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MAGNETIC FIELD
the earth's magnetic field
• The magnetic field is the central concept used in describing magnetic phenomena.
• A region or a space surrounding a magnetized body or current-carrying circuit in which
resulting magnetic force can be detected.
• A magnetic field consists of imaginary lines of flux coming from moving or spinning
electrically charged particles. Examples include the spin of a proton and the motion of
electrons through a wire in an electric circuit.
Magnetic field or lines of flux of a moving charged particle
MAGNETIC FORCE
The magnetic field of an object can create a magnetic force on other objects with magnetic
fields. That force is what we call magnetism.
When a magnetic field is applied to a moving electric charge, such as a moving proton or the
electrical current in a wire, the force on the charge is called a Lorentz force.
Attraction
When two magnets or magnetic objects are close to each other, there is a force that attracts
the poles together.
Force attracts N to S
Magnets also strongly attract ferromagnetic materials such as iron, nickel and cobalt.
Repulsion
When two magnetic objects have like poles facing each other, the magnetic force pushes
them apart.
Force pushes magnetic objects apart
Magnetic and electric fields
The magnetic and electric fields are both similar and different. They are also inter-related.
Electric charges and magnetism similar
Just as the positive (+) and negative (−) electrical charges attract each other, the N and S
poles of a magnet attract each other.
In electricity like charges repel, and in magnetism like poles repel.
Electric charges and magnetism different
The magnetic field is a dipole field. That means that every magnet must have two poles.
On the other hand, a positive (+) or negative (−) electrical charge can stand alone. Electrical
charges are called monopoles, since they can exist without the opposite charge.
• Monopole – a single magnetic pole or electric charge
• Dipole – a pair of opposite poles
• The so-called magnetic moment is the measure of the strength of the dipole.
The magnetic moments are expressed as multiples of Bohr Magnetons. A Bohr magneton
has a value of 9.27 x 10-24 joules/tesla.
FORMS OF MAGNETISM
DIAMAGNETISM
– is the property of an object which causes it to create a weak magnetic field in opposition
of an externally applied magnetic field. It is a form of magnetism that is only exhibited by a
substance in the presence of an externally applied magnetic field.
PARAMAGNETISM
– is a form of magnetism which occurs only in the presence of an externally applied
magnetic field. Paramagnetic materials are attracted to magnetic fields, hence have a
relative magnetic permeability greater than one (or, equivalently, a positive magnetic
susceptibility).
MOLECULAR MAGNET
– A single-molecule magnet or SMM is an object that is composed of molecules each of
which behaves as an individual super paramagnet. This is distinct from a molecule-based
magnet, in which a group of molecules behave collectively as a magnet.
FERROMAGNETISM
– is the “normal” form of magnetism, with which most people are familiar, as exhibited in
horseshoe magnets and refrigerator magnets. It is responsible for most of the magnetic
behavior encountered in everyday life. The attraction between a magnet and ferromagnetic
material is “the quality of magnetism first apparent to the ancient world, and to us today,”
according to a classic text on ferromagnetism.
ANTIFERROMAGNETISM
– the magnetic moments of atoms or molecules, usually related to the spins of electrons,
align in a regular pattern with neighbouring spins (on different sublattices) pointing in
opposite directions.
FERRIMAGNETISM
– a ferrimagnetic material is one in which the magnetic moment of the atoms on different
sublattices are opposed, as in antiferromagnetism; however, in ferrimagnetic materials, the
opposing moments are unequal and a spontaneous magnetization remains.
METAMAGNETISM
– is the increase in the magnetization of a material with a small change in an externally
applied magnetic field. The metamagnetic behavior may have quite different physical causes
for different types of metamagnets.
SUPERPARAMAGNETISM
– is a form of magnetism which occurs only in the presence of an externally applied
magnetic field.
MAGNETIC FIELDS and FORCES
The same situations which create magnetic fields (charge moving in a current or in an atom,
and intrinsic magnetic dipoles) are also the situations in which a magnetic field has an
effect, creating a force. Following is the formula for moving charge; for the forces on an
intrinsic dipole, see magnetic dipole.
When a charged particle moves through a magnetic field B, it feels a force F given by the
cross product:
where is the electric charge of the particle, is the velocity vector of the particle, and is
the magnetic field. Because this is a cross product, the force is perpendicular to both the
motion of the particle and the magnetic field. It follows that the magnetic force does no
work on the particle; it may change the direction of the particle’s movement, but it cannot
cause it to speed up or slow down. The magnitude of the force is where the angle between
the vectors is.
One tool for determining the direction of the velocity vector of a moving charge, the
magnetic field, and the force exerted is labelling the index finger “V”, the middle finger “B”,
and the thumb “F” with your right hand. When making a gun-like configuration (with the
middle finger crossing under the index finger), the fingers represent the velocity vector,
magnetic field vector, and force vector, respectively. See also right hand rule.
Lenz’s law gives the direction of the induced electromotive force (emf) and current resulting
from electromagnetic induction. German physicist Heinrich Lenz formulated it in 1834.