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Electric Fields and Potentials. Joey Multari, Shannon Burt, Katie Abbott. Electric Force. Electricity exerts a force similarly to gravity. F e = kq 1 q 2 r 2

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electric fields and potentials

Electric Fields and Potentials

Joey Multari, Shannon Burt, Katie Abbott

electric force
Electric Force

Electricity exerts a force similarly to gravity.

Fe = kq1q2

r2

where q1 and q2 represent the amount of charge in Coulombs (6.24 x 1018), r is in meters and k is the electrical constant (9 x 109 Nm2 /C2)

1 Coulomb of electrons travels through a 100-W lightbulb in about one second

review
Review
  • How many coulombs travel through a 100W light bulb in 3 seconds?
  • What type of behavior does charge have?

A-Attracting

B-repelling

C-both

D- Fast movement

E- Slow movement

electric fields
Electric Fields

Just like gravity field, charges have a force field (E) as well, measured in force per unit charge

E = F = kQ

q r2

where Q is a positive test charge

Direction of fields – away from a positive charge, toward a negative charge

force field lines
Force Field Lines
  • Fields have strength and direction
  • Field is determined by the force and direction of motion of a positive test charge
  • Field is strongest where the force is the strongest – where the lines are the most concentrated
electrical potential
Electrical Potential

Just like gravity—the potential (possibility) of falling to earth, charges have the potential to move toward or away from each other

electrical potential1
Electrical Potential
  • Force of attraction/repulsion causes the potential
  • Potential is energy divided by charge—since charge is usually small, potential can be relatively large—5000 volts on a charged balloon
  • A larger amount of charge makes larger potential
voltage electrical potential
Voltage – Electrical Potential

Voltage = PE/Q

PE in Joules and Q in Coulombs

100 Volts

0.000001-J/0.00000001-C

100-J/ 1-C

1,000,000-J/10,000-C

electric shielding
Electric Shielding

Electrons repel toward the outside of any conducting surface

Net charge inside is zero

Electrons flow outward evenly, but pile up on sharp corners

Shielding is important in electronic devices such as televisions and computers

faraday cage
Faraday Cage
  • Faraday stated that the charge on a charged conductor resided only on its exterior
  • To demonstrate this fact he built a room coated with metal foil, and allowed high-voltage discharges from an electrostatic generator to strike the outside of the room
  • He used an electroscope to show that there was no excess electric charge on the inside of the room's walls.
slide24

Person in a car hit by artificial lightning. The lightning strikes the car and jumps to the ground bypassing the front tire arcing from the axle to the ground.

storing charges
Storing Charges

Capacitors can store charges on plates which are separated — as in Franklin’s Leyden jars

storing charges1
Storing Charges
  • A capacitor is a device that stores electric charge
  • A capacitor consists of two conductors separated by an insulator
slide27

capacitor

capacitor

capacitors and capacitance
Capacitors and Capacitance

A capacitor in a simple

electric circuit.

Charge Q stored:

The stored charge Q is proportional to the potential difference V between the plates. The capacitance C is the constant of proportionality, measured in Farads.

Farad = Coulomb / Volt

parallel plate capacitor
Parallel-Plate Capacitor
  • A simple parallel-plate capacitor consists of two conducting plates of area A separated by a distance d.
  • Charge +Q is placed on one plate and –Q on the other plate.
  • An electric field E is created between the plates.
capacitor applications
Capacitor Applications
  • .
  • Computer RAM memory and keyboards.
  • Electronic flashes for cameras.
  • Electric power surge protectors.
  • Radios and electronic circuits.
  • Power supplies
van de graaf generator
Van de Graaf Generator

This machine is capable of producing very high electrostatic potential differences in the order of millions of volts

It works by friction of the belt with the rollers and separates charges at combs which take the charges to the dome and picks them up from the ground at the base

van de graff generator
Van de Graff Generator

http://demoroom.physics.ncsu.edu/movies.html

van de graff generator1
Van de Graff Generator

http://demoroom.physics.ncsu.edu/movies.html

van de graff generator2
Van de Graff Generator

http://demoroom.physics.ncsu.edu/movies.html

electrical force

Woman is touching negatively-charge sphere

Electrical Force

Electrical force is more powerful than gravity

review1
Review
  • Which two signs attract? Which two repel?
  • A) Unlike signs attract, like signs repel
  • B) unlike signs repel, like signs attract
  • C) like signs repel and attract
  • D) vary depending on amount of charge
  • E) No charges attract, they only repel
structure of the atom

Neutron

Proton

Electron

Energy Levels

or Orbits

Structure of the Atom
charge
Charge
  • Electrons and protons have an attribute called charge
    • Electrons have a negativecharge
    • Protons have a positivecharge
      • 1800 times more massive than electrons
    • Neutrons have no charge
      • 1800 times more massive than electrons
charge conservation
Charge Conservation
  • Charge is neither created or destroyed.
  • What we call charging is either
    • Transfer of charges, or
    • Internal rearrangement of charge carrying units
  • Uncharged (neutral) objects have equal amounts of positive and negative charge
  • An object with unequal number of electrons and protons is electrically charged
    • Negative – Electrons > Protons
    • Positive– Protons > Electrons
charge quantization
Charge Quantization
  • Charge is always an integer multiple of a constant.
    • Six billion billion electrons is  - 1 Coulomb of charge
    • Six billion billion proton is + 1 Coulomb of charge
  • Q=Ne, where e is the unit electrical charge
  • Electrons have –e charge, protons have +e.
  • Millikan’s Oil Drop experiment
coulomb s law
Coulomb’s Law
  • One Coulomb = 6.24 x 1018 electrons
  • Electrons have a negative charge
    • qe = -1.6 x 10-19 Coulomb
  • Protons have a positivecharge
    • qp = +1.6 x 10-19 Coulomb
  • Electrical Force can be positive or negative
    • Positive – repulsive force
    • Negative – attractive force
example
Example

One pair of charges of 1 C each are 1 m apart

F = kq1q2/ d2

F = (9 x 109 N m2/C2)(1 C)(1 C)/(1-m)2

F = 9 x 109 N m2/C2)(1 C2)/1-m2

F = 9 x 109 N (repulsive)

10 times the weight of a battleship

review2
Review
  • The charge on an electron is 1.6 X 10^-19 C. How many electrons make a charge of 1C?
conductors
Conductors
  • An electrical conductor is a substance through which electrical current flows with small resistance
  • Metals are generally excellent electrical conductors
  • The electrons in conductors lie in an ‘loose’ outer orbit – the so-called "valence band"
valence bonds
Valence Bonds
  • In a periodic table the columns represent number of valence bonds
  • Often times, the valence bonds in two combining elements will add up to eight with rare exceptions.
insulators
Insulators
  • An electrical insulator is a substance with an extremely high resistance to the flow of charge
  • Most nonmetals solids are generally excellent insulators
  • Most atoms hold on to their electrons tightly and are insulators
covalent bonds
Covalent Bonds
  • In a tightly knit molecular bond the atoms are held together and therefore hang onto their electrons, creating excellent insulators.
review3
Review
  • 1. How much resistance does a conductor have?
  • A- large amount
  • B- small amount
  • C- None
  • D- Average amount
  • E- infinite
review4
Review
  • Where are valence electrons represented on the periodic table?

A- columns

B-rows

C- atomic number

D- atomic mass

E- grouping

charging by touching
Charging by Touching
  • Charging by actually touching object
charging by induction
Charging by Induction
  • Two charged objects placed on opposite sides create a charge in third object.
polarization
Polarization

Electrons surrounding nucleusmay be thought of as a cloudin which the total negativecharged is smeared out.

polarization1
Polarization

Unpolarized atom

Put negatively charged rod on the right side...

Center of electron cloud shifts to the left.

neutral objects are attracted to charged objects
Neutral Objects are Attracted to Charged Objects

Charged comb attracts neutralbits of paper

charge distributions

Charge on Metals

  Metal Ball

Charge on Metal Points

Lightning, lightning rods

Charge Distributions
lightning
Lightning
  • As the negative charges collect at the bottom of the cloud it forces the negative charges in the ground to be forced away from the surface.  This leaves the ground positive.
  • A streamer of negative charges is repelled by the bottom of the cloud and attracted by the ground.
  • As this streamer of negative charges approaches the ground, a streamer of positive charges is repelled by the ground and attracted to the negative streamer.
lightning1
Lightning
  • When the two streamers connect, they have created a fairly conductive path which allows a sudden down surge of electrons to jump to the ground.  This is the lightning.  
  • The rapidly moving electrons excite the air along the path so much that it emits light.   It also heats the air so intensely that it rapidly expands creating thunder.
  • One thing to notice is that the positive charges that make up both the cloud and the ground do not move.  Even the positive streamer launched by the ground is really only made up of positively charged air particles because the electron(s) left the particle.  
slide77

Electroscope

Conductor

Insulator

Metal Leaves

bibliography
Bibliography
  • The textbook