Please Be Courteous To Others Turn off all cell phones and pagers now.

Magnetism Kyle Thornton DMI 50B

Lecture Objectives Define magnetism Identify the magnetic properties Define magnetic moment, magnetic domains Explain how a magnet is able to exert force through space List the three different types of magnets and give an example of each Classify different materials with regard to their magnetic susceptibility

What Is Magnetism? The ability of certain metallic substances to attract iron, cobalt, and nickel Magnetism was discovered as early as the 6th century, BC First artificial magnet was discovered in China around 1 AD by stroking certain materials with iron The first technological application of magnetism is believed to be the compass There are many similarities between magnetism and electricity

Application Of Magnetism In Radiology Used to produce images in Magnetic Resonance Imaging (MRI) Utilizes very strong magnetic fields Used in the circuitry of the X-ray machine and other x-ray apparatus

Magnetic Properties All magnets have two poles North South If cut in two - each half will develop north and south pole Like magnetic poles repel, opposite poles attract The force of attraction between two magnetic poles is directly related to the strength of the magnets and inversely related to the square of the distance between them

Magnetic Lines of Force http://micro.magnet.fsu.edu/electromag/java/magneticlines/index.html http://micro.magnet.fsu.edu/electromag/java/magneticlines2/index.html

More Magnetic Properties Some materials can be easily magnetized, others cannot Magnetic fields exist around magnets The strength of a magnet can be adversely affected by heating or mechanically jarring it Materials can be magnetized by leaving them near a strong magnet or stroking them with another magnet

Atoms - The Smallest Magnet Any charged particle in motion creates a magnetic field Spinning electric charges such as electrons induce a magnetic field Magnetic domains A group of atoms Each atom behaves like a tiny magnet These atoms determine the overall magnetic properties of the magnet Each atom has a north and south pole

Magnetic Domains Order of magnetic domains Unmagnetized bar Domains in random order Magnetized bar Domains are aligned in the same direction This can be accomplished by dragging a strong magnet over a steel or iron bar Domains can be knocked out of alignment by physical force or heating This will destroy a permanent magnet

How Does MRI Work? In the nucleus of an atom, a proton spinning creates a nuclear magnetic dipole This is called a magnetic moment The magnetic moments of hydrogen atoms in the body is the basis for MRI

Magnetic Fields A magnet has the ability to exert force over distance This is accomplished by magnetic fields Direction of magnetic fields Flow from north to south outside the magnet Flow from south to north inside the magnet

Three Principal Types of Magnets Naturally Occurring Magnets Artificially Induced Permanent Magnets Electromagnets

Unit of Magnetic Strength The magnitude of magnetic force exerted on an object Tesla Gauss 10,000 Gauss = 1 Tesla MRI units are generally around 1.5 Tesla

Classification of Magnetic Materials Dimagnetic Paramagnetic Ferromagnetic High magnetic susceptibility

Review questions What is magnetism? Name two applications of magnetism in Radiology What will occur if two magnetic poles that are positive are brought together? If the distance between two magnets is doubled, what will happen to the strength of the magnetic field? How are the magnetic domains aligned in an object that has been magnetized? What is the term for a magnet that has been wrapped in a coil of wire? How would the substance gadolinium be classified?