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Transcript
Chapter 14:
Magnetism
Brent Royuk
Phys-110
Concordia University
Magnets
•
Magnets are caused by moving charges.
–
•
•
Permanent Magnets vs. Electromagnets
Magnets always have two poles, north and
south.
Like poles repel, opposites attract.
2
Magnets
•
•
•
•
•
North means north-seeking, so Earth’s
north pole is what kind of pole?
But any pole attracts metal: Why?
Bar magnets are dipoles. Can there be
a monopole?
History: lodestones and magnetic
compasses. Remember Magnesia?
Permanent magnets vs. electromagnets:
More later
3
What’s Wrong With This Picture?
4
Earth’s Magnetic Field
• Probably caused by currents of
molten core
• Drift and reversals
– Last reversal: 780,000 years ago
5
Magnetic Fields
• The magnetic field
B surrounds
magnets
analogously to the
electric field
• Is there an analog
to Coulomb’s
Law? No, the Bfield is more
complicated.
6
B-Field Lines
• Field line mapping: What defines a
field line?
• The direction of the line is always
from N to S.
N
S
7
Electromagnetism
• H. C. Oersted, 1820
– Current-carrying wires exert a force
on each other
F
2 kI1I 2

l
r
– k’ = 1 x 10-7 N/A2

QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
8
Electromagnets
•
The Long, Straight Wire
–
How long is it?
 oI
B
2 r
The Permeability of
Free Space:
Another RightHand Rule:
o = 4 x 10-7 Tm/A
Demo
9
Long Straight Wire
• What direction is the B-field a)
above both wires, b) below both
wires, and c) between the wires?
10
Magnetic Force on a Moving Charge
• A moving charge moving in a
perpendicular direction through a
B-field experiences a force
perpendicular to its motion
• Qualitative: F  qvB sin 
11
Electric Field Units
• [B] = [F/qv]
• 1 N/Am  1 tesla (T)
–
–
–
–
–
–
Neutron star: 108
Big magnet: 1.5
Small bar magnet: .01
Earth’s magnetic field: 5 x 10-5
Interstellar space: 10-10
Magnetically shielded room: 10-13
12
Magnetic Force on a Moving Charge
•
F = qvB sin 
–
•
•
•
Note that the force is maximum when
perpendicular, minimum at parallel. Weird.
What is the significance of a field line for
a moving charge?
Example: An electron moves at right
angles to a magnetic field of 0.12 T.
What is its speed if the force exerted on
it is 8.9 x 10-15 N?
Have you ever brought a magnet near a
CRT screen?
13
Direction of the Magnetic Force
The Right Hand Rule
–
–
–
Wrap or Point from v to B
In/Out conventions
Positive vs. Negative
14
What’s Wrong With This Picture?
15
Force on a Wire
• In the picture below, the wire is
deflected downward. Which side of
the magnet is a north pole?
• The monstrosity
16
Loops & Solenoids
•
•
•
Loops concentrate
magnetic effects.
What is the direction of
the B-field in the vicinity
of a current-carrying
loop?
Solenoids contain
multiple loops.
–
–
Solenoids increase
magnetic fields.
Solenoids increase the
force felt by currentcarrying wires.
17
Application: Loudspeakers
•
A modulated current is sent to a voice
coil, which experiences a force from a
magnet that is transmitted onto a
speaker cone.
18
Application: The Galvanometer
• Torque on a coil of current loops is
balanced by a spring.
• Galvanometers can be configured
as voltmeters or ammeters.
19
Motors
•
Problem: How do you run a motor with
DC electricity?
–
The commutator
20
Motors
Increase the
flux with
multiple
armatures
21
Faraday’s Law
•
What does this mean?

E
t
• “What good is a baby?”
• “One day, sir, you may tax it.”

24
Magnetic Flux
• Water pipe analogy
– Flow through a butterfly net
•  = BA
• Can be visualized as the number of
field lines passing through a
current loop
– Orientation matters too.
• Unit: 1 weber (Wb) = 1 T m2
• Ways flux can change
– Relative motion, Changing field
strength, Changing orientation,
Changing area of loop
25
Changing Flux
• Faraday: 1830, an induced emf is
produced by a changing flux in a
circuit loop.
• Demo
26
Changing Field Strength
•
•
When is current induced?
Demo
27
Applications
•
Dynamic (Induction) Microphone
28
Applications
•
Guitar Pickups
29
Lenz’s Law
•
Lenz’s Law: An induced current always
flows in a direction that opposes the change
that caused it.
30
Eddy Currents
•
•
Magnet in tube
Monstrosity
31
Eddy Currents
32
Generators
What’s the difference between a motor
and a generator?
33
Back EMF
•
•
There is a “braking effect” caused by a
generator that is a voltage that resists the
changing current, and it’s called Back EMF.
When motors are spun by electricity, they
generate a back EMF
–
–
•
Maximum current occurs during the startup of an
electric motor.
“Cold-cranking amps.”
Generators have a counter torque.
–
Hand-crank generator
34
Transformers
•
•
•
Place two solenoids side-by-side.
How can a DC voltage in one produce a
voltage in the other?
How can an AC voltage in one produce a
voltage in the other?
35
Transformers
•
•
Get two coils to share the same
changing flux and their voltages will
differ by the number of turns in the
coils.
The transformer relations:
N1
N2


V1
V2

I2
I1
36
Transformers
•
•
•
Step-Up vs. Step-Down
Isolation Transformers
Suppose that our neon transformer draws 4 A of current. How much
current does it supply to the discharge tube?
–
Neon transformers have an inductor in series with the transformer. Why?
37
Transformer Energy Loss
•
Losses can come from flux leaks, selfinduction, resistive heating.
–
•
Mechanical losses: Transformer hum
Eddy currents can be minimized with laminated
cores.
38
The Power Grid
•
Edison vs. Westinghouse
39
The Power Grid
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
40
The Power Grid
41