Transcript Ch33 - Siena College

Field Around Magnet
• Use a compass to map the direction of the
magnetic field surrounding a magnet.
• White board your results. In particular:
– how does the strength of the field vary with
distance from the wire?
– how does the field direction relate to the poles of
the magnet?
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Magnetism
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Activity: Map Field of Magnets
• Use iron filings to map the field of a
– bar magnet
– horseshoe magnet
• White board results
– draw field lines.
– how might magnets generate magnetic fields?
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Magnetic Field Lines
• direction of magnetic field, B, is parallel to
field line
• number of lines per area is proportional to
strength of field
•field lines point
from N to S
•field lines form
closed loops
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Magnetism
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Magnetism
No magnetic monopoles!
General Physics 2
Magnetism
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Magnets are similar to Electric Dipoles
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Magnetism
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Ferromagnetism
• Ferromagnetic material
– iron or other materials that can be
made into magnets
Random
Preferentially
downwards
• You can make a magnet from
iron by placing it in a strong B
field
– individual domains become
aligned with external B field
• Loss of magnetism from:
– dropping
– heating
• Curie temperature
– 1043 K for iron
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Cross Product – Right Hand Rule
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Specifying 3 Dimensions
• out of page
• tip of arrow
General Physics 2
• into page
• tail of arrow
Magnetism
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Force on a moving charge
• Right Hand Rule (#2)
– qv = fingers
– B = bend fingers
– F = thumb
•
Find the direction of the force on a negative charge for
each diagram shown.
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Think-Pair-Share
• Derive an expression
for the radius of an
e-’s orbit in a
uniform B field.
Express your answer
in terms of me, v, qe,
and B.
Turn in your
solution!
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Earth’s Magnetic Field
• magnetic declination
– angular difference
between geographic
north and magnetic
north
– varies with latitude
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Tactics: Right-hand rule for fields
The Source of the Magnetic Field: Moving
Charges
The magnetic field of a charged particle q moving with
velocity v is given by the Biot-Savart law:
where r is the distance from the charge and θ is the angle
between v and r.
The Biot-Savart law can be written in terms of the cross
product as
EXAMPLE 33.1 The magnetic field of a
proton
QUESTION:
EXAMPLE 33.1 The magnetic field of a
proton
EXAMPLE 33.1 The magnetic field of a
proton
EXAMPLE 33.1 The magnetic field of a
proton
The Magnetic Field of a Current
The magnetic field of a long, straight wire carrying current
I, at a distance d from the wire is
The magnetic field at the center of a coil of N turns and
radius R, carrying a current I is
EXAMPLE 33.4 The magnetic field strength
near a heater wire
QUESTION:
EXAMPLE 33.4 The magnetic field strength
near a heater wire
Practice Problems
• Magnetism: Worksheets 1 and 2
• Finish before next class
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Tactics: Finding the magnetic field direction
of a current loop
Magnetic Dipoles
The magnetic dipole moment
of a current loop enclosing an
area A is defined as
The SI units of the magnetic
dipole moment are A m2. The
on-axis field of a magnetic
dipole is
EXAMPLE 33.7 The field of a magnetic dipole
QUESTIONS:
EXAMPLE 33.7 The field of a magnetic dipole
Tactics: Evaluating line integrals
Ampère’s law
Whenever total current Ithrough
passes through an area bounded
by a closed curve, the line
integral of the magnetic field
around the curve is given by
Ampère’s law:
The strength of the uniform magnetic field inside a solenoid
is
where n = N/l is the number of turns per unit length.
The Magnetic Force on a Moving Charge
The magnetic force on a charge q
as it moves through a magnetic
field B with velocity v is
where α is the angle between v
and B.
Magnetic Forces on Current-Carrying Wires
Consider a segment of wire of length l carrying current I in
the direction of the vector l. The wire exists in a constant
magnetic field B. The magnetic force on the wire is
where α is the angle between the direction of the current
and the magnetic field.
EXAMPLE 33.13 Magnetic Levitation
QUESTION:
EXAMPLE 33.13 Magnetic Levitation
General Principles
General Principles
General Principles
Applications
Applications
Applications
Does the compass needle rotate clockwise
(cw), counterclockwise (ccw) or not at all?
A. Clockwise
B. Counterclockwise
C. Not at all
Does the compass needle rotate clockwise
(cw), counterclockwise (ccw) or not at all?
A. Clockwise
B. Counterclockwise
C. Not at all
The magnetic field at the position P points
A. Into the page.
B. Up.
C. Down.
D. Out of the page.
The magnetic field at the position P points
A. Into the page.
B. Up.
C. Down.
D. Out of the page.
The positive charge is
moving straight out of the
page. What is the direction
of the magnetic field at the
position of the dot?
A. Left
B. Right
C. Down
D. Up
The positive charge is
moving straight out of the
page. What is the direction
of the magnetic field at the
position of the dot?
A. Left
B. Right
C. Down
D. Up
What is the current
direction in this loop?
And which side of the
loop is the north pole?
A. Current counterclockwise, north pole on bottom
B. Current clockwise; north pole on bottom
C. Current counterclockwise, north pole on top
D. Current clockwise; north pole on top
What is the current
direction in this loop?
And which side of the
loop is the north pole?
A. Current counterclockwise, north pole on bottom
B. Current clockwise; north pole on bottom
C. Current counterclockwise, north pole on top
D. Current clockwise; north pole on top
An electron moves perpendicular to a
B direction
magnetic field. What is the
of ?
A. Left
B. Into the page
C. Out of the page
D. Up
E. Down
An electron moves perpendicular to a
B direction
magnetic field. What is the
of ?
A. Left
B. Into the page
C. Out of the page
D. Up
E. Down
What is the current direction in the loop?
A. Out of the page at the top of the
loop, into the page at the bottom.
B. Out of the page at the bottom of the
loop, into the page at the top.
What is the current direction in the loop?
A. Out of the page at the top of the
loop, into the page at the bottom.
B. Out of the page at the bottom of
the loop, into the page at the top.
Which magnet or magnets
produced this induced
magnetic dipole?
A.
B.
C.
D.
E.
a or d
a or c
b or d
b or c
any of a, b, c or d
Which magnet or magnets
produced this induced
magnetic dipole?
A.
B.
C.
D.
E.
a or d
a or c
b or d
b or c
any of a, b, c or d