Magnetism
AP-Physics
Magnetism
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In physics, magnetism is one of the phenomena
by which materials exert attractive or repulsive
forces on other materials.
Some well-known materials that exhibit easily
detectable magnetic properties are nickel, iron,
cobalt, gadolinium and their alloys; however, all
materials are influenced to greater or lesser
degree by the presence of a magnetic field.
Magnets
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By convention, in
physics we say that
magnetic fields leave
the magnetic north pole
of a magnet and enter
the magnetic south
pole.
Magnets
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Iron filings can be placed
on clear film with a bar
magnet placed below the
film to reveal the magnetic
field lines of the magnet.
As you can see, the field
lines look very similar to
the electric field lines
between two oppositely
charged particles
The Earth as a Magnet
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The Earth has a magnetic
field associated with it due
to the flow of its molten
iron core.
Notice that the North Pole
is really the magnetic south
pole.
Compass needles point
toward the direction of the
Earth’s magnetic field, thus
pointing north.
Source of Magnetism
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Every electron, on account of
its spin, is a small magnet.
In most materials, the
countless electrons have
randomly oriented spins,
leaving a net magnetic effect
of zero on average.
However, in magnetic
substances, a majority of the
electron spins are aligned in
the same direction, so they act
cooperatively, creating a net
magnetic field.
Sources of Magnetism
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Electron spin is not the only
source of magnetism.
Hans Christian Oersted is
credited with being the first
person to deduce that moving
currents also create magnetic
fields.
While preparing for an
evening lecture on April 21,
1819, Oersted developed an
experiment which provided
evidence that surprised him.
Sources of Magnetism
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As he was setting up his
materials, he noticed a compass
needle deflected from magnetic
north when the electric current
from the battery he was using
was switched on and off.
This deflection convinced him
that magnetic fields radiate from
all sides of a wire carrying an
electric current, just as light and
heat do, and that it confirmed a
direct relationship between
electricity and magnetism.
Charged Particles in a Magnetic Field
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When moving through a magnetic field, a charged particle
experiences a magnetic force.
This force is greatest when the particle enters the field
perpendicular to the direction of the field (B). (θ = 90)
The force is zero when it moves along the field lines.
The force on the particle can be found with the formula:
F = qvBsinθ
F = force on particle
q = charge on particle
v = velocity of particle
B = magnetic field strength (measured in Teslas or N/A*m)
Small electric fields are sometimes measured in Gauss’s
1 T = 1 x 104 G
A Few Typical B Values
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Conventional laboratory magnets
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Superconducting magnets
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25000 G or 2.5 T
300000 G or 30 T
Earth’s magnetic field
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0.5 G or 5 x 10-5 T
Finding the Direction of Magnetic
Force
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The direction of the
magnetic force is
always perpendicular
to both B and V
Fmax occurs when B is
perpendicular to V.
F = 0 when B is
parallel to V.
Open Hand Right Hand Rule
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If we point our fingers in
the direction of the
magnetic field and let our
thumb point in the
direction of the moving
charge, the magnetic force
will leave our palm.
(you must use right hand
and will only work for
positive charges – negative
charges will go out the
back of your hand)
RHR Practice
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Determine the direction of the force on a positive test charge
on each of the 6 diagrams below. Use up page, down page,
left, right, out of page, into page and no force as your possible
answers.
Answers
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A. out of page
B. Right
C. up the page
D. Right
E. Down
F. No force
Example Problem #1
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A proton moving to the right at 6 x 105 m/s enters a
magnetic field of 8 Teslas directed into the page at
an angle of 90°. What is the magnitude and
direction of the force on the proton?
V = 6 x 105 m/s
B=8T
Θ = 90°
q = 1.6 x 10-19 C
F = (1.6 x 10-19)(6 x 105)(8)(sin90)
F = 7.68 x 10-13 N up the page
Example Problem #2
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A proton is moving vertically upward with a velocity of 4.2 x
105 m/s. The magnetic field is north with a strength of 2 T.
What is the magnitude and direction of the force?
(change in terminology common in different texts)
Vertically upward – out of page
Vertically downward – into page
North – up page
South – down page
West – left
East - right
F =1.34 x 10-13 N to the left
Example #3
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An electron is moving west. It enters a
magnetic field directed into the page. In what
direction is the force on the charge?
North or up the page.
Particle in Magnetic Field
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A charged particle
moving in a plane
perpendicular to a
magnetic field will
move in a circular
orbit with the magnetic
force playing the role
of centripetal force.
The direction of the
force is given by the righthand rule.
Example Problem #4
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Derive an equation for the radius of a charged
particle moving in an magnetic field.
F = qvBsinθ
Fc = mac
ac = v2/r
mv2/r = qvBsinθ
r = mv/qB
Magnetism in a Wire
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Because current is moving
charges, we can conclude
that there must be a
magnetic force acting on a
wire carrying an electric
current.
The magnetic field due to a
current carrying wire
circulates around the wire
in a direction given by
what is usually called the
closed right hand rule.
Closed Right Hand Rule
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To determine the
direction of the
magnetic field around
a wire, you just need
to visualize yourself
wrapping your right
hand around the wire
and pointing your
thumb in the
direction of the
current flow.
Force on Wire in Magnetic Field
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F = BILsinθ
F = magnetic force
acting on wire
B = magnetic field
strength
I = current in wire
L = length of wire
Θ = angle between I
and B
Magnitude of the Field of a Long
Straight Wire
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The magnitude of the
field at a distance r from a
wire carrying a current of
I is:
µo = 4  x 10-7 T.m / A
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µo is called the
permeability of free space
Direction still found with
RHR.