Download Electric Charge, Fields, Potentials, and Current

Electric Fields
and Potentials
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
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
Electric Shielding
Shielding is
important in
electronic devices
such as
televisions and
computers
Electrons repel toward the
outside of any conducting
surface
Net charge inside is zero
Electrons flow outward
evenly, but pile up on
sharp corners
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
0.000001-J/0.00000001-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
A capacitor in a simple
electric circuit.
Charge Q stored:
Q  CV
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.
+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
http://demoroom.physics.ncsu.edu/movies.html
Van de Graff Generator
http://demoroom.physics.ncsu.edu/movies.html
Van de Graff Generator
http://demoroom.physics.ncsu.edu/movies.html