Download Magnetism

Magnetism
Chapters 36 & 37
Magnetism
A brief history
Lodestones were found in Greece
some 2000 years ago.
The Chinese later used them for
navigating ships.
In the 18th century, Charles Coulomb
conducted a study of the forces between
lodestones.
Magnetism
A brief history:
Until early 19th century, electricity and
magnetism were considered to be
separate fields.
Hans Christian Oersted, in 1820,
discovered a relationship between the two
during a classroom demonstration.
This led to new technology that would
bring electric power, radio and television.
Magnetic Poles
Magnets apply forces on each other similar
to charges.
Magnets can attract and repel each other.
Magnets have poles that are the regions in
the magnet that apply forces.
Magnetic poles are not positive and
negative, but rather North and South.
Magnetic Poles
There is a rule when it comes to the
poles of magnets:
Like poles repel; opposite poles attract.
Magnetic Poles
What would happen if you were to cut a bar
magnet in half?
Magnetic Field
Every magnet produces a magnetic field.
A magnet’s magnetic field is similar to a
planet’s gravitational field.
When another magnet is near, or even a
compass, it will lie in a line with the
magnetic field.
Similarly, iron filings become tiny bar
magnets in the presence of a magnetic
field.
Magnetic Field
What did we learn in our lab about the shape of
a magnetic field around a bar magnet?
Magnetic Field
What did we learn about the direction of the
magnetic field lines around a bar magnet?
Field lines go out from the North and into the South.
Magnetic Field
What would happen to the magnetic fields
of two like poles placed next to each
other?
N
N
Magnetic Field
What would happen to the magnetic fields
of two like poles placed next to each
other?
N
N
Magnetic Field
What would happen to the magnetic fields
of two opposite poles placed next to each
other?
N
S
Magnetic Field
What would happen to the magnetic fields
of two opposite poles placed next to each
other?
N
S
Magnetic Field
What can you tell about the two magnets in
each of these situations?
Earth’s Magnetic Field
This is a drawing of Earth’s magnetic field
and its direction. What do you notice?
So which magnetic pole is which?
Magnetic Domains
Magnetic domains
= a microscopic
cluster of atoms
with their
magnetic fields
aligned.
Magnetic Domains
In our lab yesterday, how were you able to pick
up the paperclips with the nail?
What did the magnet do to the nail?
Induced magnetism = metals (particularly iron)
exhibiting magnetic properties due to contact
with another magnet.
Electric Current & Magnetic Field
Remember Oersted? What did he discover
during a classroom presentation?
A moving charge/current produces a
magnetic field, deflecting a compass.
No current:
Current:
Electromagnetism
• First Right-Hand
Rule
I
– Thumb points in
direction of
current
– Fingers follow
magnetic field
lines (direction of
magnetic field)
I
Electric Current & Magnetic Field
These are examples of a current-carrying wire, a
current-carrying loop and a coil of loops.
Electric Current & Magnetic Field
If a current-carrying wire is bent into a loop, the
magnetic field lines bunch up. If you add another
loop and another, the magnetic field becomes more
and more concentrated. This coil is called an
electromagnet.
Electromagnetism
I
• What about a
coil of wire?
– The RHR still
applies!
Electromagnets
• Coil has a field like any
permanent magnet with N and S
poles
• Advantage: can be turned off
and on
Electromagnets
• 2nd Right-Hand Rule
–Determine magnetic field of
electromagnets
–Fingers follow current as it curls in the
coil
–Thumb points in direction of N pole
Magnetic Force
A magnetic field will also apply a force on a
current-carrying wire.
To determine direction, we use the Right Hand Rule.
Forces caused by
Magnetic Fields
• Vectors
• Perpendicular to magnetic
field lines and current
Forces caused by
Magnetic Fields
• 3rd Right-Hand Rule
– Determine direction of Force on a currentcarrying wire in a magnetic field
I
N
S
Magnetic Force
Let’s try another one…
Thumb points in direction of current.
Fingers point in direction of magnetic field.
Palm points in direction of force.
Magnetic Force
Give this one a try:
I
N
S
Force: into the page
Magnetic Force
N
S
I Force: out of the page
Magnetic Force
N
S
X
I
Force: down
Forces caused by
Magnetic Fields
• F = BIL
–B = strength of magnetic field
–I = current in the wire
–L = length of wire in magnetic field
–We know how to measure F, I and L,
but not B so instead we use…
Forces caused by
Magnetic Fields
• B = F / (IL)
–Magnetic induction – strength of
the magnetic field
• Units: Tesla (T)
• 1 T is very strong
• Most lab magnets are 0.01 T
• Earth’s magnetic field is 5 X 10-5 T
A Simple DC Motor
A Simple DC Motor
Important Definitions
• Magnetic flux
–Number of magnetic field lines
passing through a surface
Electromagnetic Induction
Faraday discovered that
electric current could
be produced in a wire
simply by moving a
magnet in and out of a
coil of the wire.
This is called
electromagnetic
induction.
Electromagnetic Induction
The greater the number of loops of wire that move
in a magnetic field, the greater the induced
voltage and the greater the current in the wire.
Magnetic Force
A magnetic field applies a force on a moving charge.
Force on a single charged
particle
• Cathode ray tube – TV!
–Electrons deflected by magnetic
fields to form pictures
Cathode Ray Tube
Electric fields pull electrons off atoms,
then more electric fields gather, and
focus electrons into a beam.
• Magnetic fields deflect electrons
side to side and up and down across
the screen
• Screen coated with phosphorous
that glows when struck
Force on a single charged
particle
• F = BIL
• F = B(qv/L)L
• F = Bqv
–q = charge of electron
–v = particle velocity
Magnetic Force
The magnetic field of Earth deflects many
charged particles that make up cosmic
radiation.
Van Allen Radiation Belts
• Electrons trapped in Earth’s magnetic
field
– Solar storms send high-energy charged
particles toward Earth
– They knock electrons off VA belts
– The electrons excite nitrogen and oxygen in
the atmosphere creating a “halo”
– The halo surrounds geomagnetic north
Van Allen Belts
• The Van Allen radiation belts are formed as a
result of earth’s magnetic field and shield us
from radiation. We can see the aurora borealis
as a result.