Magnetism
Chapter 7
J. C. Rowe
Course Instructor
A Brief History Of Electromagnetism
About 2500 yrs ago, Greeks discovered that when Amber resin
was rubbed together it attracts leaf pieces
Around the 18th century it was discovered that there are 2 types
of electricity; Negative & positive electric currents e.g. rub
amber = -ve charge while rub glass = +ve charge
Sir Ben Franklin suggested that all substances possess certain
“electric fluids”…
600 BC – The lodestone (Fe3O4) is discovered
Sushruta, ~ 625 BC - 545 BC, Indian surgeon mentioned its use
Magnets Cure:
 Heart Disease
 Gout
 Restores marital harmony
 increase elegance & Charm
1175 - First Reference to a Compass
Alexander Neckem an English monk of St. Albans describes the
workings of a compass.
1269 - First Detailed Description of a Compass
Petrus Peregrinus de Marincourt, a French Crusader, describes a
floating compass and a compass with a pivot point.
1600 - Static Electricity (De Magnete)
W. Gilbert 3 ways to magnetize a steel needle: by touch with a
loadstone; by cold drawing in a North-South direction; and by
exposure for a long time to the Earth's magnetic field while in a
North-South orientation.
1819 with work by Hans Orsted - An understanding of the
relationship between electricity and magnetism began its was
discovered more or less by accident that an electric current could
influence a compass needle
Magnetism Defined
1 - phenomenon associated with the motion of electric
charges.
2 - Magnetism is also associated with elementary
particles, such as the electron, that have a property
called spin.
This motion can take many forms. It can be an electric
current in a conductor or charged particles moving
through space, or it can be the motion of an electron
in atomic orbit.
Basic Facts:
• Magnet originates from the
Greek word “magnetite”
• A magnetite is a naturally
magnetic ore.
• Natural elements that are
strongly attracted to magnets are
called ferromagnetic e.g.. Iron,
nickel, cobalt
Are you Curious About:
What is a magnetic field?
The region around a magnetic material or a moving electric
charge within which the force of magnetism acts
What is a magnetic force?
This is the attraction or repulsion that arises between electrically
charged particles that are in motion. While only electric
forces exist among stationary electric charges, both electric
and magnetic forces exist among moving electric charge
What is the relationship between magnetism and electricity?
Magnetic properties are due to atomic charges, all electrons
are charges in motion. Therefore all moving electrons can act
like tiny magnets. But this isn't true because the magnetic
fields produced by individual electrons are very weak but
when tiny magnetic fields overlap in proper orientation in
areas called magnetic domains, they become a bigger and
stronger magnetic field. In natural magnets enough magnetic
fields work together to produce a noticeable magnetic
properties that we can actually see
Magnetic Poles
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A magnet has 2 poles
South pole or negative pole
North pole or positive pole
Like poles repel while opposite poles attract
If you break a bar magnet you will end up 2
magnets
All magnets make a magnetic field. You can not see
this field but you can map it by placing a piece of
paper over a magnet and pour iron fillings on top. You
would something like the picture below.
Earth and Magnetism
• The south pole of earths magnetic field is actually
located near the earths geographic north pole.
• The earths magnetic poles change on a daily basis
So
• From a magnetic stand point when we head in the direction
of the geographic north, are we headed to the magnetic
North or Magnetic South ?
Magnetic field around current carrying wires
Danish physist Hans Christian Oersted discovered the
connection between electricity and magnetism. He completed
a circuit which caused a deflection of a near by compass
Determining the Direction of the Current by Right Hand Rule:
1. Point the thumb of your hand in the conventional currents
direction (into or out of the page) opposite to the electron
flow if it is given.
2. Curl fingers in. that is the direction of the magnetic field
around the wire
Deduce the Magnetic Field
Direction
If you are told electron flow is to the Right of
this slide… where is the current direction ?
If you are told the electron flow is towards the
ceiling… where is the current flow? Where is
the direction of the magnetic flow ? Use the
Right Hand Rule…
What if the electron flow is pointing to the
ground ?
Solenoids
a coil of wire, usually surrounding a movable iron core, that
acts as a magnet when carrying a current: used as an
electromagnetic switch or relay
a long, thin loop of wire, which is
usually wrapped around a metal
and causes a uniform magnetic field
to be generated when an electric
current is passed through it.
Magnetic Fields Around a Solenoid
A solenoid is a coil around a piece of metal.
Ex. A wire around a nail.
• Creates a stronger magnetic field
• Solenoid acts as a bar magnet (develops south/north pole)
Right Hand Rule for Solenoids:
1. Find conventional flow direction from the front of 1 of
the loops
2. Point your finger in the direction of the conventional
current
3. Your fingers will be pointing south
4. Curl fingers to show direction of magnetic field
Magnetic Field Strength
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All magnets are a different shape, size, and strength
The magnitude of the force experienced by a charged object
moving perpendicularly through a magnetic field
F=qvB
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F is force
Q is particle charge
V is velocity
B is strength of magnetic field
When it is not perpendicular
F = qsinX
B=F/qvsinX
HOW TO FIND THE MAGNITUDE OF THE FORCE
OF A WIRE IN THE MAGNETIC FIELD:
F= I L B SINX
F = Force
I = Current
L = Length
B = Magnetic Field
X = angle
(when force is parallel magnetic force is equal to zero)
Right Hand Rule for Straight Wires:
1. Point thumb in the direction of velocity for positive
opposite direction for negative
2. Point index finger to the direction of magnetic field
3. Point middle finger of your right hand in the direction
that is perpendicular to both thumb and the index. This
gives the direction of force
Extended Right Hand Rule
This is used for Straight Wires
Electromagnetic Induction:
Experiments showed that moving or turning a wire
will induce a current. found by Michael Faraday
and Joseph Henry. This was one of the biggest
discoveries in history.
Magnetic flux:
Is the change in current by increasing the number of
loops or turning the wire.
Magnetic flux= magnetic field x area where field
passes x Cos of angle
Theta  angle = angle between the field lines & the
surface of the area they pass through
Faraday’s Law
The instantaneous EMF resulting from magnetic
induction equals the rate of change of magnetic flux
Or….
Any change in the magnetic environment of a coil of
wire will cause a voltage (EMF) to be "induced" in the
coil. .. This induced EMF is = rate of change of flux
Faradays law:
Lenz’s Law:
An induced EMF (voltage) gives rise to a current
whose magnetic field opposes the change in
magnetic flux that produced it.