Physics I
Class 23
The Magnetic Field
Rev. 17-Apr-03 GB
23-1
Magnetism in Ancient Times
The ancient Greeks knew that the mineral
magnetite, named for the Magnesia region
of Asia Minor, attracts small iron objects
like nails. Magnetite is also known as
lodestone.
Ancient Chinese discovered the compass.
The Chinese compass seen at the left uses
a piece of magnetite shaped like a spoon.
The handle of the spoon points south.
23-2
Peter Peregrinus
Peter Peregrinus (Pierre de Maricourt) was
a member of the army of the King of Sicily
and probably served as an engineer. In 1269
he described everything known about
lodestones and how to make instruments
using these magnets. He discovered the
existence of magnetic poles and was the first
to use the word "poles." (Latin polus.) He
did experiments referring to how the north
and south poles attract or repel.
He also described how to make a perpetual motion machine using
magnets. People are still trying this (unsuccessfully) today!
23-3
William Gilbert
Gilbert published De Magnete (On the Magnet)
in 1600. This book became a standard reference
on electricity and magnetism in Europe. Gilbert
was the first to distinguish the electric force
(named for the Greek word for amber) from the
magnetic force. Gilbert debunked many folk
myths about the curative properties of magnets.
William Gilbert, 1544-1603
Based on his observations of the preferred directions of thin iron
needles near spherical lodestones, and the similarity of this
phenomenon to the tendency of compass needles to tilt with
respect to the horizontal plane as well as point north-south,
Gilbert deduced that the earth itself must be a giant magnet.
23-4
Magnetic Forces Between Poles
“Opposites Attract”
S
S
N
N
attract
repel
S
N
N
S
23-5
Why Is A Magnet Attracted to
Some Materials (Refrigerators)?
S
N
S
N
S
N
S
N
S
N
S
N
The permanent magnet causes tiny magnetic
domains in the nearby material to line up.
attract
S
N
 This effect is most noticeable in ferromagnetic
metals: iron, cobalt, nickel (& some “rare earth”
elements).
 In paramagnetic materials (like liquid oxygen), the
magnetic domains line up weakly.
 In diamagnetic materials (like silver), the domains
line up against the permanent magnet and cause a
weak repulsion.
23-6
What Happens If You Try to
Separate North and South Poles?
S
N
saw cut
S
N
S
N
Scientists have searched in
 iron ore
 moon rocks
 cosmic rays
 high-energy colliders
 and many other places
but “magnetic monopoles” have
never been found.
23-7
Magnetic Field Lines
From N to S.
In direction of
compass
needle.
Try this web site:
http://www.walter-fendt.de/ph11e/mfbar.htm
23-8
Electromagnetism
Oersted discovered in 1820 that electric current
creates a magnetic field like a permanent magnet.
This is a Physics 2 topic, but we will use this effect
in the Physics 1 activities involving magnetism.
Hans Christian Oersted
1777-1851
23-9
Units of Magnetic Field
Inventions: a telephone repeater, rotating magnetic field
principle, polyphase alternating-current system, induction
motor, alternating-current power transmission, Tesla coil
transformer, wireless communication, radio, fluorescent
lights, and more than 700 other patents.
Nikola Tesla
1856-1943
The SI unit of magnetic field (actually magnetic flux density, but we
don’t make a distinction in Physics 1) is the tesla, T.
The magnetic field in the gap between the rotor and stator of an electric
motor is about 1 T.
We also sometimes use gauss. 1 T = 10,000 gauss.
The magnetic field in a typical home environment is about 0.1-1.0 gauss.
23-10
Class #23
Take-Away Concepts
1.
The magnetic force was known in ancient times.
2.
Magnets have two poles, N and S.
3.
Opposite poles attract, like poles repel.
4.
N and S poles are always in pairs, never alone.
5.
Electric currents also cause magnetic fields.
6.
Magnetic field lines start at N and go to S,
following the direction of a compass needle.
7.
Units of tesla (T) and gauss. 1 T = 10,000 gauss.
23-11
Class #23
Problems of the Day
___1. A scientist claims that he has discovered magnetic
monopoles, and he sends you a sample cube of material
that he claims contains only north magnetic poles. Which
test below would help you verify or dispute his claim?
A) Check if all sides of the cube stick to a refrigerator door.
B) Measure the magnetic flux density (magnitude) near the
surface to check if it exceeds the strongest normal magnet.
C) Hang the cube on a string and see if it is attracted to or
repelled by a sphere with a negative electrical charge.
D) See if the south pole of a compass needle points toward all
sides of the cube when placed near the respective sides.
E) No test would help you unless you had other samples of pure
north and south poles to check for attraction and repulsion.
23-12
Answer to Problem 1 for Class #23
The answer is D. Here’s why:
A. It would not be hard to magnetize a cube so that every side sticks
to a refrigerator.
B. The strength of the magnetic field by itself would not prove or
disprove the existence of monopoles. However, the direction of
the field would be critical (answer D).
C. An electric charge would not attract or repel a magnetic
monopole.
D. We are looking for magnetic field lines that all point out from
the cube with none pointing in. (More in Physics 2.)
E. Since D is a possible test, we don’t need pre-existing monopoles.
23-13
Class #23
Problems of the Day
___2. If magnetic monopoles have never been discovered, what
is/are the source(s) of magnetic fields?
A) Some elementary particles, like electrons, are tiny magnetic
dipoles.
B) Moving electric charges create magnetic fields.
C) Since magnetic fields exist, there must be magnetic
monopoles. We just haven’t discovered them yet.
23-14
Answer to Problem 2 for Class #23
All magnetic fields presently known are created either by A and/or B.
When many electron magnetic dipoles align in a material, we have a
permanent magnet. The more that align per unit volume, the stronger
the magnet.
Circulating electrical current creates an electromagnet.
Sometimes, the two effects work together: for example, in an
electromagnet with an iron core, the magnetic field created by
circulating current induces the electron spins in the iron to line up.
Magnetic monopoles aren’t needed to explain all known magnetic
phenonena.
23-15
Activity #23
Introduction to Magnetic Fields
Objective of the Activity:
1.
2.
3.
Think about magnetic fields.
Investigate the magnetic field created by a bar magnet.
Learn how to measure magnetic fields using a Hall probe.
23-16