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Magnetism

Kyle ThorntonKyle Thornton

DMI 50BDMI 50B

Lecture Objectives

Define magnetismDefine magnetism Identify the magnetic propertiesIdentify the magnetic properties Define magnetic moment, magnetic domainsDefine magnetic moment, magnetic domains Explain how a magnet is able to exert force Explain how a magnet is able to exert force

through spacethrough space List the three different types of magnets and give List the three different types of magnets and give

an example of eachan example of each Classify different materials with regard to their Classify different materials with regard to their

magnetic susceptibilitymagnetic susceptibility

What Is Magnetism?

The ability of certain metallic substances to attract iron, The ability of certain metallic substances to attract iron, cobalt, and nickelcobalt, and nickel

Magnetism was discovered as early as the 6th century, BCMagnetism was discovered as early as the 6th century, BC First artificial magnet was discovered in China around 1 First artificial magnet was discovered in China around 1

AD by stroking certain materials with ironAD by stroking certain materials with iron The first technological application of magnetism is The first technological application of magnetism is

believed to be the compassbelieved to be the compass There are many similarities between magnetism and There are many similarities between magnetism and

electricityelectricity

Application Of Magnetism In Radiology Used to produce images in Magnetic Used to produce images in Magnetic

Resonance Imaging (MRI)Resonance Imaging (MRI) Utilizes very strong magnetic fieldsUtilizes very strong magnetic fields

Used in the circuitry of the X-ray machine Used in the circuitry of the X-ray machine and other x-ray apparatusand other x-ray apparatus

Magnetic Properties

All magnets have two polesAll magnets have two poles NorthNorth SouthSouth

If cut in two - each half will develop north and south poleIf cut in two - each half will develop north and south pole Like magnetic poles repel, opposite poles attractLike magnetic poles repel, opposite poles attract

The force of attraction between two magnetic poles isThe force of attraction between two magnetic poles is directly related to the strength of the magnets and directly related to the strength of the magnets and inversely related to the square of the distance between inversely related to the square of the distance between themthem

Magnetic Lines of Force

http://micro.magnet.fsu.edu/electromag/javahttp://micro.magnet.fsu.edu/electromag/java/magneticlines/index.html/magneticlines/index.html

http://micro.magnet.fsu.edu/electromag/http://micro.magnet.fsu.edu/electromag/java/magneticlines2/index.htmljava/magneticlines2/index.html

More Magnetic Properties

Some materials can be easily magnetized, Some materials can be easily magnetized, others cannotothers cannot

Magnetic fields exist around magnetsMagnetic fields exist around magnets The strength of a magnet can be adversely The strength of a magnet can be adversely

affected by heating or mechanically jarring itaffected by heating or mechanically jarring it Materials can be magnetized by leaving them Materials can be magnetized by leaving them

near a strong magnet or stroking them with near a strong magnet or stroking them with another magnetanother magnet

Atoms - The Smallest Magnet Any charged particle in motion creates a magnetic fieldAny charged particle in motion creates a magnetic field Spinning electric charges such as electrons induce a Spinning electric charges such as electrons induce a

magnetic field magnetic field Magnetic domainsMagnetic domains

A group of atomsA group of atoms Each atom behaves like a tiny magnetEach atom behaves like a tiny magnet These atoms determine the overall magnetic properties These atoms determine the overall magnetic properties

of the magnetof the magnet Each atom has a north and south poleEach atom has a north and south pole

Magnetic Domains

Order of magnetic domainsOrder of magnetic domains Unmagnetized barUnmagnetized bar

Domains in random orderDomains in random order Magnetized barMagnetized bar

Domains are aligned in the same directionDomains are aligned in the same direction This can be accomplished by dragging a strong This can be accomplished by dragging a strong

magnet over a steel or iron barmagnet over a steel or iron bar Domains can be knocked out of alignment by Domains can be knocked out of alignment by

physical force or heatingphysical force or heating This will destroy a permanent magnetThis will destroy a permanent magnet

How Does MRI Work?

In the nucleus of an atom, a proton spinning In the nucleus of an atom, a proton spinning creates a nuclear magnetic dipolecreates a nuclear magnetic dipole This is called a magnetic momentThis is called a magnetic moment

The magnetic moments of hydrogen atoms The magnetic moments of hydrogen atoms in the body is the basis for MRIin the body is the basis for MRI

Magnetic Fields

A magnet has the ability to exert force over A magnet has the ability to exert force over distancedistance This is accomplished by magnetic fieldsThis is accomplished by magnetic fields

Direction of magnetic fieldsDirection of magnetic fields Flow from north to south outside the magnetFlow from north to south outside the magnet Flow from south to north inside the magnetFlow from south to north inside the magnet

Three Principal Types of Magnets Naturally Occurring MagnetsNaturally Occurring Magnets Artificially Induced Permanent MagnetsArtificially Induced Permanent Magnets ElectromagnetsElectromagnets

Unit of Magnetic Strength

Magnetic StrengthMagnetic Strength The magnitude of magnetic force exerted on an The magnitude of magnetic force exerted on an

objectobject TeslaTesla GaussGauss 10,000 Gauss = 1 Tesla10,000 Gauss = 1 Tesla MRI units are generally around 1.5 TeslaMRI units are generally around 1.5 Tesla

Classification of Magnetic Materials DimagneticDimagnetic ParamagneticParamagnetic FerromagneticFerromagnetic

High magnetic susceptibilityHigh magnetic susceptibility

Review questions

What is magnetism?What is magnetism? Name two applications of magnetism in RadiologyName two applications of magnetism in Radiology What will occur if two magnetic poles that are positive are What will occur if two magnetic poles that are positive are

brought together?brought together? If the distance between two magnets is doubled, what will If the distance between two magnets is doubled, what will

happen to the strength of the magnetic field?happen to the strength of the magnetic field? How are the magnetic domains aligned in an object that How are the magnetic domains aligned in an object that

has been magnetized?has been magnetized? What is the term for a magnet that has been wrapped in a What is the term for a magnet that has been wrapped in a

coil of wire?coil of wire? How would the substance gadolinium be classified?How would the substance gadolinium be classified?