ANSHS Physics Classroom – MAGNETISM

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 superparamagnet. 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 neighboring 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      is the angle between the     and      vectors.
One tool for determining the direction of the velocity vector of a moving charge, the magnetic field, and the force exerted is labeling 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.