Conceptual Physics

1
Conceptual Physics

• Chapter Thirty Six Notes
• Magnetism

2
July 1820 Oersted and electromagnetism   H
ans Christian Oersted
By the end of the 18th century, scientists had
noticed many electrical phenomena and many
magnetic phenomena, but most believed that these
were distinct forces. Then in July 1820, Danish
natural philosopher Hans Christian Oersted
published a pamphlet that showed clearly that
they were in fact closely related. During a
lecture demonstration, on April 21, 1820, while
setting up his apparatus, Oersted noticed that
when he turned on an electric current by
connecting the wire to both ends of the battery,
a compass needle held nearby deflected away from
magnetic north, where it normally pointed. The
compass needle moved only slightly, so slightly
that the audience didnt even notice. But it was
clear to Oersted that something significant was
happening.
3

• Even in this day and age, most of the public
  regards magnetism as a mystery. That has led to
  magnetic bracelets and similar "health products,"
  to magnets taped to fuel lines for better gas
  mileage, and to widespread worries about possible
  reversal of the Earth's field, encouraged by
  Hollywood movies.
•     In the minds of most Americans magnetism is
  forever associated with specially treated iron,
  with patterns of iron filings and with the way
  the compass needle lines up with the north-south
  direction. Few schools teach much more, because,
  (1) physics is an elective, and (2) magnetism is
  covered near the end of the textbook, the school
  year is short, and teachers are happy if they
  just make it to Ohm's law.
•     Some people may also know that a
  current-carrying wire coil wrapped around an iron
  bar turns it into a magnet, and about use of
  electromagnets in electric machinery. But it's
  always with iron, or with some magnetic
  substance. Why sunspots would be magnetic remains
  completely unclear.
•     In ancient times, both Greeks and Chinese
  knew about natural magnets, rare chunks of
  iron-rich mineral known as lodestones. The
  Chinese also knew that if you rubbed a steel
  needle against a lodestone, in a fixed direction,
  it also became a magnet. Around the

4

• year 1000, they furthermore found that if a
  magnet or lodestone was placed on a little "boat"
  floating in a bowl of water, it always pointed in
  a fixed direction--and for a magnetized iron bar,
  that direction was always north-south. You could
  rotate the bowl, but the magnet would keep
  pointing in the same
•     The reason, we now know, is that the Earth,
  too, is magnetic. From that came the magnetic
  compass, quickly copied by Arab navigators and
  then by Europeans. We may wonder today--if
  lodestones did not exist, the compass might have
  stayed undiscovered for a long time, and would
  Columbus have ventured so far from land without
  it?

5
36.1 Magnetic Poles

• Every magnet has two poles. This is where most of
  its magnetic strength is most powerful. These
  poles are called north and south or north-seeking
  and south seeking poles. The poles are called
  this as when a magnet is hung or suspended the
  magnet lines up in a north - south direction.
  When the north pole of one magnet is placed near
  the north pole of another magnet, the poles are
  repelled. When the south poles of two magnets are
  placed near one another, they also are repelled
  from one another. When the north and south poles
  of two magnets are placed near one another, they
  are attracted to one another.
• The attraction and repelling of two magnets
  towards one another depends on how close they are
  to each other and how strong the magnetic force
  is within the magnet. The further apart of the
  magnets are the less they are attracted or
  repelled to one another.

6

• The magnetic and electric fields are both similar
  and different. They are also inter-related.
• Similar Just as the positive () and negative
  (-) electrical charges attract each other, the N
  and S poles of a magnet attract each other.
• In electricity like charges repel, and in
  magnetism like poles repel.
• Different The magnetic field is a dipole field.
  That means that every magnet must have two poles.
• On the other hand, a positive () or negative (-)
  electrical charge can stand alone. Electrical
  charges are called monopoles, since they can
  exist without the opposite charge.
•  Monopole a single magnetic pole or electric
  charge
•  Dipole a pair of opposite poles
•  The so-called magnetic moment is the measure of
  the strength of the dipole. The magnetic moments
  are expressed as multiples of Bohr Magnetons. A
  Bohr magneton has a value of 9.27 x 10-24
  joules/tesla.
• When a magnet is broken into little pieces, a
  north pole will appear at one of the broken faces
  and a south pole. Each piece, regardless of how
  big or small, has its own north and south poles.

7

• The area around a magnet can also behave like a
  magnet. This is called a magnetic field. The
  larger the magnet and the closer the object to
  the magnet, the greater the force of the magnetic
  field.
• Magnetic Materials
• The term magnetism is derived from Magnesia, the
  name of a region in Asia Minor where lodestone, a
  naturally magnetic iron ore, was found in ancient
  times. Iron is not the only material that is
  easily magnetized when placed in a magnetic
  field others include nickel and cobalt.

8
36.2 Magnetic Fields

•  The magnetic field is the central concept used
  in describing magnetic phenomena.

9
36.3 The Nature of a Magnetic
Field

•  A region or a space surrounding a magnetized
  body or current-carrying circuit in which
  resulting magnetic force can be detected.
•  A magnetic field consists of imaginary lines of
  flux coming from moving or spinning electrically
  charged particles. Examples include the spin of a
  proton and the motion of electrons through a wire
  in an electric circuit.
• Magnetic field or lines of flux of a moving
  charged particle

10

• A magnetized bar has its power concentrated at
  two ends, its poles they are known as its north
  (N) and south (S) poles, because if the bar is
  hung by its middle from a string, its N end tends
  to point northwards and its S end southwards. The
  N end will repel the N end of another magnet, S
  will repel S, but N and S attract each other. The
  region where this is observed is loosely called a
  magnetic field.

• Either pole can also attract iron objects such as
  pins and paper clips. That is because under the
  influence of a nearby magnet, each pin or paper
  clip becomes itself a temporary magnet, with its
  poles arranged in a way appropriate to magnetic
  attraction.

• But this property of iron is a very special type
  of magnetism, almost an accident of nature!
• Out in space there is no magnetic iron, yet
  magnetism is widespread. For instance, sunspots
  consist of glowing hot gas, yet they are all
  intensely magnetic. The Earth's own magnetic
  powers arise deep in its interior, and
  temperatures there are too high for iron magnets,
  which lose all their power when heated to a red
  glow. What goes on in those magnetized regions?
• 
  It is all related to
  electricity.

11

• MAGNETIC FORCE
• The magnetic field of an object can create a
  magnetic force on other objects with magnetic
  fields. That force is what we call magnetism.
• When a magnetic field is applied to a moving
  electric charge, such as a moving proton or the
  electrical current in a wire, the force on the
  charge is called a Lorentz force.
• Attraction
• When two magnets or magnetic objects are close to
  each other, there is a force that attracts the
  poles together.
• Force attracts N to S
• Magnets also strongly attract ferromagnetic
  materials such as iron, nickel and cobalt.

12

•  Repulsion
• When two magnetic objects have like poles facing
  each other, the magnetic force pushes them apart.
• Force pushes magnetic objects apart
• Magnetic and electric fields
• The magnetic and electric fields are both similar
  and different. They are also inter-related.
• Each atom that makes up a substance is a time
  magnet. When atoms are arranged not in random
  directions but all in the same direction, the
  substance is a permanent magnet. Magnetism and
  electricity are very closely related, so that the
  flow of electricity through a conductive wire
  generates a magnetic field, and conversely a
  change in a magnetic field produces a flow of
  current in a conductor.

13

• How is Magnetism Produced?
• The electrons in an atom spin as they rotate
  about the nucleus. This spinning motion creates a
  magnetic effect in each electron, which together
  forms a magnetic field around the atom.
•  

Normally, the atoms in any substance are oriented in random fashion throughout the substance. This means that their individual magnetic fields cancel each other, and the substance as a whole does not appear magnetically charged. In a permanent magnet, all of the electrons are oriented in the same direction. This means that the substance as a whole acts as a magnet. The lines that show the direction of the magnetic field are called magnetic lines.
14
36.4 Magnetic Domains

• Even when the atoms are oriented randomly,
  exposure to a nearby magnet may cause them to
  line up with the magnetic field. Substances that
  do this easily, such as iron or nickel, can be
  'magnetized.
• Other substances, in which the atoms remain
  randomly oriented even when exposed to a magnet,
  such as copper, wood or plastic, cannot be
  magnetized.

15
When a coil is wrapped around an iron bar and
electric current is passed through the coil, the
iron bar picks up a magnetic field, and becomes
an 'electromagnet.' The strength of the magnetic
field is proportional to the size of the current
flow. The relation is similar to the way wind
passing through a windmill (current flow) flows
at right angles to the plane of rotation of the
windmill blades (creation of the magnetic
field).
If the wind reverses
direction,
the windmill will
also rotate in
the opposite
direction.

• If a permanent magnet is inserted and withdrawn
  through the center of the coil, it causes an
  electric current to flow in the coil. The
  direction of the current flow is in opposition to
  the change in the magnetic field, so that the
  current is reversed each time the permanent
  magnet is inserted or withdrawn. This is the
  principle of the electric generator. The relation
  is similar to the way a windmill revolves
  (current flows) in response to the wind passing
  through it (movement of the magnet).

16
(No Transcript)
17
36.5 Electric Currents and
Magnetic Fields

• The connection between electric current and
  magnetic field was first observed when the
  presence of a current in a wire near a magnetic
  compass affected the direction of the compass
  needle. We now know that current gives rise to
  magnetic fields, just as electric charge gave
  rise to electric fields.
• With positive current, point
• your thumb in the
• direction of the current
• and your fingers wrap
• around the wire in the
• direction of the B field.
• B-field Magnetic field

Compass near a current-carrying wire
18

• Form a loop with current carrying wire, and the
  concentration of the magnetic field within the
  loop is much stronger. Double the number of
  loops, and the magnetic field is twice as strong.
  The magnetic field intensity increases with the
  number of loops. A current carrying coil of wire
  with many loops is an electromagnet.
• Sometimes a piece of iron is placed inside
• the coil of an electromagnet. The magnetic
• domains of the iron are induced into
• alignment, increasing the magnetic field
• intensity. Beyond a certain limit, the
• magnetic field in the iron saturates, so
• iron is not used in the cores of the
• strongest electromagnets, which are made
• of superconducting material (section 34.4)

19
36.6 Magnetic Forces on Moving
Charged Particles

• A charged particle moving in a plane
  perpendicular to a magnetic field will move in a
  circular orbit with the magnetic force playing
  the role of centripetal force.
• The direction of the force is given by the
  right-hand rule.
• Equating the centripetal force with the magnetic
  force and solving for R the radius of the
  circular path we get
• mv2 / R q v B and
•  R m v / q B

Orbit of charged particle in a magnetic field
20

• Right Hand Rule

21
36.7 Magnetic Forces on Current-

Carrying Wires

• Since a charged particle moving through a
  magnetic field experiences a deflecting force, a
  current of charged particles moving through a
  magnetic field also experiences a deflecting
  force. The direction of that deflection is
  dependent upon the direction of the current.

22
36.8 Meters to Motors

• The basic galvanometer, devised by William
  Sturgeon in 1825, allows all of the various
  combinations of current and magnetic needle
  direction to be tried out. By making suitable
  connections to the screw terminals, current can
  flow to the right or to the left, both above and
  below the needle. Current can be made to travel
  in a loop to double the effect, and, with the aid
  of two identical external galvanic circuits, the
  currents in the two wires can be made parallel
  and in the same direction. Note that the wires
  are insulated from each other where they cross.

23

• Electric Motor
• A current-carrying loop in a B field is the basis
  of an electric motor.
• Using the right-hand rule one can see that the
  forces acting on the wire will cause the loop to
  rotate. Changing the current direction at the
  right time will cause the loop to continue
  rotating on the motor shaft.

24

• DC motor

25
36.9 Earths Magnetic Field

•     In ancient times, both Greeks and Chinese
  knew about natural magnets, rare chunks of
  iron-rich mineral known as lodestones. The
  Chinese also knew that if you rubbed a steel
  needle against a lodestone, in a fixed direction,
  it also became a magnet. Around the year 1000,
  they furthermore found that if a magnet or
  lodestone was placed on a little "boat" floating
  in a bowl of water, it always pointed in a fixed
  direction--and for a magnetized iron bar, that
  direction was always north-south. You could
  rotate the bowl, but the magnet would keep
  pointing in the same direction.
•     The reason, we now know, is that the Earth,
  too, is magnetic. From that came the magnetic
  compass, quickly copied by Arab navigators and
  then by Europeans. We may wonder today--if
  lodestones did not exist, the compass might have
  stayed undiscovered for a long time, and would
  Columbus have ventured so far from land without
  it?

26

• Robert Norman and an early scientific experiment
• Figure 4
•     This is mainly about explaining a very
  fundamental concept in science--the experiment. A
  scientific experiment is a way of testing nature,
  to learn how it behaves.
•     By 1580, the use and manufacture of compass
  needles was a well known art. The maker would
  take a flat steel needle, find its middle by
  balancing it, install a pivot there, and then
  magnetize it by stroking it against a magnet or a
  lodestone. But that was not enough. The
  north-pointing end always seemed heavier, and a
  tip had to be snipped off, to make the needle
  balance again.
•  

27

•   The story goes that a compass maker named
  Robert Norman once snipped off too much and
  ruined a needle, so he devised an experiment, to
  find what was happening. Before magnetizing the
  needle, he balanced it not on a vertical pivot
  but on a horizontal one, lined up in the
  east-west direction. (Figure 4, above). Before
  the needle was magnetized, it stayed horizontal.
  Afterwards, its north end slanted down. (For some
  reason, the needle in Figure 4 points straight
  down, as it would at the magnetic pole.) Aha! The
  north-pointing magnetic force on the needle was
  not horizontal, but pointed into the Earth.
• 
  Slide 5

    It was a classical scientific experiment, one
of the first, and was published in 1581. Norman's
contemporary was William Gilbert, distinguished
physician and later physician to Queen Elizabeth
I. Gilbert devoted much of his energy and money
to study magnetism, and in 1600 published his
research in a book "De Magnete" (Latin for "On
the Magnet"). The preceding visualization of the
downward "dip angle" of the magnetic force (Slide
5) is from this book.
28

•     Gilbert devised an experiment which suggested
  a reason for the properties of the compass the
  Earth itself was a giant magnet. Using as model
  for the Earth a lodestone fashioned into a sphere
  (he named it "terrella" or "little Earth"),
  Gilbert reproduced not only the north pointing
  properties of the horizontal needle, but also the
  downward slanting of the needle which Robert
  Norman made.

    You will find two reviews of Gilbert's book
and a lot more, including most of what we are
telling you here today, in a web course on Earth
magnetism, "The Great Magnet, the Earth" With
home page by Dr. David P. Stern
http//www.phy6.org/earthmag/demagint.htm .
29
Earth's Inconstant Magnetic Field Our planet's
magnetic field is in a constant state of change,
say researchers who are beginning to understand
how it behaves and why.

• I found the following story, and thought it was
  extremely interesting! If you would like to read
  the entire story go to the following website
• http//science.nasa.gov/headlines/y2003/29dec_magn
  eticfield.htm
• December 29, 2003 Every few years, scientist
  Larry Newitt of the Geological Survey of Canada
  goes hunting.
• His quarry is Earth's north magnetic pole.
• At the moment it's located in northern Canada,
  about 600 km from the nearest town Resolute Bay,
  population 300, where a popular T-shirt reads
  "Resolute Bay isn't the end of the world, but you
  can see it from here.
• Scientists have long known that the magnetic pole
  moves.
• Sometimes the field completely flips. The north
  and the south poles swap places. Such reversals,
  recorded in the magnetism of ancient rocks, are
  unpredictable.

30

• At the heart of our planet lies a solid iron
  ball, about as hot as the surface of the sun.
  Researchers call it "the inner core." It's really
  a world within a world. The inner core is 70 as
  wide as the moon. It spins at its own rate, as
  much as 0.2 of longitude per year faster than
  the Earth above it, and it has its own ocean a
  very deep layer of liquid iron known as "the
  outer core."
• At the heart of our planet lies a solid iron
  ball, about as hot as the surface of the sun.
  Researchers call it "the inner core." It's really
  a world within a world. The inner core is 70 as
  wide as the moon. It spins at its own rate, as
  much as 0.2 of longitude per year faster than
  the Earth above it, and it has its own ocean a
  very deep layer of liquid iron known as "the
  outer core."
• Earth's magnetic field comes from this ocean of
  iron, which is an electrically conducting fluid
  in constant motion. Sitting atop the hot inner
  core, the liquid outer core seethes and roils
  like water in a pan on a hot stove. The outer
  core also has "hurricanes"--whirlpools powered by
  the Coriolis forces of Earth's rotation. These
  complex motions generate our planet's magnetism
  through a process called the dynamo effect.
• The next page has some of the figures pertaining
  to the information
• given above. If you
  would like more, go to the website!

31

Right a schematic diagram of Earth's interior.
The outer core is the source of the geomagnetic
field.
The movement of Earth's north magnetic pole
across the Canadian arctic, 1831--2001.
Supercomputer models of Earth's magnetic field.
On the left is a normal dipolar magnetic field,
typical of the long years between polarity
reversals. On the right is the sort of
complicated magnetic field Earth has during the
upheaval of a reversal.