General Physics PHY 2140

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General Physics (PHY 2140)
Introduction

• Syllabus and teaching strategy
• Electricity and Magnetism
• Properties of electric charges
• Insulators and conductors
• Coulombs law

Lecture 1. Chapter 15
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Syllabus and teaching strategy
Lecturer Prof. Alexey A. Petrov, Room 260
Physics Building, Phone [phone], or
313-577-2720 (for messages) e-mail
apetrov_at_physics.wayne.edu, Web
http//www.physics.wayne.edu/apetrov/ Office
Hours MWF 1040 AM -1135 AM, General Lectures,
Room 150 Tuesday 100-200 PM, on main campus,
Physics Building, Room 260, or by
appointment.  Grading Reading Quizzes
(bonus) 5 Quiz section performance/Homework
10 Best Hour Exam 25 Second Best Hour
Exam 25 Final 40 Reading Quizzes
It is important for you to come to class
prepared! BONUS POINTS Reading
Summaries Homework and QUIZ Sessions The
quiz sessions meet once a week 5 quizzes will
count towards your grade. BONUS POINTS
will be assigned by your quiz instructor. Hour
Exams and Final Exam There will be THREE (3)
Hour Exams and one Final Exam. Additional
BONUS POINTS will be given out for class
activity. Online Content Lecture Online will
be made available to you as a supplemental
reference.  
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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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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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15.3 Coulombs Law - Observation

• Charles Coulomb discovered in 1785 the
  fundamental law of electrical force between two
  stationary charged particles.
• An electric force has the following properties
• Inversely proportional to the square of the
  separation, r, between the particles, and is
  along a line joining them.
• Proportional to the product of the magnitudes of
  the charges q1 and q2 on the two particles.
• Attractive if the charges are of opposite sign
  and repulsive if the charges have the same sign.

r
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15.3 Coulombs Law Mathematical Formulation

• ke known as the Coulomb constant.
• Value of ke depends on the choice of units.
• SI units
• Force the Newton (N)
• Charge the coulomb ( C).
• Current the ampere (A 1 C/s).
• Distance the meter (m).
• Experimentally measurement ke 8.9875109
  Nm2/C2.
• Reasonable approximate value ke 8.99109
  Nm2/C2.

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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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15.3 Coulombs Law Remarks

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

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Mini-Quiz

• Name the first action at a distance force you
  have encountered in physics so far.

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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 (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.

• 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 -1.60x1019 C
• q2 1.60x1019 C
• r 5.3x1011 m

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QuestionThe 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.

• 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
• Solution
• Attractive force with a magnitude of 8.2x10-8 N.

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Superposition Principle

• From observations one finds that whenever
  multiple charges are present, the net force on a
  given charge is the vector sum of all forces
  exerted by other charges.
• Electric force obeys a superposition principle.

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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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Consider three point charges at the corners of a
triangle, as shown below. Find the resultant
force on q3.

• Observations
• 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.

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Consider three point charges at the corners of a
triangle, as shown below. Find the resultant
force on q3.
5.00 m
Solution