Electric Fields and Potentials Electric Force Electricity exerts a force similarly to gravity. F e =...
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Transcript of Electric Fields and Potentials Electric Force Electricity exerts a force similarly to gravity. F e =...
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
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
–.
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