Electric Fields and Potentials
Electric Force Electricity exerts a force similarly to gravity. F e = kq 1 q 2 r 2 where q 1 and q 2 represent the amount of charge in Coulombs (6.24 x ), r is in meters and k is the electrical constant (9 x 10 9 Nm 2 /C 2 ) 1 Coulomb of electrons travels through a 100-W lightbulb in about one second
Electric Fields Just like gravity field, charges have a force field (E) as well, measured in force per unit charge E = F = kQ q r 2 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 positiveField 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
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 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 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 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 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
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
Capacitors and Capacitance 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 A capacitor in a simple electric circuit.
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. +Q-Q +Q-Q
Capacitor Applications Computer RAM memory and keyboards. Electronic flashes for cameras. Electric power surge protectors. Radios and electronic circuits. Power supplies –.–.
capacitor
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