Electrical Fundamentals: Magnetism Overview

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Questions and Answers

What is the primary relationship between electricity and magnetism?

Electricity cannot exist without magnetism.

Electricity and magnetism are independent phenomena.

Understanding one requires knowledge of the other. (correct)

Magnetism is a type of electricity.

Which of the following concepts is NOT related to magnetism?

Voltage drop (correct)

Eddy currents

Coercive force

Magnetic flux density

In Weber’s Theory of Magnetism, what are magnetic substances assumed to contain?

Single large magnets

Magnetic fields

Molecular magnets (correct)

Metallic particles

What is the term for the maximum point a magnetic material can be magnetized?

Saturation point Signup and view all the answers

Which statement best describes magnetic shielding?

It involves containing the magnetic field to protect sensitive equipment. Signup and view all the answers

What does coercive force represent in magnetism?

The required strength to demagnetize a material. Signup and view all the answers

Which of the following describes the hand clasp rule?

A way to ascertain the direction of the magnetic field around a conductor. Signup and view all the answers

What is the term for the force that causes magnetic flux to occur?

Magnetomotive force Signup and view all the answers

What does the term 'reluctance' refer to in magnetism?

The resistance to magnetic field lines in a material. Signup and view all the answers

Which property describes a material's ability to retain magnetization after the external magnetic field is removed?

Retentivity Signup and view all the answers

What technique can be used to minimize magnetic interference in sensitive electronic devices?

Magnetic shielding Signup and view all the answers

Which term describes the measure of a material's opposition to magnetic flux?

Reluctance Signup and view all the answers

What does the hand clasp rule determine regarding a current-carrying conductor?

Direction of magnetic field lines Signup and view all the answers

What are eddy currents primarily produced by?

Moving conductors in a changing magnetic field Signup and view all the answers

What effect do eddy currents have on materials?

They can cause magnetic and heating effects Signup and view all the answers

How do eddy currents flow in a conductor?

In loops perpendicular to the magnetic flux Signup and view all the answers

What is the recommended storage method for a horseshoe magnet?

Keep it with a keeper, such as a soft iron bar Signup and view all the answers

What should personnel with pacemakers avoid related to magnets?

Being in close proximity to magnets Signup and view all the answers

What happens to magnets when they are knocked or heated?

They lose some of their effective magnetism Signup and view all the answers

Eddy currents are named due to their resemblance to what phenomenon?

Swirling eddies in streams Signup and view all the answers

What type of current is known to flow microscopically in conductors?

Eddy currents Signup and view all the answers

What happens to the strength of a magnetic field as the distance from a conductor increases?

It diminishes with distance. Signup and view all the answers

In the Left-Hand Grasp Rule, what does the thumb represent?

Direction of conventional current. Signup and view all the answers

What occurs when two conductors are placed in parallel with currents flowing in the same direction?

They will attract each other. Signup and view all the answers

What is the effect of coiling a wire in relation to its magnetic field?

It strengthens the magnetic field around it. Signup and view all the answers

How is the direction of the magnetic field around a conductor determined?

By the direction of electrical current in the wire. Signup and view all the answers

What visual aid is used to show the direction of the magnetic field when looking at a conductor end on?

A cross indicating the tail of the arrow. Signup and view all the answers

In which scenario does a magnetic field cease to exist?

When there is no current flowing. Signup and view all the answers

What principle explains the interaction of two magnetic fields from parallel conductors?

Like magnetic poles repel each other. Signup and view all the answers

What is the primary purpose of using magnetic materials for shielding?

To redirect flux away from sensitive areas Signup and view all the answers

Which statement correctly differentiates between a permanent and a temporary magnet?

Permanent magnets are made from materials with high reluctance and low permeability. Signup and view all the answers

What does it mean when a ferromagnetic material is said to be magnetically saturated?

No further magnetization can increase flux. Signup and view all the answers

Which method is NOT used for magnetizing materials?

Heating the material to a high temperature Signup and view all the answers

In the context of hysteresis, what is meant by 'lag of an effect after its cause'?

The material retains some magnetization after removing the magnetic field. Signup and view all the answers

What characteristic of magnetic material is described by its permeability?

Its capacity to channel magnetic lines of force Signup and view all the answers

How does the shape of a hysteresis loop vary among different magnetic materials?

The shape indicates the energy lost in magnetization cycles. Signup and view all the answers

Which factor is primarily responsible for a material becoming magnetically saturated?

Exposure to a constant external magnetic field Signup and view all the answers

What are artificial magnets primarily made from?

Special iron or steel alloys Signup and view all the answers

What is the primary characteristic of permanent magnets?

They retain a great deal of magnetism after being magnetized. Signup and view all the answers

Which of the following materials is typically used to create temporary magnets?

Silicon steel Signup and view all the answers

What is meant by the term 'residual magnetism'?

The amount of magnetism remaining in a temporary magnet after demagnetization. Signup and view all the answers

What is reluctance in the context of magnets?

The opposition a material offers to magnetic lines of force. Signup and view all the answers

How is the strength of a magnetic field affected within a magnet?

It is strongest at both ends of the magnet. Signup and view all the answers

Which term describes the ability of a material to retain an amount of magnetism after being magnetized?

Retentivity Signup and view all the answers

What distinguishes temporary magnets from permanent magnets?

Temporary magnets lose their magnetic strength after the magnetizing force is removed. Signup and view all the answers

Study Notes

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

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Description

Explore the fundamental concepts of magnetism as it relates to electricity in this quiz. Understand the relationship between magnetism and various electrical devices, along with key theories such as Weber's and Domain Theory. Strengthen your knowledge of this essential topic in electrical fundamentals.