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Magnetism
Magnets, Magnetic Poles, and
Magnetic Field Direction
Magnets have two
distinct types of poles;
we refer to them as
north and south.
Magnets, Magnetic Poles, and
Magnetic Field Direction
Like magnetic poles repel, and unlike poles
attract.
Magnets, Magnetic Poles, and
Magnetic Field Direction
Two magnetic poles of opposite kind form a
magnetic dipole. All known magnets are
dipoles; magnetic monopoles could exist but
have never been observed.
A magnet creates a magnetic field:
The direction of a magnetic field (B) at any location is
the direction that the north pole of a compass would
point if placed at that location.
Magnets, Magnetic Poles, and
Magnetic Field Direction
North magnetic poles are attracted by south
magnetic poles, so the magnetic field points
from north poles to south poles.
The magnetic field may be represented by
magnetic field lines.
The closer together (that is, the denser) the B field
lines, the stronger the magnetic field. At any location,
the direction of the magnetic field is tangent to the
field line, or equivalently, the way the north end of a
compass points.
Magnetic Field Strength and
Magnetic Force
A magnetic field can exert a
force on a moving charged
particle.
Magnetic Field Strength and
Magnetic Force
The magnitude of the force is proportional
to the charge and to the speed:
Or
F = qvB
SI unit of magnetic field: the tesla, T
B = Magnetic Field (T)
F = Force (N)
Q = charge (C)
V = velocity (m/s)
Magnetic Field Strength and
Magnetic Force
In general, if the particle is moving at an
angle to the field,
The force is perpendicular to both the
velocity and to the field. So if the particle
is not moving perpendicular to the field,
this formula must be used. If the particle
is moving parallel to the field, then there
is no force on it.
Magnetic Field Strength and
Magnetic Force
For a clearer picture,
check page 623 in your text book.
Magnetic Field Strength and
Magnetic Force
• A) No magnetic force acts on a charge
moving with a velocity v that is parallel to a
magnetic field B.
• B) The charge experiences a maximum force
F when the charge moves perpendicular to
the field.
• C) If the charge travels at an angle θ with
respect to B, only the velocity component
perpendicular to B gives rise to a magnetic
force F, which is smaller than the one in B).
This component is v sin θ
Magnetic Field Strength and
Magnetic Force
A right-hand rule gives the
direction of the force.
THE RIGHT HAND RULE #1
Extend the right hand so the
fingers point along the
direction of the magnetic field
B and the thumb points along
the velocity v of the charge.
The palm of the hand then
faces in the direction of the
magnetic force F that acts on
a positive charge.
Magnetic Field Strength and
Magnetic Force
• If the moving charge is negative instead of
positive, the direction of the magnetic force
is opposite to that predicted by the Right
Rule #1.
Always assume the charge is positive when
applying the rule, and then simply change
the direction if it is actually a negative
charge.
Magnetic Forces on Current-Carrying
Wires
• Any charged particle moving in a
magnetic field will experience a
magnetic force.
• Since electric current is composed of
moving charges, a current carrying
wire will also experience a magnetic
force when it is placed in a magnetic
field.
Magnetic Forces on CurrentCarrying Wires
The magnetic force on a current-carrying wire
is a consequence of the forces on the
charges. The force on an infinitely long wire
would be infinite; the force on a length L of
wire is:
θ is the angle
between I
and B.
**Note L = v/t
Magnetic Forces on Current-Carrying
Wires
• If the current is parallel to or directly opposite the
magnetic field, then the force on the wire is zero.
• If the current is completely perpendicular to the to
the magnetic field, then the equation becomes
F = ILB
Where I is the current (A)
L is the length of the wire (m)
B is the magnetic field (T)
F is the force on the wire (N)
The strength of a magnetic field is directly proportional
to the amount of current in the wire.
Magnetic Forces on CurrentCarrying Wires
The direction of the force is given by a righthand rule:
When the fingers of the right hand are pointed in the
direction of the conventional current I and then curled
toward the vector B, the extended thumb points in the
direction of the magnetic force on the wire.
Magnetic Forces on Current-Carrying
Wires
• You can also use the ‘same’ right hand
rule but replace the velocity with
current
• When the fingers of the right hand are
extended in the direction of the magnetic field
and the thumb pointed in the direction of the
conventional current I carried by the wire,
the palm of the right hand points in the
direction of the magnetic force on the wire
Magnetic Forces on Current-Carrying
Wires
• If we have two parallel straight wires that carry
current in the same direction, the force between
them will be attractive
• If the wires have current that is in the opposite
direction of each other, the force between them will
be repelling
Extra: Charged Particles in Magnetic
Fields
A cathode-ray tube, such as a television or
computer monitor, uses a magnet to direct a
beam of electrons to different spots on a
fluorescent screen, creating an image.
Extra: Geomagnetism: The Earth’s
Magnetic Field
The Earth’s magnetic
field is similar to that of
a bar magnet, although
its origin must be in the
currents of molten rock
at its core.
Its magnitude is
approximately 10–5 to
10–4 T.
Extra: Geomagnetism: The Earth’s
Magnetic Field
The magnetic poles are
not in exactly the same
place as the geographic
poles; when navigating
with a compass, you
need to know the angle
between them, called
the declination, at your
position.
Extra: Geomagnetism: The Earth’s
Magnetic Field
Charged particles can become trapped around
magnetic field lines. Such trapping of solar wind
particles has resulted in bands of charged
particles around the Earth called Van Allen belts.