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Magnetism
Magnetism
Chapter 19 Problems
19-3 1,2,5,7
19-4 11,15,17
19-5 19,21,23
19-7 30-33,
19-8 36,38,39
19-9 42,43
19-11 45,47
Magnetism
OBJECTIVES
After studying the material of this chapter, the student should be able to
1. Draw the magnetic field pattern produced by iron filings sprinkled on
paper placed over different arrangements of bar magnets.
2. Determine the magnitude of the magnetic field produced by both a
long, straight current carrying wire and a current loop. Use the right hand
rule to determine the direction of the magnetic field produced by the
current.
3. Explain what is meant by ferromagnetism, include in your explanation
the concept of domains and the Curie temperature.
4. State the conventions adopted to represent the direction of a magnetic
field, the current in a current carrying wire and the direction of motion of
a charged particle moving through a magnetic field.
Objectives
 5. Apply the right hand rule to determine the direction of the force on
either a charged particle traveling through a magnetic field or a current
carrying wire placed in a magnetic field.
 6. Determine the magnitude and direction of the force on a current
carrying wire placed in a magnetic field and a charged particle
traveling through a magnetic field.
 7. Determine the torque on a current loop arranged in a magnetic field
and explain galvanometer movement.
 8. Explain how a mass spectrograph can be used to determine the
mass of an ion and how it can be used to separate isotopes of the same
element
KEY TERMS AND PHRASES
 bar magnet
galvanometer movement
 north pole
magnetic moment
 south pole
permeability of free space
 magnetic field
ferromagnetism
 magnetic field strength
right hand rule
 mass spectrograph
Curie temperature
MAGNETS AND MAGNETIC
FIELDS
 Two BAR MAGNETS exert a force on one another. If two
NORTH POLES (or SOUTH POLES) are brought near,
a repulsive force is produced. If a north pole and a south
pole are brought near then a force of attraction results.
Thus, "Like poles repel, unlike poles attract”’
MAGNETS AND MAGNETIC
FIELDS
 The concept of a field is applied to magnetism as
well as gravity and electricity. A MAGNETIC
FIELD surrounds every magnet and is also
produced by a charged particle in motion relative
to some reference point. The presence of the
magnetic field about a bar magnet can be seen by
placing a piece of paper over the bar magnet and
sprinkling the paper with iron filings.
The magnetic field produced by certain arrangements of bar
magnets are represented in the diagrams shown below
MAGNETS AND MAGNETIC
FIELDS
 The magnetic field lines drawn to represent the magnetic field
produced by certain arrangements of bar magnets are represented in
the diagrams shown below
Notice the similarity
between the lines of this
slide and the previous slide
particle distribution
FORCE ON A CHARGED PARTICLE MOVING
IN A MAGNETIC FIELD
An electrically
charged particle (q)
moving through a
magnetic field (B) at
speed v may be acted
upon by a force (F).
The magnitude of the
force F on the particle
is
FORCE ON A CHARGED PARTICLE MOVING
IN A MAGNETIC FIELD
An electrically charged particle (q) moving through a magnetic
field (B) at speed v may be acted upon by a force (F). The
magnitude of the force (F on the particle is
FORCE ON A CHARGED PARTICLE MOVING
IN A MAGNETIC FIELD
 is the angle between the direction of motion of the particle
and the direction of the magnetic field. If  = O, then the
particle is traveling parallel to the field and no force exists on
the particle (sin  = 0). If  = 90'. then sin 90'= 1, the
particle is traveling perpendicular to the magnetic field and
the force is a maximum.
FORCE ON A CHARGED PARTICLE MOVING
IN A MAGNETIC FIELD
FORCE ON A CHARGED PARTICLE MOVING
IN A MAGNETIC FIELD
ELECTRIC CURRENTS
PRODUCE MAGNETISM
 A wire carrying a current (I) produces a
magnetic field. The magnitude of the
magnetic field strength (B) a perpendicular
distance r from a LONG, STRAIGHT
WIRE the wire is given by
ELECTRIC CURRENTS
PRODUCE MAGNETISM
 The direction of the
magnetic field produced by a
current carrying wire can be
predicted by using the
RIGHT HAND RULE. The
thumb of the right hand
points in the direction of the
conventional current in the
wire. The fingers encircle the
wire in the direction of the
magnetic field.
ELECTRIC CURRENTS
PRODUCE MAGNETISM
 The magnitude of the strength of the
magnetic field (B) at the center of a
LOOP OF WIRE of radius r which carries
a current I is
ELECTRIC CURRENTS
PRODUCE MAGNETISM

The direction of the magnetic field at the center of the loop can again
be predicted by using the right hand rule. The thumb is placed tangent
to a point on the loop and is directed in the same direction as the current
in the loop at that point. The fingers
encircle the wire in the same
direction as the magnetic field.
CONVENTIONS
Certain CONVENTIONS have been adopted in order to
represent the direction of the magnetic field and the
current in a wire. A magnetic field directed into the paper
is represented by a group of x's, while a magnetic field
out of the paper is represented by a group of dots.
CONVENTIONS
A current carrying wire which is arranged perpendicular
to the page is represented by a circle. If the current is
directed into the paper then an x is placed in the center of
the circle. If the current is directed out of the paper then
a dot is placed in the center of the circle.
FORCE ON A CURRENT CARRYING
WIRE IN A MAGNETIC FIELD
 A current (I) in a wire
consists of moving
electrical charges and
a force (F) may be
produced when a
current carrying wire
of length l is placed in
a magnetic field. The
magnitude of the force
is given by the
equation:
FORCE ON A CURRENT CARRYING
WIRE IN A MAGNETIC FIELD
B is the MAGNETIC FIELD
STRENGTH in tesla (T).
Other units for magnetic
field strength include
newtons per ampere meter
(N/A*m), newtons per
coulomb meters per second
(N/C*m/s), webers per
square meter (wb/m2) and
gauss (G).
FORCE ON A CURRENT CARRYING
WIRE IN A MAGNETIC FIELD
 is the angle between the directions of the current in the
wire and the magnetic field. The force on the wire is zero if
FORCE ON A CURRENT CARRYING
WIRE IN A MAGNETIC FIELD
A SECOND RIGHT HAND RULE is used to predict the
direction of the force on the wire: "First you orient your right
hand so that the outstretched fingers point in the direction of
the (conventional) current; from this position when you bend
your fingers they should then point in the direction of the
magnetic field lines, if they do not, rotate your hand and arm
about the wrist until they do, remembering that your
straightened fingers must point in the direction of the
current. When your hand is oriented in this way, then the
extended thumb points in the direction of the force on the
wire."
FORCE ON A CURRENT CARRYING
WIRE IN A MAGNETIC FIELD
FORCE ON A CURRENT CARRYING
WIRE IN A MAGNETIC FIELD
FORCE ON A CURRENT CARRYING
WIRE IN A MAGNETIC FIELD
Torque on a current loop
Torque on a current loop
Torque on a current loop
=r x F
F= I Bl, l = a, & r =b/2
so
ab =A (area of coil)
Torque on a current loop (Magnetic Moment)
If there are N
loops then
IF we allow
then
Torque on a current loop (Magnetic Moment)
The Solenoid
Magnetic Field due to a straight wire
 The direction of the magnetic
field produced by a current
carrying wire can be predicted
by using the RIGHT HAND
RULE.
 Careful experiments show that
the magnetic field is
proportional to the current and
inversely proportional to the
distance from the wire
L
Magnetic Field due to a straight wire and the force
between two parallel wires
 Combining the two formulas
yields
By using the right hand
rule, the direction of the
force is found to be
towards I1 (attractive)
L
Magnetic Field due to a straight wire and the force
between two parallel wires
 The value of the constant
o= 4 x 10 -7 T*m/A is called the
permeability of free space
is the equation
for the magnetic field caused
by I1 acting on I2
L
The force per unit length acting
on I2 is
Magnetic Field due to a straight wire and the force
between two parallel wires