This document discusses magnetism and electromagnetism. It begins by defining magnets and their properties such as polarity and attraction/repulsion. It then discusses how magnetic fields are created by magnets and electric currents. The strength of magnetic fields depends on factors like the current, number of coil turns, and the core material. A changing magnetic field can induce an electric current based on electromagnetic induction. Electromagnets and their applications in devices like speakers and computer monitors are also covered.

1. Magnetism,
Electromagnetism, and its
Applications

2. Magnetism
 What is a magnet?
– Phenomenon of magnetism has been
known since ancient times…
– Certain rocks (called lodestones) attracted iron
– When iron was rubbed with lodestone, the iron
became magnetized
– If a thin magnet was floated on water, one end of
the magnet always pointed in the northern
direction
 A magnet, then, is any substance
that possesses these properties.

3. Magnetism
 Magnets have polarity
– The end that points northward is the
north pole of a magnet.
– The end that points southward is the
south pole of a magnet.
 Like poles repel, and unlike poles
attract.

4. Magnetism
 If a piece of metal is placed in the
vicinity of a magnet, the metal itself
will become magnetized.
– If the metal retains its magnetism after
the original magnet is removed, it is
called a permanent magnet. (example
alloy called ALNICO)
– Otherwise, it is called a temporary
magnet. (example soft iron)

5. Magnetism
 Demo – Bar magnet, nail, and paper
clip.

6. Magnetism

7. Magnetism

8. Magnetism
 Why do certain substances have
magnetic properties?

9. Magnetism

10. Magnetism
 Domains –
microscopic
magnets created
by groups of
atoms that align
their unpaired
electrons so that
they spin in the
same direction.

11. Magnetic Fields
 Lab - Magnetic Field Lines

12. Magnetic Fields

13. Magnetic Fields
 Note that field lines form closed
loops inside the magnet.
– As a consequence, magnetic monopoles
are not believed to exist.

14. Magnetic Fields

15. Magnetic Fields
 Each magnetic flux line has been
standardized and has the SI unit of 1
weber (Wb).
 The strength of the magnetic field,
known as the magnetic induction,
is given by the concentration of the
flux lines (i.e. the number of flux
lines per unit area)

16. Magnetic Fields
 Magnetic induction is a vector
quantity because it has magnitude
and direction.
 We represent magnetic induction by
the letter B, and its unit is the weber
per square meter, also known as the
tesla (T).

17. Magnetic Fields
 The Earth’s magnetic field is weak,
and has a magnetic induction of ~5 x
10-5 tesla.
 A field of 1 tesla is extremely strong
– Magnetic resonance imaging

18. Electromagnetism
 In 1820, the Danish physicist Hans
Oersted discovered that a wire
carrying current produced a
magnetic field as follows… (on board)
– Note that the magnetic field is circular
and that its plane is perpendicular to
the direction of the wire carrying the
current.

19. First Hand Rule
 The thumb of the
right hand is pointed
in the direction of
the conventional
current. The fingers
of the right hand
(from wrist to
fingertips) will curl in
the direction of the
magnetic field.
(Note: if you use
electron-flow instead
of conventional
current, use your left
hand instead of your
right)

20. Electromagnetism
 To represent the direction of a
magnetic field in two dimensions, we
use dots (•) to indicate that the
direction is out of the plane of the
paper and X’s (x) to indicate that the
direction is into the plane of paper

21. Magnetic Field Around a Coil
 Second Hand Rule
– The fingers of the right hand are
wrapped in the direction of the
conventional current. The thumb will
point to the end of the coil which is the
north pole.

22. Solenoid Magnetic Field Strength
 Moving charges create magnetic
fields. Each electron moving in a
conductor creates its own magnetic
field. As electrons move through the coil
of wire, the magnetic field of one electron
adds to the field of any others moving in
the same direction. The faster a charge
moves, the stronger the magnetic field it
creates. For this reason alone, a higher
current implies an electron is moving
faster, and as a result, it would create a
stronger magnetic field.

23. The animation above shows the
magnetic field created by a solenoid
with a small current from a 3V source.
This animation shows the magnetic
field created by the same solenoid
with a higher current created by a 6V
source. Notice that the magnetic field
is shown twice as strong. (twice the
number of field lines) Doubling the
current doubles the field strength.
In conclusion, we could say the magnetic field strength of a solenoid is directly
related to the current through the coil.

24. Solenoid Magnetic Field Strength
 Since the magnetic field is created by
moving electrons we could argue that the
more electrons are moving, the stronger
the magnetic field would be. A given
length of wire contains a certain number
of electrons. Twice that length will contain
twice as many electrons. If a solenoid is
made with more "turns" or "wraps" of
wire, then it must create a stronger
magnetic field.

25. This solenoid has only three turns or
wraps of wire around it. Its magnetic
field is not very strong.
This solenoid has 6 turns of wire around
it. If all else is constant, the magnetic
field should be twice as strong since it
has twice as many turns.
In conclusion we could say that the number of turns of wire around a solenoid is
directly related to the magnetic field strength of the solenoid.
In reality it is not quite direct, since doubling the amount of wire would increase the
resistance. The increased resistance would lead to less current from the same
source. This animation ignores this effect.

26. Solenoid Magnetic Field Strength
 Some materials are more susceptible
to magnetic fields than others. We
say they are more "permeable" to
the magnetic field.

27. This is diagram illustrates the magnetic
field in the air between two poles of a
horseshoe magnet
This diagram is for the same magnet
with a piece of soft iron placed
between the poles. The soft iron is
more permeable to the magnetic field
than the air is. Notice how the soft iron
seems to focus the magnetic
field. This is a result of the iron
actually becoming a magnet while
placed in the existing field. As a result,
when a compass is moved to map the
field it is influenced by the iron. Thus
showing a more "focused" or stronger
field.

28.  The solenoid above
would be considered
an "air core" solenoid
since the coils are
wrapped around a
hollow core.
 This solenoid has a
soft iron core which is
more permeable and
as a result, creates a
much stronger
magnetic field. As a
result, it is a much
stronger
electromagnet.

29. Magnetic Field Around a Coil
 Factors that affect the magnetic induction
of a coil (summary)
– The current in the coil (the more current, the
stronger the field)
– The number of turns per unit length of the coil
(the more turns the stronger the field)
– The nature (material) of the core.
 Demo with wood core and metal core

30. Forces on a Current Carrier in a
B Field
 If a wire carrying a current is placed in a
magnetic field so that the direction of the
current is perpendicular to the direction of
a magnetic field, the magnetic field of the
wire will interact with the magnetic field of
the magnet to produce a force on the
wire.
Demo
Illustration Page 502 of text.

32. Third Hand Rule
 Thumb (right hand) points in direction of
conventional current
 Fingers point in direction of the magnetic
field
 Direction of the force on the current
carrying wire points away from the palm.
 Electron-flow users, left hand, please.

33. Forces on a Current Carrier in a
B Field
 Note that if the current is parallel to
the magnetic field, no force
develops.
 When the direction of the current is
perpendicular to the magnetic field,
the magnitude of the force on the
wire can be found from:
F = I l B

34. Electromagnetic Induction
 Electromagnetic induction is the
process of generating a potential
difference in a conductor due to
relative motion between the
conductor and a magnetic field.
– Overhead diagram
– Apply third hand rule

35. Electromagnetic Induction
 If a conductor “cuts” across magnetic flux
lines, a magnetic force acts on the
electrons in the conductor, causing them
to move from one end to the other.
– This results in a potential difference,
specifically called an induced potential
difference.
– If the conductor is part of a complete circuit,
an electric current is produced.
– If the conductor is moved parallel to the lines
of flux, no potential difference is induced.
– This is the principle behind electric generators

36.  In this movie you see an
electron moving and then
entering a magnetic field
(B-field). The direction of
the B-field is into the
screen. Notice that the
force created by the
magnetic field ends up
always pointing to the
center of the curve. This
force is considered to be a
centripetal force (see more
in the motion in a plane
unit ) because it causes
the electron to travel in a
circular path.

37.  If the charge on the
electron were greater,
the force would be
greater.
 If the speed of the
electron were greater,
the force would be
greater.
 If the magnetic field
strength were greater,
the force on the
electron would be
greater.

38.  If the mass of the
electron were
greater, it would
have no effect on
the force, but the
circular path would
be larger.

39.  The screen of your computer
(unless it is an LCD) and your
television screen both use
magnetic fields to deflect
electrons fired from an
"electron gun". The moving
electrons are directed toward
different areas of the screen
by magnetic fields created by
electromagnets. Wherever the
electrons strike the screen,
they cause phosphorus to give
off light. If millions of
electrons are directed to the
screen in the right places, the
little bursts of light leave an
impression our eyes which
forms an image. An instant
later (1/24th of a second) a
million more electrons strike
the screen and form a new
image. Our mind pieces them
together to create the
impression of smooth motion.