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Chapter 15
Electricity and Magnetism
History of Electrostatics
• Electrostatics is the study of the nature,
behavior, and uses of static electricity.
• Benjamin Franklin is famous for his study
of electricity!
The Study of Static Electricity
The Electric Charge:
We know that all matter consists
of tiny particles called atoms which contain
protons, neutrons, and electrons.
Protons and electrons have a
certain amount of “electric charge.”
This electric charge enables them
to attract and repel each other.
The Study of Static Electricity
The Electric Charge cont…
Protons are positive (+)
Electrons are negative (-)
Electricity is the result of the transfer of
electrons.
The Study of Static Electricity
Ordinarily – the charges are in equal balance and
the atom is said to be “neutral.”
If an atom loses an electron or gains an electron, it
is “charged”..
Too many electrons? It is negatively charged.
Too many protons? It is positively charged.
The Study of Static Electricity
Opposite charges neutralize each other!!!
One of the ways to cause an atom to gain or
lose electrons is with friction.
When an objects “rub” against each other,
the electrons can “rub off” one object onto
another.
The Study of Static Electricity
• Whenever charges
are built up in two
objects by friction,
one object is charged
negatively and the
other is charged
positively.
Electric Fields
• We cannot see electric fields – we CAN
map them with imaginary lines.
• These lines are called “lines of Force”
Measuring Charge
• The more electrons an object gains or
loses, the stronger its charge will be.
• The unit of charge is the “coulomb”
• One coulomb equals 6.24 billion billion
electrons. (6.24 X 1018)
Electrostatic Laws
• Law of electric charges: opposite charges
attract each other, like charges repel each
other
• Law of electric force: strong charges
attract strongly, weak charges attract
weakly
Electrostatic Laws
• Coulomb’s Law of Electric Force
- strength of the attraction or repulsion
is directly related to the strength of the
charges and inversely related to the
distance between them.
Coulomb’s Law of Electric Force
The force between two
charges is 100 Newtons
1. What happens if the
distance d is doubled?
2. Double both charges?
3. Double d and charges?
Sharing a Charge
Contact – neutral object touches a
charged object – charged
object transfers the charges.
This can happen with positive and
negatively charged objects.
Charging by contact is
permanent!
The transfer of charges is
conserved – that is the amount
of charge is the same before
and after the transfer.
Transferring Charges
• An object’s charge
can be taken away
completely by
bringing it into contact
with the ground!
• The earth is so “huge”
it can soak up an
object’s electrical
charge.
• This is called
grounding!
Inducing a Charge
• A temporary transfer
happens without direct
contact.
• This is called “induction”
• Charging by induction
produces two “new”
charges instead of one.
• The example of this is the
balloon on your hair!
Detecting Charges
• Small charges can be detected by means
of an electroscope.
• When uncharged,
the foil leaves stay
down.
• When charged, the foil
leaves rise up.
Electrostatic Generators
• The Van de Graaff Generator
• The charges move
outward!
Protection from Lightning
• Benjamin Franklin
knew that if the
charge from a
lightning strike could
be directed into the
ground, then the
strike would be
harmless.
• So, He invented the
Lightning Rod.
Protection from Lightning
• Franklin’s famous experiment is VERY
DANGEROUS!
• Numerous people over the years have
tried to duplicate it and have died from
being struck by lighting!
• Do not attempt!
Video on Static Electricity
http://streaming.discoveryeducation.com/sea
rch/assetDetail.cfm?guidAssetID=bbb0d71
7-46f7-4fed-94350db35d27cae6&tabDisplay=myContent
Magnets and Magnetism
• The first magnet ever
used was actually a
lodestone which is a
natural magnet.
• It is a form of the
mineral magnetite!
Magnets and Magnetism
The Vikings were
the first to use
lodestones for
navigation.
The Nature of Magnets
• Magnets come in various sizes and
strengths.
• Magnets are used in televisions and
speakers and computers.
Magnetic Fields
• The property of attracting
objects by the magnetic
force is known as
magnetism.
• The region around a
magnet that other objects
are attracted to are the
magnetic field.
• Compasses are affected
by these fields.
• The iron filings represent
lines of force.
Law of Magnetic Poles
• All magnets possess poles.
• One pole always seeks North.
• One pole always seeks South.
• The Law of Magnetic Poles states:
Like poles repel and unlike poles
attract.
Law of Magnetic Poles
http://www.windows.ucar.edu/tour/link=/physi
cal_science/magnetism/bar_magnet_inter
active.html
Permeability
• Some substances can absorb or channel
the lines of magnetic force better than
others.
• The ability to do this is permeability.
• Highly permeable substances include iron,
cobalt, and nickel.
The Earth as a Magnet
• The earth’s magnetic
field reaches out into
space thousands of
miles!
• We don’t know why
we are a
magnet…scientists
think it may be that
we have electricity
circulating in the core.
The Earth as a Magnet
• The magnetic poles
do not coincide with
the geographic north
and south poles.
• The poles wander
about 5 miles every
year!
The Magnetosphere
• The earth’s magnetic
field out in space is
called the
magnetosphere.
• The magnetosphere
is distorted by the
solar wind.
• The magnetosphere
protects the earth
from the harmful
effects of the sun.
Van Allen Radiation
• Some of the sun’s
radiation is trapped in
the magnetosphere.
• These trapped
particles are called
Van Allen belts.
Aurora Borealis
• Some particles of solar
wind near the poles
sometimes funnel down
toward the earth.
• They crash into particles
of air and produce brilliant
shimmering lights known
as the Aurora Borealis.
• The frequency of these
depends on the solar
winds.
Aurora Borealis
Understanding Magnets
• Magnets are caused
by the spin of
electrons around an
atom.
• As a rule, the more
unpaired electrons an
atom has the more
likely it is magnetic.
Understanding Magnets
• Domains
– A group of aligned
atoms having a single
magnetic field is a
domain.
– A small bar magnet
will have thousands of
domains.
– A magnetic substance
forms a magnet only if
its domains are mostly
aligned.
Types of Magnetic Materials
• Diamagnetic – no unpaired electrons
• Paramagnetic – one unpaired electron
• Ferromagnetic – several unpaired
electrons.
Methods of Magnetizing
• Magnetizing by contact – may be
temporary or permanent.
• Magnetizing by induction – no contact
needed.
• Magnetizing by electricity – electricity
flows through a wire and produces a
magnetic field.
The Electromagnet
• An iron core with a
wire wrapped around
it can become a
magnet if the wire has
a current flowing
through it.
The End!!!!