Download Magnetic Field of Force

Law of Magnetic Poles
Lodestone, or magnetite
is magnetic and will align
itself in a north-south position.
Currently iron, nickel and
cobalt are used for magnets.
Law of Magnetic Poles
Opposite magnetic poles attract
Similar magnetic poles repel
Drawing Fields (Magnetic Field of Force,  )
1. Lines close together signify greater force
2. Lines are concentrated at the poles (3D)
3. Conventionally lines go from [S] to [N] inside magnet
4. Lines do not cross
Magnetic Materials
Ferromagnetic materials have
dipoles that can all be aligned
under the influence of a
magnetic field to form a
magnetic domain. This
causes an unmagnetized
metal to develop "poles".
Magnetic induction of soft iron
doesn't last long, but hard iron
contains more carbon and will
maintain the magnetic qualities
longer. Dropping or heating the metal can accelerate the
dipoles to return to random orientations and demagnetize
the metal. Polarities can be reversed using strong
magnets.
Oersted's Discovery
Hans Christian Oersted (1777-1851)
accidentally discovered that a current
running through and conductor
generated a magnetic field that could
influence a compass.
Principle of Electromagnetism
Whenever an electric current moves through a conductor,
a magnetic field is created in the region around the
conductor.
Right-Hand Rule #1
If the conductor is held in the
right hand, the direction of the
current follows the thumb while
the direction of the curled fingers
point in the direction of the
magnetic field.
If you look directly down the length of
the
conductor, a current travelling toward you
is conventionally drawn as you would see an arrow coming
at you!
Toward you (head)
- out of page
Away from you (tail)
- into page
Right-Hand Rule for a Coil RHR#2
If a coil is grasped in the right hand with the curled fingers
representing the direction of the electrical current, the
thumb points in the direction of the magnetic field inside the
coil.
Magnetic Fields  : Coils and Solenoids
If a straight conductor is coiled, the magnetic field inside
the coil is intensified while it is conducting electricity. If the
coils are wound close together, it is called a solenoid.
Inside the solenoid the magnetic field is uniformly
distributed and acts in the same direction. If the direction
of the current is reversed, the poles of the magnetic field
are reversed. Electromagnets can be useful because they
can be turned on and off.
Factors
1. Current - directly proportional in
an air coil
2. Number of Loops - directly
proportional per length
3. Type of Core - directly
proportional to the relative
magnetic permeability (K) which
is a ratio of the magnetic field
strength in material/vacuum
Magnetic Permeability Classifications
1. Ferromagnetic - become strong induced magnets
when placed in a coil (Fe, Ni, Co)
2. Paramagnetic - become slight induced magnets when
placed in a coil (O2, Al)
3. Diamagnetic - cause a decrease in magnetic field
strength when placed in a coil (Cu, Ag, and H2O)
8.2 Magnetic Force on a Moving Charge
A current can exert a force on a magnet and a magnet can
exert a force on a current.
A moving electric charge or current produces a magnetic
field, B (Units: Tesla, T = kg/(Cs). (scientist: Oersted). Do you
know why it has to be moving?
If a charged particle enters a magnetic field at an angle to
the magnetic field lines, it experiences a magnetic force,
F , which makes the path of the particle curve.
The magnitude of the magnetic force is given by:
M
FM  qvB sin 
When v and B are at 90 degrees (   90 ) FM is at
maximum. Remember the circular motion of satellites in
orbit? The centripetal force was caused by gravity. We
have a similar situation with the magnetic force causing the
centripetal force for a moving charged particle in a
magnetic field when v and B are at 90 degrees.
The direction of the magnetic force is given by Right Hand
Rule #3 (motor principle – remember from Grade 11?).
.
Recall:The Motor Principle: (Scientist: Faraday)
A current-carrying conductor (or moving charge) that cuts
across an external
magnetic field line
experiences a force
perpendicular to both
the magnetic field and
the direction of the
electrical current. The
magnitude of the force
depends on the current
and magnetic field as
well as the angle
between the conductor
and the magnetic field.
Right-Hand Rule #3 for the Motor Principle
If the fingers of the open right hand point in the direction of
the external magnetic field, and the thumb represents the
direction of the electrical current, the force on the
conductor will be in the direction in which the right palm
faces.
Remember, magnetic field lines are always drawn from
north to south (outside). The north "pointer" of a compass
pointing to the north pole - actually consists of a south
"pole".
Do practice questions using RHR#3 to determine direction
of magnetic force.
Example 1: An electron accelerates from rest in a
horizontally directed electric field through a potential
difference of 50. Volts. The electron then leaves the electric
field and enters a magnetic field of 0.30 Tesla directed info
the page.
a) Find the initial velocity of the electron as it enters the
magnetic field.
b) Find the magnitude and direction of the magnetic force
on the electron.
c) Find the radius of the electron’s circular path.
Example 2:Find the mass of chlorine-35 ions (charge of
1.60x10-19C) accelerated into a mass spectrometer through
a potential difference of 2.50x102V into a 1.00 Tesla
magnetic field. The radius of the curved path is 1.35 cm.
8.3 Magnetic Force on a Conductor
Since moving charges are really like current we can
represent the formula FM  qvB sin  in terms of current
F  IlB sin 
by M
where I is the current and l is the length
of the conductor.
Example: What length of conductor, running at right angles
to a 0.033 T magnetic field and with a current of 20.0 A,
experiences a force of 0.10 N?
Charge to Mass Ratio (Thompson’s discovery)
Forces can combine to change the direction of an
electron’s motion.
After this discovery, Millikan found the elemental charge, e,
and combined with Thomson’s discovery they were able to
find the mass of an electron.
Application
This led to the development of mass spectrometers (to
separate different particles/ions by mass) where particles
are accelerated into a magnetic field perpendicular to their
velocity. The particles follow different paths depending on
their mass and charge.
Lenz's Law
"The direction of the induced current is such as to oppose the change causing it."
Which means that when a magnet is brought near a coil,
the coil develops an induced polarity OPPOSITE to the
magnet!
This causes an electric current to flow within the coil while
the magnet is moving - an INDUCED CURRENT.
When the magnet is withdrawn from the coil, the polarity of
the coil is reversed, and so is the direction of the current.
The size of the current
depends on how fast the
magnet moves in or out of
the coil, and the number of
loops in the coil.
The phenomenon of
inducing a current by
changing the magnetic field
in a coil of wire is known as
ELECTROMAGNETIC
INDUCTION.
This phenomenon underpins
the design of all electric
generators.
www.physchem.co.za/Current10/Magnetic3.htm