Magnetism

Magnetism – is the ability to attract iron, nickel and cobalt.

Magnetism is perhaps more difficult to understand than other characteristic properties of matter, such as mass, energy, and electric charge, because magnetism is difficult to detect and measure. We can feel mass, visualize energy, and be shocked by electricity, but we cannot sense magnetism.

Classification of magnets:

• Natural magnets• Artificial permanenet• Electromagnets

Natural Magnet

Natural magnets-lodestone

Lodestones became magnetized by lying in earth for thousands of years

Artificial permanent magnets• Hard steel• Magnetic compass• Alnico

Hard steel

Magnetic compass

The magnetic compass was brought to Europe in the Middle Ages from the Chinese who had been using the compass for over 1500 years

Alnico is an acronym referring to iron alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni) and cobalt (Co). Alnico alloys are ferromagnetic, with a high resistance to loss of magnetism and are used to make permanent magnets. Before the development of rare earth magnets in the 1970s, they were the strongest type of magnet.

Aluminium cannot be magnetized

Electromagnets – magnets produced by means of electric.

The magnetic field of a charged particle such as an electron in motion is perpendicular to the motion of that particle. The intensity of the magnetic field is represented by imaginary lines

If the electron's motion is a closed loop, as with an electron circling a nucleus, magnetic field lines will be perpendicular to the plane of motion.Electrons behave as if they rotate on an axis clockwise or counterclockwise. This rotation creates a property called electron spin. The electron spin creates a magnetic field, which is neutralized in electron pairs. Therefore, atoms that have an odd number of electrons in any shell exhibit a very small magnetic field.

The lines of a magnetic field do not start or end as the lines of an electric field do. Such a field is called bipolar or dipolar; it always has a north and a south pole. The small magnet created by the electron orbit is called a magnetic dipole

Spinning electric charges also induce a magnetic field. The proton in a hydrogen nucleus spins on its axis and creates a nuclear magnetic dipole called a magnetic moment. This forms the basis of MRI.

MRI electromagnet

Laws of magnetism:

• Every magnet has two poles• Like magnetic poles repel unlike attract• The force of attraction/repulsion follows inverse square

law (The magnetic force is proportional to the product of the magnetic pole strengths divided by the square of the distance between them.)

Every magnet has two polesThe lines of a magnetic field are always closed loops.

Like magnetic poles repel

Unlike poles attract

Breaking a magnet will result in each magnet becoming a whole

Magnetic domain –accumulation of many dipoles.

The magnetic dipole in bar magnet generate imaginary lines of magnetic field.

Characteristics of the lines of force:

• From N to F outside of the magnet• From S to N inside of the magnet• Lines in the same direction will repel each other• Magnetic field is distorted by magnetic material

but not affected by non-magnetic materials

Lines of force

LINES OF FORCE

Magnetic field is distorted by magnetic material but not affected by non-magnetic materials.

This principle is employed with many MRI systems that use an iron magnetic shield to reduce the level of the fringe magnetic field. Ferromagnetic material acts as a magnetic sink by drawing the lines of the magnetic field into it.

A magnetic element can exist in magnetized or non-magnetized form.

Magnetization:

• Induction• Touch

MAGNETIC INDUCTION (MAGNETIZATION)

Magnetization and demagnetization

Magnetic permeability

Ease with which the material can be magnetized. Soft iron

Magnetic retentivity

Material resistance to magnetization.

Hard steel

State Material Characteristics

Nonmagnetic Wood, glass, copperUnaffected by a magnetic field

Diamagnetic Water, plasticWeakly repelled from both poles of a magnetic field

Paramagnetic GadoliniumWeakly attracted to both poles of a magnetic field

Ferromagnetic Iron, nickel, cobaltCan be strongly magnetized

Four Magnetic States of Matter

The SI unit of magnet field strength is the tesla. An older unit is the gauss. One tesla (T) = 10,000 gauss (G).

The use of a compass might suggest that the Earth has a strong magnetic field, but it does not. The Earth's magnetic field is approximately 50 μT at the equator and 100 μT at the poles. This is far less than the magnet on a cabinet door latch, which is approximately 100 mT.