Magnetism
What is magnetism?
Magnetism is the properties and interactions of
magnets
The earliest magnets were found naturally in the mineral
magnetite which is abundant the rock-type lodestone.
These magnets were used by the ancient peoples as
compasses to guide sailing vessels.
Magnets produce magnetic forces and have magnetic
field lines
Iron Filings lab
• Go to the various tables and make
sketches of the images you see with iron
filings and magnetic fields.
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Magnets have two ends or poles, called north and
south poles. At the poles of a magnet, the magnetic field
lines are closer together. The more lines the greater the flux.
Unlike poles of magnets attract
each other and like poles of
magnets repel.
Magnetic Field Lines
• Are vector, have the symbol B, and the SI
units are the Tesla (T). Gauss is another
unit sometimes used.
• The arrows go from North to South
• They can up, down,left, right, into the
page, or out of the page. Look at the
examples drawn on the smart board…
The earth is like a giant magnet!
The nickel iron core of the earth gives the earth a
magnetic field much like a bar magnet.
How do compasses work?
Section Review p. 769
• Answer problem 1-3
What are magnetic domains?
Magnetic substances like iron, cobalt, and nickel are
composed of small areas where the groups of atoms are
aligned like the poles of a magnet. These regions are
called domains. All of the domains of a magnetic
substance tend to align themselves in the same direction
when placed in a magnetic field. These domains are
typically composed of billions of atoms.
How do
you
make a
magnet?
Magnetic Field of a current
carrying wire
• A wire that has an electric current going
through it will have a magnetic field around
it. The direction of the current determines
the direction of the field.
• The right hand rule will show you the field.
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Electricity and Magnetism – how
are they related?
When an electric current passes through a wire a
magnetic field is formed.
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Right hand rule in a current
carrying wire
• See image on page 770
Magnetic Field of a current loop
• See images on page 771
• Solenoids – continuous loops of wire, they
form strong magnets on the inside.
• How can you tell North / South?
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What is an electromagnet?
When an electric current is passed through a coil of wire
wrapped around a metal core, a very strong magnetic
field is produced. This is called an electromagnet.
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Charged Particle in a Magnetic Field
• Stationary charges don’t interact with
magnetic fields, but moving charges do.
• This force is maximum when the charges
move perpendicular to the magnetic field.
Right Hand Rule
• B Magnetic Field
4 fingers
• V velocity of moving charge
thumb
• F force on a positive charge palm of hand A
negative charge would have the opposite
direction of force.
• Arrows into a page are drawn as X’s, arrows out
of the page are drawn as points.
• Study Image on page 774
• Answer questions on page 783 #35
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B, to the right
V, up the page
What would be the direction of force on an electron? A proton?
X
Force is into the
page, Magnetic field
is down the page.
What is the direction
of velocity for a
positive charge?
What is the direction
of velocity for a
negative charge?
Fmag= qvB
• Fmag = magnetic force on a charged particle (N)
Newton.
• Q= magnitude of charge (C) Coulomb
• V = velocity of charged particle(m/s)
• B= Magnetic Field (T) Tesla.
• Sample prob. P.774
• Practice probs 21A p.775 (1-3)
Magnetic Force on a current
carrying wire
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http://www.youtube.com/watch?v=tUCtCYty-ns
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http://www.youtube.com/watch?v=43AeuDvWc0k
2 parallel wires carrying current, they attract if the current
is in the same direction
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2 current carrying wires repel if their currents run in
opposite directions.
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Loudspeakers work by this idea
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Fmag = BIL
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Fmag = magnetic force (N)
B = magnetic Field (T) Tesla
I = current (A) Ampere
L = length of conducting wire in the
magnetic field.
• Sample prob. P. 778
• Prac. Probs 21B page 778 (1-3)
What is a galvanometer?
A galvanometer is an electromagnet that interacts with a
permanent magnet. The stronger the electric current
passing through the electromagnet, the more is interacts
with the permanent magnet.
Galvanometers are
used as gauges in
cars and many other
applications.
The greater the current passing through the wires, the stronger
the galvanometer interacts with the permanent magnet.
Galvanometer
• A simple instrument designed to detect
electric current.
• When calibrated to measure current, it is
an ammeter.
• When calibrated to measure voltage, it is a
voltmeter.
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Electromagnetic Induction
• The production of an emf (electro-motive
force – kinda like voltage) in a conducting
circuit by a change in the strength,
position, or orientation of an external
magnetic field.
• Further explained on page 794
Faradays Law
• Hans Christian Oersted discovered magnetism
and electricity are related.
• Magnetism is produced by the motion of
electrons.
• Voltage is induced in a loop of wire if there is a
change in the magnetic field in the loop.
• If a magnets in and out motion through a coil of
wire is doubled, the voltage is doubled.
Faradays Law
• If the number of loop is doubled, the
induced voltage is doubled.
• A rapidly changing magnetic field in any
region of space induces a rapidly changing
electric field. Which in turn induces a
rapidly changing magnetic field.
• This generation and regeneration of
electric and magnetic fields makes up
electromagnetic waves.
Faradays Law
• The induced emf (electromotive force) in
any closed circuit is equal to the time rate
of change of the magnetic flux through the
circuit.
• It is the operating principle of transformers,
inductors, many types of motors and
generators.
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Lenz Law
• An induced current is always in
such a direction as to oppose
the motion or change causing it
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Induction
• Wires spinning in magnetic fields is what
underlies all electric motors and electric
generators.
• Most of the rest of chapter 22 deals with
applications of this. Electric motors,
electric generators, and transformers will
be as far as we go down this road. Take
AP Physics next year to get more.
What are electric motors?
An electric motor is a device which changes electrical
energy into mechanical energy.
How does an electric motor work?
Go to the next slide 
Simple as that!!
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We have seen how electricity can produce a magnetic
field, but a magnetic field can also produce electricity!
How?
What is electromagnetic induction?
Moving a loop of wire through a magnetic field produces
an electric current. This is electromagnetic induction.
A generator is used to convert
mechanical energy into electrical energy by
electromagnetic induction.
Carefully study the next diagrams:
YouTube animations
http://www.youtube.com/watch?v=Xi7o8cMPI0E
http://www.youtube.com/watch?NR=1&feature=endscreen&v=d_aTC0iKO68
Direct current versus alternating current –
AC vs DC : What’s the difference?
Direct current is electrical current which comes from a
battery which supplies a constant flow of electricity in
one direction.
Alternating current is electrical current which comes
from a generator. As the electromagnet is rotated in the
permanent magnet the direction of the current alternates
once for every revolution.
Go to this website and click the button for DC then for
AC to visually see the difference between the two.
You can see that the DC source is a battery – current
flows in one direction. The AC source is the generator
and the current alternates once for each revolution.
Mutual Inductance
• A measure of the ability of one circuit
carrying a changing current to induce an
emf in a nearby circuit.
• See page 814
• This only works in alternating current (ac)
• This explains ……..
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Transformers
• A device that changes one ac potential
difference to a different ac potential
difference. Only works for AC!
Power companies increase voltages for long
distance transmission, then they must
decrease voltages before going into your
home.
We say the voltage has been stepped up or
it has been stepped down.
Transformers
• A transformer can step up voltage but in
no way can it step up energy and power.
• A transformer operates on ac because the
magnetic field within the iron core must
continually change.
Transformers
• They have 2 sides – primary and
secondary
• Primary is the side closest (wired) to the
generator
• Secondary is the side wired to the resistor
or the consumer.
• A soft Iron core connects both sides.
Transformer Animation
http://www.youtube.com/watch?v=2-Ijjm7if5g&feature=related
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Transformer Equation
V2N1 = N2V1
V = voltage
N = number of turns or coils of wire
1 = Primary
2 = Secondary
Sample problem page 817
Practice 22D page 818 (1-3)
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This concludes our Magnetism
topic