Physics 102-002
Announcements
• WebAssign –
– Chapter 24 due next
Wednesday
• Exam #3 is graded
Average =
61.5349
Std Dev =
16.4568
Max Score =
94
Min Score =
30
Median Score =
58
– Corrections due Wed,
Apr 25
Picture: The Earth's Magnetic Field: Gary A.
Glatzmaier (UCSC) Pictured, a computer
simulation shows the resulting magnetic field
lines out to two Earth radii, with blue lines
directed inward and yellow lines directed
outward.
Class Schedule
4/9
Midterm Exam #3
4/11
Chapter 24
Magnetism, (Pg 458-470)
4/16
Chapter 26
Properties of Light
4/18
Chapter 28
Reflection and Refraction, Part 1 (Pg 530-540)
4/23
Chapter 28
Reflection and Refraction, Part 2 (Pg 540-551)
4/25
Chapter 29
Light Waves (Pg 558-568)
4/30
Chapter 30
Light Emission (Pg 558-568) Midterm Exam #4
5/2
Review
5/7
Final Exam
Note the change!!!
Chapter 24
Magnetism
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Magnetic Forces
Magnetic Poles
Magnetic Fields
Magnetic Domains
Electric Currents and Magnetic Fields
Magnetic Force on Moving Charged Particles
Magnetic Force on Current-Carrying Wires
Earth’s Magnetic Field
Magnetic Forces
In “Electrostatics” we learned about the force between charged particles that
depends on the net charge on each and on their separation distance … this is the
“Coulomb force”.
If 2 charges are in motion with respect to each other, there is an additional force
between them, the Magnetic Force, that depends on their motion.
… the Electrostatic Force and the Magnetic Force are thus related to each other.
We’ll see that there’s a reciprocity:
Moving charges (Currents) create magnetic fields …..
And …. Magnetic fields exert forces on moving charges.
Physics Place video - Oersted.
Magnetic Poles
Magnetic Poles exert a magnetic force on each other
There are 2 types of magnetic Poles:
South (S) Poles
North (N) Poles
(similar to electric charges)
Like poles repel
Opposite poles attract
The strength of the repulsion or attraction obeys an inverse square law
(The closer the poles are to each other, the stronger the force)
The North pole points “north” in the earth’s magnetic field.
The South pole points “south”
A “horseshoe magnet” is just a bar magnet
Bent in a U shape. It has 2 poles too.
Cut a bar magnet in 2 pieces, you get 2 bar magnets:
N
S
N
S
+
N
S
You can’t find a magnetic NORTH pole without finding a SOUTH pole partnered with it.
There is no such thing as a “magnetic monopole” – scientists have spent a lot of energy
looking!
Magnetic Fields
Also similar to electrostatics … the space between 2 magnetic poles is filled
by a MAGNETIC FIELD.
You can visualize the magnetic field by sprinkling iron filings
around a magnet.
The direction of the field is from the North pole to the South both
inside and outside of the magnet. Outside the magnet, the field
starts on the North pole and circles around to the South Pole.
The earth has a naturally occuring magnetic field (caused by motion of
the electrons making up the earth’s subterranean structure). The
needle of a compass will line up with the earth’s magnetic field. If the
needle starts out not lined up, the magnetic field exerts a torque on the
needle and forces it to “get In line”,
Here is the magnetic field of a pair of bar magnets. The field
is caused by the motion of the electrons in the Iron atoms
making up the magnet.
Electrons are in motion 2 ways:
- They’re “spinning”
- They’re “orbiting” the nucleus
of the atom
Physics Place figure
Most common magnets are made from Iron, cobalt, and nickel
Question 1
Question 1 Answer
Magnetic Domains
Iron becomes magnetized when large clusters of its atoms become lined up in the same direction.
Such a cluster of atoms when aligned is called a Magnetic Domain.
There are millions of atoms in a domain, and there are many domains in a single crystal. Here is
a cartoon depiction of magnetic domains.
Domains can also be aligned (or anti-aligned)
with each other.
The magnetic strip on the back of your LOBO card is a large collection of magnetic domains used to
record your personal information as a series of binary digits.
To make a magnet (or to “magnetize” a piece of metal) we have to get a significant number of the
domains within it to line up. We can do that by subjecting it to an external magnetic field from another
magnet, or by beating on it (depending on the softness of the metal).
Physics Place figure
Electric Currents and Magnetic Fields
We said a moving charge creates a magnetic field.
This means that every current-carrying wire is surrounded by a magnetic field!!
There are several ways to demonstrate this is true … some of them will be shown in class. One is by
surrounding a wire with compasses and then passing a current through the wire … the compasses
line up with the magnetic field which circles around the wire.
If the wire is bent into a loop, the field can be built up so that it circles through the middle of the
loop. If you loop the wire many times, you can build up a strong magnetic field that exists inside
the coil of wire. This is called a Solenoid.
An Electromagnet is built by winding the wire around an iron core. The magnetic field inside the
windings forces the iron’s domains into alignment and turns it into a very strong magnet.
Magnetic Force on Moving Charged
Particles
A charged particle that is not moving will not be affected by a magnetic field. But if the charged
particle begins moving in a magnetic field, the magnetic field will exert a force on the particle,
deflecting it from its “normal” path. This is the principle behind devices such as the Mass
Spectrometer.
If that path is inside a wire, the magnetic field will deflect the electrons moving through the wire, and
will make the wire move.
Because of the nature of the field, though, the field exerts no force on the particle if the particle is
moving parallel to the magnetic field lines. The force on the particle is maximum if it moves
perpendicular to the field lines.
The odd thing is that the direction of
the deflecting force is perpendicular
to BOTH the field’s direction and
the direction of motion of the
particle.
If the particle moves continuously in the magnetic field (and doesn’t exit the field for some reason),
the particle can be made to move in a circle! This is the concept behind the cyclotron.
Physics Place figure
Magnetic Force on Current Carrying Wires
If charges are moving through a wire when they enter a magnetic field, the wire will be deflected
to one side.
The direction that the wire gets
deflected is determined by the
direction the current is flowing
and the direction of the magnetic
field, and the wire is deflected
perpendicular to both of those
directions.
So electric currents deflect magnets, and magnets deflect electric currents.
This is the fundamental relationship that lets us build electric motors (and
electric meters)!!
Electric meter
(a galvanometer)
Really just a compass inside a coil of
wires that lets us measure current,
voltage, and many other things.
Physics Place figure
Electric Motors
A motor is really just a
galvanometer in which
the “compass” is
allowed to turn all the
way around. The forces
between the moving
current and the
magnetic field provide
the push to keep the
motor turning.
Question 2
Question 2 Answer