Knowledge of electricity dates back to Greek antiquity 700 BC.

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Introduction

• Knowledge of electricity dates back to Greek
  antiquity (700 BC).
• Began with the realization that amber (fossil)
  when rubbed with wool, attracts small objects.
• This phenomenon is not restricted to amber/wool
  but may occur whenever two non-conducting
  substances are rubbed together.

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15.1 Properties of Electric Charges - Discovery

• Observation of Static Electricity
• A comb passed though hair attracts small pieces
  of paper.
• An inflated balloon rubbed with wool.
• Electrically charged
• Rub shoes against carpet/car seat to charge your
  body.
• Remove this charge by touching another person/a
  piece of metal.
• Two kinds of charges
• Named by Benjamin Franklin (1706-1790) as
  positive and negative.
• Like charges repel one another and unlike charges
  attract one another.

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15.1 Properties of Electric ChargesNature of
Electrical Charge

• Origin of charge is at the atomic level.
• Nucleus robust, positive.
• Electrons mobile, negative.
• Usual state of the atom is neutral.
• Charge has natural tendency to be transferred
  between unlike materials.
• Electric charge is however always conserved in
  the process.
• Charge is not created.
• Usually, negative charge is transferred from one
  object to the other.

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15.1 Properties of Electric ChargesQuantization

• Robert Millikan found, in 1909, that charged
  objects may only have an integer multiple of a
  fundamental unit of charge.
• Charge is quantized.
• An object may have a charge e, or 2e, or 3e,
  etc but not say 1.5e.
• Proton has a charge 1e.
• Electron has a charge 1e.
• Some particles such a neutron have no (zero)
  charge.
• A neutral atom has as many positive and negative
  charges.
• Units
• In SI, electrical charge is measured in coulomb (
  C).
• The value of e 1.602 19 x 10-19 C.

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15.2 Insulators and Conductors Material
classification

• Materials/substances may be classified according
  to their capacity to carry or conduct electric
  charge
• Conductors are material in which electric charges
  move freely.
• Insulator are materials in which electrical
  charge do not move freely.
• Glass, Rubber are good insulators.
• Copper, aluminum, and silver are good conductors.
• Semiconductors are a third class of materials
  with electrical properties somewhere between
  those of insulators and conductors.
• Silicon and germanium are semiconductors used
  widely in the fabrication of electronic devices.

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Example

• 1e -1.60 10-19 c
• Takes 1/e6.6 1018 protons to create a total
  charge of 1C
• Number of free electrons in 1 cm3 copper 1023
• Charge obtained in typical electrostatic
  experiments with rubber or glass 10-6 C 1 mc
• A very small fraction of the total available
  charge

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Mini-quiz

• Identify substances or materials that can be
  classified as
• Conductors ?
• Insulators?

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15.2 Insulators and Conductors Charging by
Conduction.

• Consider negatively charge rubber rod brought
  into contact with a neutral conducting but
  insulated sphere.
• Some electrons located on the rubber move to the
  sphere.
• Remove the rubber rod.
• Excess electrons left on the sphere. It is
  negatively charged.
• This process is referred as charging by
  conduction.

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15.2 Insulators and Conductors Earth/Ground.

• When a conductor is connected to Earth with a
  conducting wire or pipe, it is said to be
  grounded.
• Earth provides a quasi infinite reservoir of
  electrons can accept or supply an unlimited
  number of electrons.

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15.2 Insulators and Conductors Charging by
Induction.

• Consider a negatively charged rubber rod brought
  near a neutral conducting sphere insulated from
  the ground.
• Repulsive force between electrons causes
  redistribution of charges on the sphere.
• Electrons move away from the rod leaving an
  excess of positive charges near the rod.
• Connect a wire between sphere and Earth on the
  far side of the sphere.
• Repulsion between electrons cause electrons to
  move from sphere to Earth.
• Disconnect the wire.
• The sphere now has a positive net charge.
• This process is referred as charging by
  induction.
• Charging by induction requires no contact with
  the object inducing the charge.

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15.2 Insulators and Conductors Charging by
Induction.

• Consider a negatively charged rubber rod brought
  near a neutral conducting sphere insulated from
  the ground.
• Repulsive force between electrons causes
  redistribution of charges on the sphere.
• Electrons move away from the rod leaving an
  excess of positive charges near the rod.
• Connect a wire between sphere and Earth on the
  far side of the sphere.
• Repulsion between electrons cause electrons to
  move from sphere to Earth.
• Disconnect the wire.
• The sphere now has a positive net charge.
• This process is referred as charging by
  induction.
• Charging by induction requires no contact with
  the object inducing the charge.

Q How does this mechanism work is we bring is a
positively charged glass rod instead?
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15.2 Insulators and Conductors Polarization.

• Polarization is realignment of charge within
  individual molecules.
• Produces induced charge on the surface of
  insulators.
• how e.g. rubber or glass can be used to supply
  electrons.

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Mini-quiz

• A positively charged object hanging from a string
  is brought near a non conducting object (ball).
  The ball is seen to be attracted to the object.
• Explain why it is not possible to determine
  whether the object is negatively charged or
  neutral.
• What additional experiment is needed to reveal
  the electrical charge state of the object?

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Explain why it is not possible to determine
whether the object is negatively charged or
neutral.

• Two possibilities
• Attraction between objects of unlike charges.
• Attraction between a charged object and a neutral
  object subject to polarization.

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-

-

-

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What additional experiment is needed to reveal
the electrical charge state of the object?

• Two Experiments
• Bring known neutral ball near the object and
  observe whether there is an attraction.
• Bring a known negatively charge object near the
  first one. If there is an attraction, the object
  is neutral, and the attraction is achieved by
  polarization.

-
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ELECTRIC FORCES EQUILIBRIUM

• SABINO PHYSICS
• April 27th 2009

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TODAYS OBJECTIVES

• Calculate electric force using Coulombs law
• in one dimension
• In two dimensions (Superposition Principle)
• Compare magnitude of electric gravitational
  forces
• Determine equilibrium for a third charge acted on
  by two other charges (If time permits)

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17-2 Coulombs Law

• Charles Coulomb discovered in 1785 the law of
  electrical force between two charged particles.
• He discovered electric force has the following
  properties
• Inversely proportional to the square of the
  separation between the particles, and is along a
  line joining them.
• Proportional to the product of the charges q1
  and q2 on the two particles.

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17-2 Coulombs Law

• Relationship between Fe and q1q2 is DIRECT.
• Double one charge and Fe will be doubled.
• Double both charges (x4) and Fe will be
  quadrupled.

• Relationship between Fe and distance is an
  Inverse Square Law.
• Double the distance and Fe will be 1/4 as
  large.
• Triple the distance and Fe will be 1/9 as
  large.

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17-2 Coulombs Law

• The electrostatic force is often called Coulomb
  force.
• It is a VECTOR
• a magnitude
• a direction.
• Second example of action at a distance.

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17. 2 Coulombs Law

• ke known as the Coulomb constant
• 9 x 109 Nm2/C2
• Other SI units
• Force the Newton (N)
• Charge the coulomb ( C).
• Distance the meter (m).

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RECALL
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Example Electrical Force

• Question
• The electron and proton of a hydrogen atom are
  separated (on the average) by a distance of about
  5.3x10-11 m. Find the magnitude of the electric
  force that each particle exerts on the other.

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• Question
• The electron and proton of a hydrogen atom are
  separated by a distance of about 5.3x10-11 m.
  Find the magnitude of the electric force that
  each particle exerts on the other.

• Observations
• We are interested in finding the magnitude of the
  force between two particles of known charge, and
  a given distance of each other.
• The magnitude is given by Coulombs law.
• q1 -1.60x10-19 C
• q2 1.60x10-19 C
• r 5.3x10-11 m

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• Given Quantities
• q1 -1.60x10-19 C
• q2 1.60x10-19 C
• r 5.3x10-11 m
• Solution
• Attractive force with a magnitude of 8.2x10-8 N.

Can anyone name the first action at a distance
force you have encountered in physics earlier
this year?
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Gravitational Force Comparison

• Given Mass proton 1.7 x 10-27
• Mass electron 9.1 x 10-31

6.67 x 1027 (1.7 x 10-27)(9.1 x 10-31)
(5.3 x 10-11)2 Fg 4.9 x 10-27
Newtons verses Fe 8.2 x 10-8 Newtons
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Fg Fe Similarities Differences

• Similarities
• Both act on objects at a distance,
• i.e. no contact.
• b. always acts along a straight line between two
  objects

• Differences
• Fg is only attractive Fe can be
• attractive or repulsive
• b. Fg is a relatively weak force Fe
• is very strong.

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YOUR TURN!
What is the electric force between two charges of
1 C separated by 1 meter?
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Superposition Principle

• Find the resultant force
• Draw a picture, label the charges
• Find the magnitude of the forces on charge
  specified using Coulombs Law.
• Find net-x net-y for each force on charge
  specified (remember vector components)
• Use Pythagorean Theorem to find resultant
  magnitude.
• Use tan-1SFy/SFx ? to find direction of net
  force on charge specified.

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Example Using the Superposition Principle

• Consider three point charges at the corners of a
  triangle, as shown below. Find the resultant
  force on q3 if
• q1 6.00 x 10-9 C
• q2 -2.00 x 10-9 C
• q3 5.00 x 10-9 C

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The superposition principle tells us that the net
force on q3 is the vector sum of the forces F32
and F31.The magnitude of the forces F32 and F31
can calculated using Coulombs law.
-2.00 x 10-9 C
5.00 x 10-9 C

5.00 m
6.00 x 10-9 C
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Solution
Tan-1 SFy / SFx ?
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YOUR TURN!
Consider three charges placed at the corners of a
triangle as shown on the right. What is the
magnitude of the electric force on q1 resulting
from the electric force from the other two
charges?
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Solution

• F12 9 x 109 (-1.5 x 10-6)(-2.5 x 10-6)
  2.16 x 10-2 C
• (1.25)2
• F13 9 x 109 (-1.5 x 10-6)( 3.5 x 10-6)
  1.51 x 10-2 C
• (1.77)2
• Fx 1.51 x 10-2 cos 45
• Fy 2.16 x 10-2 C 1.51 x 10-2 sin45
• F1

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Electric Charge in Equilibrium

• Consider two charges located on the x axis
• The charges are described by
• q1 0.15 ?C x1 0.0 m
• q2 0.35 ?C x2 0.40 m
• Where do we need to put a third charge of 0.5 ?C
  for that charge to be at an equilibrium point?
• At the equilibrium point, the forces from the two
  charges will cancel.

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Electric Charge in Equilibrium

• The equilibrium point must be along the x-axis.
• Three regions along the x-axis where we might
  place our third charge

x3 lt x1 x1 lt x3 lt x2 x3 gt x2
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Electric Charge in Equilibrium
.40 m
0 m

• x3ltx1
• Here the forces from q1 and q2 will always point
  in the same direction (to the left for a positive
  test charge)
• No equilibrium
• x2ltx3
• Here the forces from q1 and q2 will always point
  in the same direction (to the right for a
  positive test charge)
• No equilibrium

0.15 ?C
0.35 ?C
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Electric Charge in Equilibrium
x1 lt x3 lt x2 Here the forces from q1 and q2 can
balance.
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Electric Charge in Equilibrium
Track the signs of the charges!!
Answer x3 0.16 m
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YOUR TURN !

• The charges are described by
• q1 -.5 ?C x1 0.10 m
• q2 -2.5 ?C x2 0.75 m

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HONORS PHYSICS
HOMEWORK
Read pages 634 639 Do Practice problems pages
636 639 Due Tomorrow