1. Magnetism
A Strangely Attractive Topic
BY
WASAY ZIA
+92-314-9030305

2. History #1
Term comes from the ancient Greek city of
Magnesia, at which many natural magnets were
found. We now refer to these natural magnets
as lodestones (also spelled loadstone; lode means
to lead or to attract) which contain magnetite, a
natural magnetic material Fe3O4.
Pliny the Elder (23-79 AD Roman) wrote of
a hill near the river Indus that was made
entirely of a stone that attracted iron.

3. History #2
Chinese as early as 121 AD knew that an
iron rod which had been brought near one of
these natural magnets would acquire and
retain the magnetic property…and that such a
rod when suspended from a string would align
itself in a north-south direction.
Use of magnets to aid in navigation can be
traced back to at least the eleventh century.

4. Basically, we knew the phenomenon existed and
we learned useful applications for it.
We did not understand it.

5. Finally, the Science
Not until 1819 was a connection between electrical and
magnetic phenomena shown. Danish scientist Hans Christian
Oersted observed that a compass needle in the vicinity of a
wire carrying electrical current was deflected!
In 1831, Michael Faraday discovered that a momentary
current existed in a circuit when the current in a nearby
circuit was started or stopped
Shortly thereafter, he discovered that motion of a
magnet toward or away from a circuit could produce the same
effect.

6. Let This Be a Lesson!
Joseph Henry (first Director of the
Smithsonian Institution) failed to publish what
he had discovered 6-12 months before Faraday

7. The Connection is Made
SUMMARY: Oersted showed that magnetic effects
could be produced by moving electrical charges;
Faraday and Henry showed that electric currents
could be produced by moving magnets

8. A Sheep in a Cow Suit?
All magnetic
phenomena result
from forces between
electric charges in
motion.

9. Looking in More Detail
Ampere first suggested in 1820 that
magnetic properties of matter were due to tiny
atomic currents
All atoms exhibit magnetic effects
Medium in which charges are moving has
profound effects on observed magnetic forces

10. For most of our discussions, we will
assume the medium is empty space,
which is a reasonable approximation of
air in this context.

11. Top Ten List
What We Will Learn About Magnetism
1. There are North Poles and South Poles.
2. Like poles repel, unlike poles attract.
3. Magnetic forces attract only magnetic materials.
4. Magnetic forces act at a distance.
5. While magnetized, temporary magnets act like permanent
magnets.

12. Top Ten continued
6. A coil of wire with an electric current flowing through it becomes
a magnet.
7. Putting iron inside a current-carrying coil increases the strength
of the electromagnet.
8. A changing magnetic field induces an electric current in a
conductor.

13. Top Ten Continued
9. A charged particle experiences no magnetic force when
moving parallel to a magnetic field, but when it is moving
perpendicular to the field it experiences a force perpendicular
to both the field and the direction of motion.
10. A current-carrying wire in a perpendicular magnetic field
experiences a force in a direction perpendicular to both the
wire and the field.

14. For Every North, There is a South
Every magnet has at least one north pole and one south pole. By
convention, we say that the magnetic field lines leave the North end
of a magnet and enter the South end of a magnet.
If you take a bar magnet and break it into two pieces, each piece will
again have a North pole and a South pole. If you take one of those
pieces and break it into two, each of the smaller pieces will have a
North pole and a South pole. No matter how small the pieces of the
magnet become, each piece will have a North pole and a South pole.
S N S N S N

15. No Monopoles Allowed
It has not been shown to be possible to end up with a single
North pole or a single South pole, which is a monopole ("mono"
means one or single, thus one pole).
S N
Note: Some theorists believe that magnetic monopoles may
have been made in the early Universe. So far, none have been
detected.

16. Magnets Have Magnetic Fields
We will say that a moving charge sets up in the space
around it a magnetic field,
and
it is the magnetic field which exerts a force on any other
charge moving through it.
Magnetic fields are vector
quantities….that is, they have a
magnitude and a direction!

17. Defining Magnetic Field Direction
Magnetic Field vectors as written as B
Direction of magnetic field at any point is defined
as the direction of motion of a charged particle on
which the magnetic field would not exert a force.
Magnitude of the B-vector is proportional to the
force acting on the moving charge, magnitude of the
moving charge, the magnitude of its velocity, and the
angle between v and the B-field. Unit is the Tesla or
the Gauss (1 T = 10,000 G).

18. Scientists Can Be Famous, Too!
Tesla

19. Famous, continued
Gauss

20. The Concept of “Fields”
Michael Faraday
realized that ...
A magnet has a
‘magnetic field’
distributed throughout
the surrounding space

21. Magnetic Field Lines
Magnetic field lines describe the structure of magnetic fields
in three dimensions.They are defined as follows. If at any
point on such a line we place an ideal compass needle, free to
turn in any direction (unlike the usual compass needle, which
stays horizontal) then the needle will always point along the
field line.
Field lines converge where the magnetic force is strong, and
spread out where it is weak. For instance, in a compact bar
magnet or "dipole," field lines spread out from one pole and
converge towards the other, and of course, the magnetic
force is strongest near the poles where they come together.

22. Field Lines Around a Magnet

23. Field Lines Around a Doughnut Magnet

24. Field Lines Around a Bar Magnet

25. Field Lines Around a Magnetic Sphere

26. Field Lines of Repelling Bars

27. Field Lines of Attracting Bars

28. Action at a Distance Explained
Although two magnets
may not be touching,
they still interact
through their
magnetic fields.
This explains the
‘action at a distance’,
say of a compass.

29. Force on the Charge
Right Hand Rule!
Put your fingers in the direction of motion of
the charge, curl them in the direction of the
magnetic field. Your thumb now points in the
direction of the magnetic force acting on the
charge. This force will bend the path of the
moving charge appropriately.

30. Watch the Bending Fingers!
QuickTime™ and a
GIF decompressor
are needed to see this picture.

31. Cyclotron
Developed in 1931 by E. O. Lawrence and
M. S. Livingston at UC Berkeley
Uses electric fields to accelerate and
magnetic fields to guide particles at very high
speeds

32. How a Cyclotron Works
Pair of metal chambers
shaped like a pillbox cut
along one of its diameters
(cleverly referred to as “D”s)
and slightly separated
Ds connected to
alternating current
Ions injected near gap
Ions are accelerated as long as they remain “in
step” with alternating electric field

33. Magnetic Force on Current-Carrying Wire
Since moving charges experience a force in a magnetic field, a current-
carrying wire will experience such a force, since a current consists of
moving charges. This property is at the heart of a number of devices.

34. Electric Motor
An electric motor, is a
machine which converts
electrical energy into
mechanical (rotational or
kinetic) energy.
A current is passed
through a loop which is
immersed in a magnetic
field. A force exists on
the top leg of the loop
which pulls the loop out
of the paper, while a
force on the bottom leg
of the loop pushes the The net effect of these forces is
loop into the paper. to rotate the loop.

35. Electromagnet (Magnetism from Electricity)
An electromagnet is simply a coil of wires which, when a
current is passed through, generate a magnetic field, as
below.

36. Magnetic Properties of Matter
In other words….materials which produce
magnetic fields with no apparent circulation of
charge.
All substances - solid, gas, and liquid - react to
the presence of a magnetic field on some level.
Remember why?
How much they react causes them to be put into
several material “types”.

37. Magnet - isms
Ferromagnetism - When a ferromagnetic material is
placed near a magnet, it will be attracted toward the region of
greater magnetic field. This is what we are most familiar with when
our magnet picks up a bunch of paperclips. Iron, cobalt, nickel,
gadolinium, dysprosium and alloys containing these elements exhibit
ferromagnetism because of the way the electron spins within one
atom interact with those of nearby atoms. They will align
themselves, creating magnetic domains forming a permanent magnet.
If a piece of iron is placed within a strong magnetic field, the
domains in line with the field will grow in size as the domains
perpendicular to the field will shrink in size.

38. Making a Magnet from a Ferromagnetic Material
• domains in which the magnetic
fields of individual atoms align
• orientation of the magnetic
fields of the domains is random
• no net magnetic field.
• when an external magnetic
field is applied, the magnetic
fields of the individual domains
line up in the direction of the
external field
• this causes the external
magnetic field to be enhanced

39. A Ferromagnet in the Middle
If we look at a solenoid, but rather than
air, wrap it around a nice iron core. What
happens to the change in flux for a given
current?
Can you see why ferromagnetic materials
are often put in the middle of current-
carrying coils?

40. More Magnet - isms
Diamagnetism - When a diamagnetic material is placed
near a magnet, it will be repelled from the region of greater
magnetic field, just opposite to a ferromagnetic material. It is
exhibited by all common materials, but is very weak. People and
frogs are diamagnetic. Metals such as bismuth, copper, gold,
silver and lead, as well as many nonmetals such as water and most
organic compounds are diamagnetic.

41. More Magnet - isms
Paramagnetism - When a paramagnetic material is
placed near a magnet, it will be attracted to the region of greater
magnetic field, like a ferromagnetic material. The difference is
that the attraction is weak. It is exhibited by materials
containing transition elements, rare earth elements and actinide
elements. Liquid oxygen and aluminum are examples of
paramagnetic materials.

42. Let’s Play!