Transcript non-conductors - The University of Iowa

L 25 Electricity & Magnetism [2]
• static electricity
– the charging process
– the van de Graff generator
– electrostatic shielding
• lightning
• batteries and frogs legs
• electric circuits
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review – electric charge
• Matter (stuff) has two basic properties
• mass  gravitational force
• charge  electric and magnetic forces
– positive charge
– negative charge
• electric forces
• like charges repel +/+ or - / • unlike charges attract + / -
• charge is measured in Coulombs [C]
• all charge is a multiple of the basic unit of
charge – we call this e = 1.6 x10-19 C
charges cannot be divided into smaller
units than this.
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Conductors and Non- Conductors
• Metals (copper, aluminum, iron) are
conductors of electricity  that means
that charge can move through them
• Plastics, wood, ceramics, and glass are
non-conductors (or insulators)  they do
not let electricity flow through them
• You should not stick a metal fork into an
electrical outlet!
• You could stick a plastic fork into an outlet
without electrocuting yourself, but don’t
do it!
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What makes conductors conduct?
• Atoms have equal numbers of positive and
negative charges, so that a chunk of stuff
usually has no net charge  the plusses
and minuses cancel each other.
• However, when you put a bunch of metal
atoms (like copper) together an amazing
thing happens  one electron from each
atom forgets which atom it belongs to.
• All the homeless electrons are free to
wander about inside the material
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Pure water is non-conducting
• clean water will not conduct electricity
• if salt or acid is added, however, it will
conduct electricity
H2O
carbon electrodes
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A salt water solution is a conductor
• When salt NaCl (sodium chloride) is
added to water H2O, the NaCl molecule
dissociates into a positive ion Na+, and a
negative ion Cl- .
• Thus the solutions contains both positive
and negative ions, both of which can
conduct electricity.
• Electric current can pass through dirty
bath water and through you also!
• we are conductors – water + Na+ + Cl–
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Gas discharges
When a high voltage is applied to a gas-filled tube, the gas
can become ionized, one or more electrons are separated from
each atom. Since positive and negative charges are present
the ionized gas conducts electricity. The gas atoms are excited
and emit light of a color characteristic of the gas.
PLASMA
Gas in
tube
not blood!
High Voltage
Source
7
examples of electrical discharges
the Aurora
fluorescent lamp
neon lights
8
Charging by friction
• If you rub plastic with cat’s fur, electrons
are rubbed onto the plastic making it
negative
• if you rub glass or plastic with silk,
electrons are rubbed off the glass making
it positive
• the charge can be transferred to other
objects.
• only the electrons can be transferred
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The charging process
• an object is charged positive (has a net
positive charge ) if electrons are removed
from it
• an object is charged negative (has a net
negative charge) if electrons are
transferred to it
• charges can be transferred from
conductors or non-conductors but they can
only move through conductors.
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Where is the charge?
• the charge is in atoms
– positive  protons
– negative  electrons
• matter is usually electrically neutral  it
has the same amount of positive and
negative charge
• electrons (not protons) can be transferred
from one object to another by rubbing
(friction)
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Non-conductors can
be charged too!
• Even though non-conductors do not have
free electrons meandering about, they can
be charged by friction
• When you move your comb through your
hair, the friction (rubbing) between the comb
and hair can pull some of the electrons out of
your hair and onto the comb
• as a result your comb ends up with a net
negative charge and attracts your hair which
is now positive.
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Example
• Object A has a charge of -5 C and Object B has
a charge of +5 C
• If -10 Coulombs of negative charge are
transferred from object A to object B. What is the
final charge on each object?
• ANSWER:
– object A has a net charge of +5 C
– object B has a net charge of -5 C.
-5 C
A
-10 C
+5 C
B
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One Coulomb is a HUGE charge
• To get a charge of one Coulomb
on an object we would have to
remove
6.250 x 1018
electrons from it!
• In the capacitor discharge demo, only
0.01 C of charge were involved.
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Attracting uncharged objects
+
+
+
+
uncharged
metal sphere
• A negatively charged
rod will push the
electrons to the far
side leaving the near
side positive.
• The force is attractive
because the positive
charges are closer to
the rod than the negative
charges
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You can bend water with charge!
The water molecule
has a positive end and
a negative end.
charged rod
When a negative rod is
brought near the stream
of water, all the positive
ends of the water molecules turn to the right
and are attracted to the
negative rod.
stream of water
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The Magic Wand
2x4
We can cause the 2 x 4 to move
with electric forces
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Seeing the effects of charge:
the electroscope
• the electroscope is a simple
device for observing the
presence of electric charge
• it consists of a small piece of
metal foil (gold if possible)
suspended from a rod with a
metal ball at its top
• If a negatively charged rod is placed near the ball,
the electrons move away because of the repulsion.
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The two sides of the metal foil then separate.
Danger High Voltage !
• The van de Graff can
charge the sphere to
more than 50,000 volts!
• This is enough to cause
discharges to the
surrounding air 
ionization or breakdown
• The sparks excite air
molecules which give off
light
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Making Sparks:
The Van de Graff Generator
• The van de Graff generator
is a device for building up a
large electrical charge on a
metal sphere.
• The charge is generated
by friction between a
rubber belt and a roller.
• the charge on the belt is
transferred to the sphere
by a brush.
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Electric Potential  voltage
9 Volt
battery
• The amount of charge on a
charged sphere can be
measured in terms of its
electric potential or voltage
• the more charge that is on the
sphere, the higher its voltage
• electric potential is measured in
VOLTS
• if I connect a 9 V battery to
the sphere and the ground, it
will have a potential of 9 V
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applications of electrostatics
• Xerox copiers use
electrostatic attraction to
put the ink droplets on the
paper
• electrostatic precipitators
use the attraction of
charged dust to remove
dust particles from smoke.
• can be used to hold
balloons on your head
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Removing soot particles
Positive
cylinder
Chimney
stack
soot
Charging units
spray electrons
on the soot
particles
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Electrostatic shielding
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Electrostatic shielding
• The effect of the high voltage on the van
de Graff generator stops on the outside of
the metal cage  Homer is SAFE!
• Being inside your car during a lightning
storm offers you some protection
• radio signals cannot penetrate through a
metal enclosure
• the metal bars (rebar) that reinforce the
concrete in walls can also interfere
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Lightning- outdoor spark





causes 80 million
dollars in damage each
year in the US
On average, kills 85
people a year in the US
is all over in a
thousandth of a second
carries up to 200,000 A
causes the thunder!
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development of a lightning bolt
charge
separation
stepped
leader
leader &
streamer
leader meets
streamer
lightning
bolt 27
Frog's leg Batteries
• in 18th century Luigi Galvani a professor of
anatomy at the University of Bologna
found that a freshly dissected frog leg
hung on a copper hook twitched when
touched by an iron scalpel.
• The two metals had to be different.
• Galvani thought that he had discovered
the secret life force
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Alessandro Volta
• Professor of Physics at the University of
Pavia realized that the electricity was not in
the frog’s leg but the twitching was the
result of touching it with two different metals
• Volta had discovered the first battery.
• Lemon battery
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Batteries  use chemical energy
to produce electricity
• two dissimilar metals immersed in a conducting
fluid (like an acid for example) cause a chemical
reaction which can produce electric current.
zinc
electrode
copper
electrode
acid
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Inside a Duracell 1.5 Volt battery
Metal Cap
plastic case
+
Carbon center
electrode
Electrolyte
paste
Zinc outer
electrode
- Bottom
electrode
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Current– flow of electric charge
If I connect a battery to the ends of the
copper bar the electrons in the copper will
be pulled toward the positive side of the
battery and will flow around and around.
 this is called current – flow of charge
copper
An electric circuit!
Duracell
+
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Electric current (symbol I)
• Electric current is the flow of electric
charge q (Coulombs)
q
• It is the amount of charge q that passes a
given point in a wire in a time t, I = q ÷ t
• Current is measured in amperes
• 1 ampere (A) = 1 C / 1 s
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Potential difference or Voltage
(symbol V)
• Voltage is what causes charge to move in
a conductor
• It plays a role similar to pressure in a pipe;
to get water to flow there must be a
pressure difference between the ends, this
pressure difference is produced by a pump
• A battery is like a pump for charge, it
provides the energy for pushing the
charges around a circuit
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Voltage and current are not the
same thing
• You can have voltage, but without a path
(connection) there is no current.
An
electrical
outlet
voltage
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