Transcript Induction versus Conduction

What is Electrostatics?
Examples of Electrostatics
Thunder and Lightning
Static shocks from carpet and
doorknobs
Rubbing balloons on hair and
sticking to the wall
Rubbing Comb and attracting
paper
Electrostatics and You!
 Wow! A balloon!
 Wow! A comb (and
bits of foil)!
 Wow! A Kleenex
tissue!
Polarization of Charge
Da-da da-da da-da, Charge!
 An object that has
electrical interactions
with its surroundings
is said to be
CHARGED.
 There are “positive”
and “negative”
charges.
 Like charges repel
(+&+; -&-)
 Opposites attract
(+&-).
Conductors and Insulators
Conductor:
Allows flow of electricity by
having free roaming electrons.
Examples:
Metals, water, humid air,
ionized gas, plasma, graphite,
wet wood
Insulator:
Hinders flow of electricity
by having tightly bound
electrons.
Examples:
Glass, ceramics, dry wood,
rubbers, plastics, cloth, air.
Electrical Forces
 Charge: Positive (+) and Negative (-) by
convention.
 Protons (+) are attracted to Electrons (-).
Neutrons have neutral (no) charge.
 Like charges repel; opposite charges attract.
Model of the Atom
A Beautiful Comparison
Newton’s Law of
Gravitation
Fg = G
m1m2
d2
Coulomb’s Law of
Attraction
F=k
q1q2
d2
Both are inversely proportional to
distance squared.
Product of masses versus product of
charges
Newton’s: Attractive only.
Coulomb’s: Attractive and repulsive.
Electroscopes
Test sample touches external ball. Charges
spread out onto foil leaves. Since all charges
are the same the leaves separate by
repulsion - either positive and positive, or
negative and negative. Pith balls also work.
Induction versus Conduction
Charging by Conduction:
Transferring charges by touching a charged object to an
uncharged one.
Example: Electroscope
Induction versus Conduction
Charging by Induction:
Transferring charges without touching a charged object
to an uncharged one.
Example: Large amounts of negative charges in storm
clouds induces separation of charge on ground and
lightning results.
Charge Induction
Induction versus Conduction
Charging by Induction or Conduction:
A balloon is charged negatively and sticks to the wall.
Is it charging the wall by induction, conduction or
something else?
Answer:
Neither. Even though it touches it is more like induction
than conduction. Few charges are transferred to the wall
since the wall is an insulator.
Van de Graaff Generator
The American physicist Robert Jemison
Van de Graaff invented the Van de Graaff
generator in 1931.
Charge transferred onto moving belt and up
to the metal dome on top.
Silicon tape on lower roller and rubber belt
cause charges to build up on belt that are
taken up and collected on the dome.
The results are shocking!
Van de Graaff Generator
Demonstration 1. Los Angeles Sparks.
Demonstraton 2. Space Invaders.
Demostration 3. Any longhaired freaks?
Jacob’s Ladder (Climbing Arc)
How Does a Jacob's Ladder Work?
The explanation is that an arc starts at
the bottom and due to the fact that hot
air rises, the arc tends to move up the
diverging rods until they are too far
apart for the voltage provided by the
power source. The circuit breaks and a
new arc is formed at the bottom.
Like a real lightning the charges jump
across the separation. Notice that
higher up the rods are pulled together
because there they are more flexible.
This basically is how lightning works.
Even the smell and generation of
Nitrides is similar.
Homemade Jacob's Ladder/ Climbing Arc
How can we move and use charges?
Michael Faraday (English physicist, 1831) found that:
 Change a magnetic field, create an electric current (see
video).
Wire in presence of strong magnet made current (see
video). Double the wires, double the current.
Many wrappings of wire around a nail with an applied
current makes a temporary magnet.
Hand Generator (The Ancient)
How Does a Hand Generator Light the Bulb?
Coils of wire on the axle are rotated between the poles of strong
magnets. This generates alternating current (try attaching a
galvanometer and observe the readings). The current is strong
enough to heat the filament in the light bulb.
Give it a try. Notice that when the circuit is opened it is much
easier to turn the crank. Why?
Hand Generator (The Modern)
These are much more efficient than the ancient ones. They have
their own storage cells too. Flashlight, AM/FM Radio, Warning
Siren.
Maybe someday we will have to ride a bicycle-like device while
we use our computers.
Put it in Reverse!
Just as motion can make electricity,
the reverse works as well.
Electricity can be used to generate
motion = electric motors
In Summary…
Objects become electrically charged in three
way:
1. By friction
2. By conduction (contact)
3. By induction (no contact).
Electrons can flow in wires when moved relative
to a magnet.
This is the basis for electric motors.