
R

• Special Cases
• x 0, E 0
• x infinite, E kQ/x2

a
a
x

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Electric Field for infinite line of charge(1)
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Infinite line(2)