Electrostatics by chenshu


 Atomic  Particles and Properties
 Interaction of Charge
 Conductors and Insulators
 Methods of Charging
 Coulomb‟s Law
 Electric Fields
 Electric Potential Energy
 Voltage (Potential Difference)
 Capacitors...
Think about it...
Try  to picture an atom...
Now, draw a picture of a
 model of an atom that
 you‟ve seen in the past.
 If you haven‟t seen one,
 make one up!
The Bohr model of the atom
Atomic Particles
All have     mass and charge
 SI units?

Fundamental        unit of
 “e” = 1.60 x 10 -19 Coulombs
  chargeis quantized; it comes in set
negatively  charged (-e)
mass = 9.11 x 10-31 kg
“orbit” the nucleus in a
 probability sphere (cloud)
  held in place by electrostatic forces
  held more weakly as “orbits”
          charged (+e)
mass = 1.67 x 10 -27 kg
 ~2,000  times the mass of an
alwaysoccur in the same
 number as electrons
 unless   it‟s an ion
mass ≈ 1.67 x 10 -27 kg
 the    same as a proton!
may  or may not occur in
 the same number as
 this   leads to isotopes
Quarks and Leptons
Of course, at this stage of our
 scientific understanding, we
 know that even protons,
 neutrons, and electrons are
 not elementary.
They are made up of quarks
 and leptons
Charge is Quantized!
   That is...electric charge will appear in packets or
                multiples of “e”. Hence,
                         q = N·e
     N is the # of charges, e is the charge of an electron
                  q is measured in Coulombs
Ex: How many protons would there be in one Coulomb of
positive charge?
         q     1.0C
       N              6.25  1018

         E 1.6  10 C

That number of basketballs would fill the Earth 11,000 times over!!!
Think and Explain...
„Think about what happens when a
balloon is rubbed against your hair and
then placed next to the wall...
„Can you explain why this happens?
 Electric   charge
   An  electrical property of matter;
    forces exist between two
    charged objects.
   Just like “mass” is an intrinsic
    property of matter and forces
    exist between two masses.
 Likecharges repel;
  opposite charges attract.
Separation of Charge
 How are electrostatic forces and
 gravitational forces alike? How are
 they different?
   Demo:       comb and paper bits…
 Charges       can be “separated”
            by rubbing two things together
   literally
   this shows how friction is an
    electrostatic force
   when charges
    move objects
    become electrified
Conservation of Charge

Law of Conservation of
Charge: During any process, the
net electric charge of an isolated
system remains the same.
    „Electrons flow from regions of negative charge to
    regions of positive charge.
    „Read the Physics of electronic ink on page 524
Conductors and Insulators
  Not only can charge exist on an object,
  it can move through an object
 Materials that readily allow the
  movement of charges are called
     metals,   water
     outer shell electrons (valence) that can move
      „freely‟ through the substance
     flows from e- surplus (-) to e- deficiency (+)
 Materials  that inhibit the movement of
    charges are called insulators
       glass, ceramic, rubber, plastic
Back to the Balloon Question…
 As the balloon is rubbed against your head electrons
  are pulled off of your hair.
 This causes the balloon to be charged negatively.
 When the negative balloon is placed near the neutral
  wall the electrons in the wall rearrange.
 The electrons in the wall are repelled giving the
  wall a local positive charge.
 Thus the negative balloon is attracted to the positive
Detecting Charge
Charge   is detected with an
The Electroscope
 Ifa charged object comes in contact
  with the electroscope, the leaves of
  the electroscope push away from
  each other. Why???
 But you can‟t tell what
  kind of charge it is with an
  electroscope. Why?
Methods of Charging
Friction       (rubbing)
   electronsget „rubbed off‟ leaving extra/not
    enough electrons (net charge)
      excess   --> negative charge -- deficiency --> positive
   carpets, static cling, the Van de Graff generator
   hard rubber & fur = negative charge
   glass & silk = positive charge
Methods of Charging
  chargingby contact
    Charges flow from a charged
     conductor to a neutral conductor
  no contact is necessary!
  Just move close to a neutral object and
   charges rearrange. Why???
A   neutral object coming close to a charged
  object can have its charges moved without
  taking on electrons.
 The object is still
  neutral while it
  has a + and – region.
(polar molecules
  in chemistry)
Coulomb’s Law
 The electrostatic force between two charged
 particles is directed along a line joining their
 centers. If the charges are similar the force
 is repulsive, otherwise it is attractive.

                              k = 8.99 x 109
   F k 2                     N·m2/C2  F is
                                in Newton's
  How is this similar to the gravitational force?
Coulomb’s Law...

    double the distance, force drops by 1/4.

  double the charges, force increases by a factor
                     of four.
Coulomb’s Law Example
Ex: How do the forces of gravity and
electrostatic repulsion compare between two
100.0 kg metal spheres separated by 10.0
cm and given a charge of 1.00 mC?
What does this say about levitation???
Coulomb’s Law Example
Ex: A gold atom normally contains 79
protons. If all of the electrons are removed
except for one, what force would it undergo
at a distance of 6 x 10-10 m from the
nucleus? Is this a very large force?
Example: Adding Forces

       20cm           30cm

q1=-6µC     q2=+2µC           q3=-4µC
 Determine the net electrostatic
 force on q2 in the figure above.
 Could charges q1 and q3 be
 positioned so that the net force is
Ex: Adding Forces in 2D
Determine the net
electrostatic force on q2
in the figure above.
Could charges q1 and
q3 be positioned so that
the net force is zero?

                            q2=+2µC          q3=-4µC
Speed of a Orbiting electron

                   planet                             nucleus

 In a hydrogen atom one electron (-e) orbits around a proton (+e) at a
 radius of 0.0529 nm. Use UCM techniques to calculate the orbital speed
 (can you calculate gamma for this speed and determine if there are
 relativistic effects?)
Electric Fields
 All charged bodies will affect other charges
  when placed near each other. Explain…
 We say that a charged body sets up an
  ELECTRIC FIELD around it (think of how a
  massive body sets up a gravitational field
  around it.)
 When a charge is placed in an electric field, it
  experiences a Coulomb attraction/repulsion
 It will accelerate in the direction of the net
  force. What would you have to do in order to
  prevent the acceleration?
The Electric Field
 Just  like gravitational fields exist wherever there is
    mass, electric fields exist wherever there is charge.
 SI   units: N/C
   The way we determine field strength is to place a (positive) point
    in the field and measure the
    applied force.
 Electricfields are vectors:
     Ex 8-11 and fig. 18.17                        E F
Electric Field Lines
                      begin on positive and end
                       on negative charges
                      directed away from positive
                       charges and toward negative
                       charges (tells which way a
                       positive test charge would
                      the density of lines indicates
                       the strength of the field
An Electric Dipole (two positive charges)

                        Where    is the field
                         the strongest?
                         The weakest?
                        Can you draw the
                         field for a dipole
                         made up of
                         opposing charges?
A more complex field…
Neat Fact:
When   electric charge is placed on a
 conductor. All the charge resides on
 the surface.
This   is behind the idea of electric
Think and Explain...
What   would
 happen to you if
 you were struck
 by lightning in
 your car???
Think and Explain...
What   must you do to lift a stone off the
 ground? What happens to the stone’s
 potential energy?
What must you do to move a positive test
 charge closer to another positive charge?
 What happens to the charge’s energy?
 Tolift a stone you must do work. The
 amount of work you do is equal to the
 stone‟s change in G.P.E.
       W = ∆G.P.E. = mgh - mgh0
 When  raised in a gravitational field a
 stone has potential energy. That
 energy depends on how high you
 raise the stone above some reference
 How would the G.P.E. of the stone compare to that
  of a large boulder raised to the same height?
 This dependence on mass could be overcome by
  dividing by mass. Let’s call this new value the
  gravitational potential (G).
               G = G.P.E./m (units?)
 Now we can determine the gravitational potential at
  any point above the earth for any sized object.
Electric Potential and Work
   Suppose we move a charge, +q, from A to B in
    the field below. The work done to move q is
    equal to the charge‟s change in electric potential
    energy (E.P.E)

       W = ∆E.P.E.
         = EPEB-EPEA

 When the positive charge is moved from A to B
  does it gain or lose EPE?
 How would the work done change for a larger
Electric Potential
 The  Electric Potential (or potential difference) is
  also called Voltage. It is found by dividing the
  Work done to move a charge by the magnitude of
  the charge moved.
             V = W/q = ∆EPE/q
 Voltage is measured in Volts (J/C)
 Voltage is the “push” that causes electrons to
  move through wires
Field Lines and Equipotentials
 Equipotential   surfaces are places in an electric field
  where the potential difference is the same
   They   are perpendicular to the electric field lines.
 In  the diagram at
  right, the field
  lines are blue and
  the equipot. Lines
  are red.
Go Figure...
How  much work must be done to
 move a 300 µC charge through a
 potential difference of 6.0 V?
1.8 x 10-3 J
If GPE is zero at ground level,
 where is EPE zero???
An infinite distance away!
Point Charges...
The   E.P.E. due a point charge
 is:               qq0
        E.P.E.  k
The   potential of a point charge
                   Vk q
Go Figure...
•Ex: A 2.00 µC point charge is
located at the center of an equilateral triangle
with sides of 50.0cm
•What is the potential at each vertex?
•What is the EPE of a 1.00 µC charge placed
at a vertex?
•6.2x10-2 J
•How much work must be done to move the
test charge from one vertex to the next?
•None! Why???
Charge Distribution and Shape

„All charge resides on the surface of a conductor
   „Charge spreads out evenly on a perfect conducting sphere
   „Charge is more confined to original area on a non
„There is no potential diff. between any points on a
   „The surface of a conductor is an equipotential surface
Charge Distribution and Shape

„Charge concentrates at points or sharp edges on a
conducting surface
   „There can be no potential difference between any
   two points on a conductor
   „If the field intensity at a sharp edge is great
   enough, gas around surface will ionize and electric
   discharge will occur
      „Spark plugs, lightning, lightning rods
St. Elmo‟s Fire
   St. Elmo's fire is a spark discharge from soaring
    buildings. It is generated by a high voltage between
    the ground and the air. St. Elmo‟s fire has been
    observed among others from steeples, masts,
    mountain tops and barbed wire fences. It is very rare.
    When you see St. Elmo‟s fire near yourself, there is a
    high danger of being struck by lightning. The high
    voltage can also show up by your hair standing on
    end. Although this may look "funny", you must
    immediately leave your position as a flash of lightning
    is imminent.
Go Figure...
Ex. How much work must be done to
place a 10.0 µC charge at each of the
corners of a square that is 50.0 cm on
each side? [Hint: place them one at a

Capacitors 1
Devices that store electric
 energy are called capacitors.
 The  most common type is made up
  of two parallel plates separated by
  some medium (called a dielectric).
  The plates are usually rolled into a
  cylindrical shape
 Energy comes from the work it took
  to separate the charge!
 Capacitors 2
The charge “stored” on a
 capacitor is given by the

                 q  CV

“C” is the capacitance of the capacitor,
 a constant that depends on its makeup.
 What are the units of C?
 Capacitors 3
The capacitance, C, of a
 capacitor can be found using:
                0A
κ is the dielectric constant, ε0 is the
 permittivity of free space, A is the area
 of the capacitor, and d is the
 separation between the two sides
Capactors 4
 Ex:The electric field
 strength between the
 plates of a capacitor
 can be found using:

          E = V/d
 where V is the voltage
 across the plates and d
 is the plate separation.
Capacitor Examples
Ex: What voltage is required to
 store 7.2 x 10-5 C of charge on
 the plates of a 6.0 mF
Ex: #40 on page 583
Capacitor Examples
Ex: How much charge can be stored by
 a parallel plate capacitor made up of
 3.0 cm x 4.0 cm sides, separated by a
 distance of 0.025 mm when a voltage
 of 12 V is placed across it and…
 a.) no dielectric is used.
 b.) ruby micra is used as a dielectric.
 c.) What would the electric field strength be
 between the plates of the capacitor?
Did you know???
Photo  Flashes use capacitors
Computer keyboards use
 capacitors (p 573)
Capacitors are used to prevent
 power surges in brown outs.
The greater the dielectric constant
 the more charge can be stored
Oil and water can be used as
 dielectrics, air too!
I’m shocked…
Can this be
the end,

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