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SPH4U1 – FIELDS – L6 Magnetism
MAGNETISM
The concept of magnetism was first discovered in a piece of iron ore in the city of
Magnesia some 2500 years ago. However, magnetism has only been the focus of
physics research for the past 200 years. Physicists are still not able to explain the
magnetic characteristics of the neutron, the proton, or the origin of the Earth’s magnetic
field. Scientists are continually discovering new ways to use the currently established
principles of magnetism and electromagnetism.
THINGS WE KNOW

A bar magnet is composed of both a north and a south pole.
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Opposite magnetic poles attract.
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Similar magnetic poles repel.
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The force felt around a magnet is caused by a magnetic force field.
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Magnetic field lines that are close together indicate a strong field.
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Magnetic field lines emerge from the N pole and curve towards the S pole.
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The magnetic field is a vector quantity, represented by the symbol 
EARTH’S MAGNETIC FIELD
The earth is a large magnet due to its iron core. (a compasses’ north pole points towards
the earth’s geographical north pole which is in fact a magnetic south pole).
SPH4U1 – FIELDS – L6 Magnetism
The angle between magnetic north and geographic north varies from position to
position on the Earth’s surface. This angle is called MAGNETIC DECLINATION. If
navigating with a compass, this angle must be known, so that a true north can be
established. The Earth’s magnetic field is three dimensional, with both horizontal and
vertical components. The angle between Earth’s magnetic field and the horizontal is
called the MAGNETIC INCLINATION. Inclination and Declination charts have to be
revised from time to time because earth’s magnetic field is slowly changing. These
changes result from the rotation of the magnetic field about the Earth’s axis.
DOMAIN THEORY
In 1819 Hans Christian Oersted discovered that an electric current produced a magnetic
field. From this, he formulated the Principle of Electromagnetism which states:
Moving electric charges produce a magnetic field
Conversely, in 1830 Michael Faraday noticed that a moving magnet in a coil of wire
generates an electric current in that coil.
http://www.youtube.com/watch?v=7MTTyWzX1EY&feature=related
RIGHT HAND RULES
Right Hand Rule for a straight conductor: If a conductor is grasped in the right hand, with
the thumb pointing in the direction of the current, the curled fingers point in the
direction of the magnetic field lines.
NOTE: Conventional current direction is being used, not electron flow direction
SPH4U1 – FIELDS – L6 Magnetism
http://www.walter-fendt.de/ph14e/mfwire.htm
A solenoid is a long conductor wound into a coil of many loops. The many loops help to
strengthen the magnetic field being produced.
Right hand rule for a solenoid: If a solenoid is grasped in the right hand, with the fingers
curled in the direction of the electric current, the thumb points in the direction of the
magnetic field lines in its core.
SPH4U1 – FIELDS – L6 Magnetism
HMWK: Pg. 391 #4,5
MAGNETIC FORCES
Electric Field


An electric charge at rest creates an electric field around it.
A force (FE = qε) acts on any charge placed in that field.
Magnetic Field
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
A moving charge or current creates a magnetic field.
A magnetic field exerts a force on any moving charge or current in that field.
A force acts on a moving charge that is within a magnetic field. When the charged
particle enters the magnetic field at an angle to the field lines, it experiences a force. If
the particle is unable to escape the field, it will follow a circular path. The magnitude of
the magnetic force on a charged particle is directly proportional to the magnitude of the


magnetic field (  ), the velocity ( v ) and the charge (q) of the particle.
FM = qvβsinθ
or
FM = qv x β (cross product)
Where
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
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q is the charge within the magnetic field
v is the velocity of the charged particle
β is the magnetic field strength, measured in Tesla (1N/Cm/s)
NOTE: The force is at its maximum when the particle moves perpendicular to the
magnetic field and vanishes when the particle moves parallel (0° or 180°).
The direction of the force can be found using another right hand rule. (thumb =
direction of current, finger =direction of magnetic field & palm = direction of force).
SPH4U1 – FIELDS – L6 Magnetism
EX 1: An electron accelerates from rest in a horizontally directed electric field through a
potential difference of 46 Volts. The electron then leaves the electric field, entering a
magnetic field of magnitude of 0.2 T directed into the page. Calculate for the initial
speed of the electron upon entering the magnetic field, the magnitude and direction of
the magnetic force on the electron and the radius of the electron’s circular path.
a)
-ΔEe = ΔEk
qV = ½mv2
v = [2qV/m]-½
v = [2(1.6 x 10-19 C)(46 V)/9.11 x 10-31 kg] -½
v = 4.0 x 106 m/s
b)
Fm = qvβsinθ
Fm = (1.6 x 10-19 C)(4.0 x 106 m/s)(0.20 kg/C.s)sin90o
Fm = 1.3 x 10-13 N
SPH4U1 – FIELDS – L6 Magnetism
c)
Fm = Fc
qvβ = mv2/r
r = mv/βq
r = 99.11 x 10-31 kg)(4.0 x 106 m/s)/(0.20 T)(1.6 x 10-19 C)
r = 1.1 x 10-4 m
SPH4U1 – FIELDS – L6 Magnetism
http://lectureonline.cl.msu.edu/~mmp/kap21/cd533capp.htm
http://webphysics.davidson.edu/applets/ibe/Velocity_X_B.html
http://www.youtube.com/watch?v=o1z2S3ME0cI&feature=related
Charge-to-Mass Ratios
J.J. Thomson’s experiment used both an electric and magnetic field (cathode ray tube) to
determine the ratio of charge to mass of an electron. First, two plates (positive and
negative) generated an electron beam. The positive plate had a slit in it. Next, the
electron beam went through both an electric and magnetic field where the forces
cancelled each other out.
Fm = Fc
evβ = mv2/r
e = charge of an electron
e/m = v/βr
also
Fm = Fe
evβ = eε
therefore
v = ε/β
Combining the two equations
e/m = ε/β2r
which has been calculated to be = 1.76 x 1011 C/kg
SPH4U1 – FIELDS – L6 Magnetism
HMWK: Pg. 396 #2-6
Pg. 402 #1,2