Electrical Fundamentals: Magnetism Overview

Module 3: Electrical Fundamentals, Topic 3.10: Magnetism

• Magnetism is necessary to understand electricity
• Magnetism and electricity are closely related
• Study of either subject is incomplete without a basic knowledge of the other
• Many electrical and electronic devices rely on magnetism, including computers, video equipment, high-fidelity speakers, and electrical motors
• Two main theories of magnetism: Weber's Theory and Domain Theory

Weber's Theory of Magnetism

• All magnetic substances are made up of tiny molecular magnets
• In unmagnetised materials, these molecular magnets' forces neutralize each other
• In magnetised materials, most of the molecular magnets line up in the same North 1 direction, with opposing South poles
• This creates a net North pole and a net South pole

Domain Theory of Magnetism

• This theory is based on the electron spin
• All matter is made up of atoms, each of which contains one or more orbital electrons
• Electrons orbit the nucleus in shells similar to planets orbiting the sun
• Each electron also spins on its axis as it orbits the nucleus
• When equal numbers of electrons spin in opposite directions, the magnetic fields cancel each other, and the atom is unmagnetised

Magnetic Materials

• Magnetism is the property of a material that enables it to attract iron
• Ferromagnetic materials are easily magnetized (iron, steel, nickel, cobalt)
• Non-magnetic materials are not attracted by magnets (paper, wood, glass, tin)
• New alloys, such as Alnico and Permalloy, can be strongly magnetised and can lift 500 times their own weight

Natural Magnets

• Ancient Greeks recognized magnetic stones
• These stones attract iron, similar to today's magnets
• The Greeks called these substances magnetite
• Chinese were aware of magnetism as early as 2600 BC
• Chinese observed naturally occurring magnets that pointed North or South
• Natural magnets have limited practical use; more powerful magnets are readily available

Artificial Magnets

• Magnets made from magnetic materials are called artificial magnets
• Commonly made from special iron or steel alloys
• Frequently magnetized electrically
• To magnetize a material, a heavy flow of electrons is passed through a coil of wire
• Artificial magnets can be permanent or temporary, depending on their ability to retain their properties after the magnetizing force is removed

Permanent Magnets

• Difficult to magnetize but retain a significant amount of magnetism
• Reluctance - the opposition a material offers to magnetic lines of force
• Permanent magnets are created from materials with high reluctance, which results in low permeability

Temporary Magnets

• Made of materials with low reluctance (soft iron or annealed silicon steel)
• Relatively easy to magnetize
• Retain only a small portion of their magnetism after the magnetizing force is removed
• Residual magnetism – the amount of magnetism that remains in the temporary magnet
• Retentivity – the capability of a material to retain residual magnetism

Magnetic Poles

• Magnetic force surrounding a magnet is not uniform
• Concentration of force is highest at the ends (poles)
• Magnets have two poles (North and South) with equal magnetic strength
• Like poles repel, unlike poles attract

Earth's Magnetic Poles

• The Earth behaves like a giant bar magnet

Magnetic Lines of Force

• Imaginary lines that represent the force around a magnet
• Lines always form closed loops and never cross
• Lines closer together indicate a stronger magnetic field
• Magnetic lines of force travel from North to South outside of the material and from South to North inside the material

Properties of Magnetic Lines of Force

• Continuous, always in closed loops, never cross
• Close lines indicate strong fields; farther lines indicate weak fields
• Travel from the North to the South pole outside and from the South to the North pole inside the material
• Enter and leave the material at right angles

Eddy Currents

• Microscopic currents flowing within a conductor due to a moving or changing magnetic field
• Can cause heating effects; may be useful or not depending on application

Precautions for Care and Storage of Magnets

• Personnel wearing pacemakers should not handle magnets
• Magnets should be stored away from sensitive electronics; personnel should be trained on handling magnets
• Avoid knocking or heating magnets
• Store horseshoe magnets using a keeper
• Store bar magnets in pairs with opposite poles together

Terminology Review

• Magnetomotive Force (MMF): The flux-producing ability of an electric current in a magnetic circuit.
• Field Strength: The amount of MMF available to create a magnetic field for each unit length of a magnetic circuit.
• Magnetic Flux Density: The number of magnetic lines of force that cut through a plane of a given area at a right angle.
• Permeability: The ability of a material to act as a path for magnetic lines of force.
• Retentivity: The amount of magnetic flux density that a material retains when the magnetizing force is removed after achieving saturation.
• Coercivity: The amount of reverse driving field required to demagnetize a material.
• Saturation Point: The point at which no additional amount of magnetization force will increase flux.
• Eddy Currents: Microscopic currents generated within a conductor by a changing magnetic field.
• Hysteresis Loop: A loop traced out by the magnetisation of a material when an alternating magnetic field is applied; the lag of an effect after its cause.

Electromagnets

• Magnets produced from magnetic materials
• Created electrically
• Current through a coil produces magnetic field; direction field determined by current flow (left-hand grasp rule)
• Strength depends on core type, core size/shape, number of turns, and current

Electromagnet Operation

• Electromagnets use a contact arrangement (relays or contactors)
• Contact arm is attracted when electromagnet is powered

Magnetomotive Force

• Also known as MMF/mmf or magnetic potential
• Represented by H
• Flux producing ability of electric current in a magnetic circuit
• Similar to electromotive force
• Standard unit of MMF is the ampere-turn (AT)

Magnetic Field Strength, Including Formulae

• Depends on the length of the coil (shorter coils produce higher intensity fields)
• Base unit for magnetic field strength is ampere-turn per meter (A t/m)
• H = MMF / Length