# Electric Fields and Potentials

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Electric Fields and Potentials
Joey Multari, Shannon Burt, Katie Abbott

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 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 Answer to number one 3 Answer to number two C

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

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 = PE/Q PE in Joules and Q in Coulombs 100 Volts J/ C 100-J/ 1-C 1,000,000-J/10,000-C

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 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.

Person in a car hit by artificial lightning
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 Capacitors can store charges on plates which are separated — as in Franklin’s Leyden jars

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

capacitor capacitor

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
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
. Computer RAM memory and keyboards. Electronic flashes for cameras. Electric power surge protectors. Radios and electronic circuits. Power supplies

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

Van de Graff Generator

Electric Forces and Charges

Like Signs Repel

Unlike Signs Attract

Electrical Force Woman is touching negatively-charge sphere
Electrical force is more powerful than gravity

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 A: Unlike signs attract, like signs repel

Conservation of Charge

Structure of the Atom Neutron Proton Electron Energy Levels or Orbits

Charge Electrons and protons have an attribute called charge
Electrons have a negative charge Protons have a positive charge 1800 times more massive than electrons Neutrons have no charge

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

Removing Electrons from Atoms
Rubber scrapes electrons from fur atoms

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 One Coulomb = 6.24 x 1018 electrons
Electrons have a negative charge qe = -1.6 x Coulomb Protons have a positive charge qp = +1.6 x Coulomb Electrical Force can be positive or negative Positive – repulsive force Negative – attractive force

Example One pair of charges of 1 C each are 1 m apart F = k q1 q2 / 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

Review The charge on an electron is 1.6 X 10^-19 C. How many electrons make a charge of 1C? 1/ (1.6 X10^-19) = 6.25 x 10^18

What happens to the magnitude of the force as the charges get farther apart?

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 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 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 In a tightly knit molecular bond the atoms are held together and therefore hang onto their electrons, creating excellent insulators.

Review 1. How much resistance does a conductor have? A- large amount
B- small amount C- None D- Average amount E- infinite B-Small columns

Review Where are valence electrons represented on the periodic table?
A- columns B-rows C- atomic number D- atomic mass E- grouping Answer- a

Net Charge Neutral

Net Charge Positive

Net Charge Negative

Charging by Touching Charging by actually touching object

Charging by Induction Two charged objects placed on opposite sides create a charge in third object.

Polarization Electrons surrounding nucleus may be thought of as a cloud in which the total negative charged is smeared out.

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
Charged comb attracts neutral bits of paper

Lightning, lightning rods
Charge Distributions Charge on Metals                             Metal Ball Charge on Metal Points                                   Lightning, lightning rods

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.

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.

Electroscope Conductor Insulator Metal Leaves

Bibliography The textbook

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