Magnetism Concepts Quiz

Module 3: Electrical Fundamentals, Topic 3.10: Magnetism

• Magnetism is a fundamental concept in understanding electricity.
• Magnetism and electricity are interconnected.
• Electrical and electronic equipment rely on magnetism to function. This includes computers, video equipment, high-fidelity speakers, and electrical motors.
• Two theories explain magnetism: Weber's Theory and Domain Theory.

Weber's Theory of Magnetism

• This theory proposes that all magnetic materials are comprised of tiny molecular magnets.
• In unmagnetized materials, molecular magnets' forces are neutralized by adjacent molecular magnets, thus eliminating any magnetic effect.
• In magnetized materials, molecular magnets align, creating a dominant north pole and a dominant south pole.
• The alignment occurs when an external magnetic field causes the molecular magnets to align in the same direction

Domain Theory of Magnetism

• This is a more contemporary theory based on electron spin.
• All matter is composed of atoms, with atoms containing one or more orbital electrons.
• Electrons orbit the nucleus in various shells, similar to the solar system. Additionally, electrons also rotate on their axis while orbiting the nucleus.
• Unmagnetized atoms have equal numbers of electrons spinning in opposite directions, which cancel out their individual magnetic fields.
• Magnetized atoms have more electrons spinning in one direction than the other, creating a net magnetic field.
• Iron atoms have 26 protons and 26 orbital electrons. 13 electrons spin clockwise with 13 counterclockwise, thus being magnetically neutral. If more than 13 electrons spin in one direction an atom is magnetized.

Magnetic Materials

• Magnetism is a material property allowing attraction of iron pieces.
• Materials attracted by magnets can become magnetised, such as iron, steel, nickel, and cobalt.
• Non-magnetic materials are not attracted by magnets, such as paper, wood, glass, and tin.
• Ferromagnetic materials are easily magnetised, including iron, steel, and cobalt.
• Alloys, like Alnico and Permalloy, can be strongly magnetised, capable of lifting significantly more weight than their own.

Natural Magnets

• Ancient Greeks identified natural magnets, called lodestones.
• Similar to modern magnets, these stones could attract small pieces of iron and oriented North-South.
• Ancient Chinese also recognised the properties of magnetism.

Artificial Magnets

• Artificial magnets are produced from magnetic materials.
• They are frequently made of special iron or steel alloys and are often magnetized electrostatically.
• To magnetize a material, a substantial electron flow is passed through a coil wrapped around the material.
• Artificial magnets are usually classified as permanent or temporary. A permanent magnet retains magnetism and a temporary magnet loses most of its strength when the magnetizing force is removed.

Magnetic Poles

• Magnets have two poles, North and South.
• Like poles repel, unlike poles attract.
• When a magnet is suspended freely, it aligns itself in a north-south direction.

Earth's Magnetic Poles

• The Earth itself acts like a giant bar magnet.
• The Earth's magnetic axis is not perfectly aligned with its rotational axis. It is offset by about 15 degrees.
• The Earth's magnetic pole is sometimes called the magnetic north pole, but actually has the properties of a south magnetic pole, attracting the north pole of a compass needle.

Magnetic Lines of Force

• Magnetic fields, while invisible, are represented by lines, which are referred to as lines of force or magnetic lines of force.
• These lines always form closed loops, beginning at the North pole and ending at the South pole. They are concentrated at the poles and less concentrated at the midpoint.
• The lines close together indicate a strong magnetic field, and the lines further apart indicate a weaker magnetic field.
• When two magnets are brought close together, the lines of force change, becoming more complicated because of attractive or repulsive forces.

Properties of Magnetic Lines of Force

• Magnetic field lines are continuous and form closed loops.
• They never cross one another.
• Closely spaced lines indicate a strong magnetic field; widely spaced, a weak magnetic field.

Eddy Currents

• Eddy currents are microscopic currents flowing within a conductor.
• They are generated when a conductor moves through or is exposed to a changing magnetic field.
• Eddy currents can produce heating effects.

Precautions and Care of Magnets

• Personnel with pacemakers should avoid handling magnets.
• To maximize durability, store magnets in a safe place and avoid knocking or applying high heat.
• Horseshoe magnets should be kept together using a keeper (soft iron bar).
• Bar magnets should be stored in pairs, ensuring opposite poles are together, to prevent loss of magnetic properties.

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 (B): The number of magnetic lines of force cutting 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 a material retains when the magnetizing force is removed after achieving saturation.
• Hysteresis Loop: If an alternating magnetic field is applied to a material, it traces a loop called a hysteresis loop. Magnetic hysteresis is the lag of an effect after the cause.
• Reluctance: Opposition that a material offers to magnetic lines of force.
• Coercive Force: Amount of reverse driving field required to demagnetise a material.
• Saturation Point: The point at which no additional amount of magnetising force will increase flux.
• Eddy Currents: Circular induced currents generated in a conductor (of any type) by a moving magnetic field, or equivalent.

Additional information

• Electromagnets can increase the strength of the magnetic field by increasing the number of coil windings or increasing the current passed through the coil.
• Electromagnets are used extensively in relays, contactors, and solenoids.