Electric Fields Electric Fields and Potentialsand Potentials

Electric ForceElectric ForceElectricity 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

Electric FieldsElectric FieldsJust 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 LinesForce Field Lines• Fields have strength and

direction

• Field is determined by the force and direction of motion of a positivepositive test charge

• Field is strongest where the force is the strongest – where the lines are the most concentrated

http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/u8l4c.html

http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/u8l4c.html

Electric ShieldingElectric ShieldingElectrons repel toward the outside of any conducting surfaceNet charge inside is zeroElectrons flow outward evenly, but pile up on sharp corners

Shielding is important in electronic devices such as televisions and computers

Faraday CageFaraday Cage• The Faraday cage is an electrical

apparatus designed to prevent the passage of electromagnetic waves, either containing them in or excluding them from its interior space

• It is named for physicist Michael Faraday, who built the first one in 1836

Faraday CageFaraday 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.

Faraday CageFaraday Cage

• A more impressive demonstration of the Faraday cage effect is that of an aircraft being struck by lightning

• This happens frequently, but does not harm the plane or passengers

• The metal body of the aircraft protects the interior.

• For the same reason, and if it were not for the highly flammable nature of petrol, a car would be a very safe place to be in a thunderstorm

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.

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

Electrical PotentialElectrical 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 PotentialVoltage – Electrical Potential

Voltage = PE/QVoltage = PE/Q PE in Joules and Q in Coulombs

100 Volts0.000001-J/0.00000001-C

100-J/ 1-C1,000,000-J/10,000-C

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

Storing ChargesStoring Charges• A capacitor is a device that

stores electric charge• A capacitor consists of two

conductors separated by an insulator

Capacitors and CapacitanceCapacitors and Capacitance

Charge Q stored:

CVQ 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

A capacitor in a simpleelectric circuit.

Parallel-Plate CapacitorParallel-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.

+Q -Q

+Q -Q

Capacitor ApplicationsCapacitor Applications• Computer RAM memory and

keyboards.

• Electronic flashes for cameras.

• Electric power surge protectors.

• Radios and electronic circuits.

• Power supplies

–.

capacitorcapacitor

Van de Graaf GeneratorVan de Graaf GeneratorThis 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 GeneratorVan de Graff Generatorhttp://demoroom.physics.ncsu.edu/movies.html

Van de Graff GeneratorVan de Graff Generatorhttp://demoroom.physics.ncsu.edu/movies.html

Van de Graff GeneratorVan de Graff Generatorhttp://demoroom.physics.ncsu.edu/movies.html