Magnetism 

 Chapter 9


 Joseph F. Alward, PhD
 Department of Physics
 University of the Pacific
                        CD reader

   

 

   Magnetic Poles
All magnets have north
and south poles.

North attracts south,
south attracts north.

North repels north,
south repels south.

 

 

 

 

 

  North Poles Cannot Be Separated from South
No matter how many times
a bar magnet is cut in half,
there is always a north and
south pole, even in the
smallest piece.

 

 

  

 

   Magnetic Field of a Bar Magnet
Un-magnetized iron
brought near a magnet
becomes magnetized.
Iron filings line up around
bar magnet.  

This shows the shape of
the magnetic field.

 

 

  

 

   Compass Needle
Compass needles are just
small magnets which twist
into alignment with
the magnetic field.

 

 

  

 

  Atomic Magnetism 
Magnetism originates
in the motion of the
electrons in iron.  

Spinning electrons
act like tiny magnets.

Cancellation of this
effect occurs in most
materials.  Iron,
nickel, cobalt are
exceptions.

 

 

  

 

   Magnetic Domains
The magnetic north-south axes of groups
of iron atoms line up in the same direction.

These groupings are called domains.

In unmagnetized iron, the domains are
randomly oriented.

An external magnetic field will twist the
domains into alignment

 

 

  

 

   Magnetization
In unmagnetized iron, the
domains are randomly oriented.

In slightly magnetized iron,
there is incomplete alignment
of domains.

In strongly magnetized irion,
virtually all of the domains are
aligned.

Even if the magnet were divided
into bits as small as a single
domain, it would still have
north and south poles.

 

 

  

 

 Magnetic Field Around a Current-Carrying Wire
Spinning or rotating electrons
are responsible for magnetism
in iron.

Moving charges set up
magnetic fields.

Compasses (bar magnets)
line up in circles around a
wire carrying current.

 

 

  

 

   Magnetic Field Around a Current-Carrying Wire
The magnetic field
of a current-
carrying wire
consists of an
infinite number
of concentric
circles of different
radii, extending
out to infinity.
The face of a
circular loop of
wire is like the
north pole of
a magnet.

 

 

  

 

   Magnetic Field Patterns around a Current Carrying Wire
Iron filings
form
concentric
circles
around
wire.

 

 

  

 

   Magnetic Levitation
Trains float
above
guideway.

 

 

  

 

 Magnetic Fields Exert Forces on Moving Charges
If charges
move at an
angle with
respect to
the direction
of the
magnetic
field, they
experience
a sideways
force.

 

 

  

 

  Cosmic Rays 
Cosmic rays are electrons
and protons streaming in
from the Sun.

Most of these particles
are deflected by the
Earth's magnetic field.

 

 

  

 

    Magnetic Force on Current-Carrying Wires
Moving
electrons
in wire
are
pushed
up, or
down,
depending
on their
direction.

 

 

  

 

 Measuring Currents

Coils of current-carrying wires set up magnetic field perpendicular to plane of coil.
Compass needle aligns itself with the field lines.

 

 

  

 

   Another Current-Measuring Apparatus
Coil carrying current
is like a magnet.

Electromagnet tends to
align its north face with
the iron magnet's south
face.

A spring (not shown)
resists this tendency
to twist; the greater
the current, the greater
the deflection of the
needle.

 

 

  

 

   The Electric Motor
Force on segments parallel to
the loop axis (broken line)
are perpendicular to
segments and in opposite
directions.

 

 

  

 

  Electromagnetic Induction
Magnets moved near--or
even inside coils of wire
induce (cause) a current
in the coil.

The more coils there are,
the more current there is.

 

 

 

 

  Electromagnetic Induction 
 

  Magnet moving inside coils induces a current; current is proportional to the number
  of loops.

 

   

 

   Metal Detectors
Metal detectors

Traffic lights:  embedded
coils of wire

Credit card magnetic strips

Recording tape

 

 

  

 

   Electromagnetic Induction
Moving wire
and keeping
the magnet
stationary is
the same as
moving the
magnet and
keeping the
wire
stationary.

 

 

 

 

  

 

    Faraday's Law Examples                  

Flux through coil changes because bar
magnet is moved up and down.           
AC current in bottom coil causes
changing B-field along iron core.

 

 

   

 

  Hand-Powered Electrical Generator
Moving a wire in a magnetic
field induces a current in the
wire.

This generator provides
direct current, current which
always travels in the same
direction through the
filament.  

Note:  the text on pages 223 and
224 wrongly describe this device
as an ac generator, when in fact
it is a dc generator.  

 

 

    

  

 

    Steam-Powered Electrical Generator

 

 

  

  

 

   

  Principles of Transformer Action                
Alternating current
in the primary coil
causes the north-
south polarity of
the electromagnet
to change.  This
is roughly the
same as flipping a
permanment
magnetic.

 

  

 

   The Transformer Equation
Alternating current in primary
causes oscillating magnetic
field in iron core.  From the
point of view of the secondary
coil, this is the same as plunging
a bar magnet in and out of
the secondary, flipping its
polarity each time.

 

 

 

  

  Power Transmission and Transformers                      
Output power = IV
Why is output at low
current and high
voltage, and not
high current and
low voltage?
Answer:  Heat loss
varies as the square
of the current.

 

 

 

 

    Automobile Ignition System                                  
When breaker points open,
current drops to zero,
magnetic field in primary
collapses, inducing a
large voltage in the
secondary coil.

 

 

 

 

 

 

   Transformer Station and Telephone Pole Transformer 

Steps down from 240,000 V to 8000 V
Steps down from 8000 V to 240 V

 

 

Stealing from the Power Company     

  How does this power-stealing device work?