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Magnetism KEY POINTS: 1.Magnets are strongest at the poles 2.The poles are North and South 3.Iron, Nickel and Cobalt are magnetic elements (steel is also magnetic, but is an alloy) Magnetic Resonance Imaging Scan 4.There is an invisible magnetic field around the magnet, which exerts a force on the magnetic materials mentioned in (3.) HISTORY This magnetic resonance imaging (MRI) scan through a normal adult head shows the brain, airways, and soft tissues of the face. MRI has become a valuable diagnostic tool, and is especially effective at imaging these areas of the body. ADVANCED The ancient Greeks, Romans and Chinese knew of Lodestone (now called Magnetite) was a type of Iron ore Magnetic materials can be classified as: The first compass was used about AD 1200 1. Diamagnetic- when placed in a magnetic field they have a moment induced which opposes the direction of the magnetic field The English physicist and doctor William Gilbert published his book Of Magnets, Magnetic Bodies, and the Great Magnet of the Earth in 1600. Gilbert applied scientific methods to the study of electricity and magnetism. He pointed out that the Earth itself behaves like a giant magnet In 1750, the English geologist John Michell invented a balance that he used in the study of magnetic forces. He showed that the attraction or repulsion between two magnetic poles decreases as the square of the distance between them increases 2. Paramagnetic- in a magnetic field the moments add to the magnetic field 3. Ferromagnetic- retains a magnetic force even when there is no external magnetic field. The Curie temperature for Iron is 770°C, it loses all magnetic properties © Microsoft Corporation. All Rights Reserved. Earth's Magnetic Field A powerful magnetic field surrounds the earth, as if the planet had an enormous bar magnet embedded in its interior. However, geophysicists believe that convection currents of charged, molten metal circulating in the earth’s core are the source of the magnetic field. A compass needle is a true bar magnet; one end of it is called “north-seeking” because it is attracted to the magnetic pole that lies within the earth in the northern hemisphere. The other end of the compass needle is a magnetic pole of the opposite kind, and is therefore attracted to the earth’s other magnetic pole. Since opposite types of magnetic pole attract each other, the pole in the northern hemisphere, although called the north magnetic pole, is itself of the south-seeking type, and vice versa for the opposite hemisphere. MAGNETISM 1. Magnets made of steel are “permanent” magnets in that they can retain their magnetism for a long time 2. Magnets made of soft iron can easily lose their magnetism, and are called “temporary” magnets, but this makes them particularly useful in electromagnets 3. When freely suspended a magnet will point to magnetic North 4. If two magnets are close to one another, their “like” poles (north-north, etc.) repel one another, and their "unlike" poles (north-south) attract one another .MAKING A MAGNET A piece of iron or steel can become magnetized in three ways: 1. By stroking it gently, from end to end, with one pole of an existing magnet 2. By tapping gently while it is lying along a magnetic field (this is how screwdrivers and other tools can become accidentally magnetized) 3. By placing it inside a long cylindrical coil of many turns of wire (a solenoid), and passing a strong direct current through the coil for a short period USES FOR MAGNETS In computers magnetized ‘bubble domains’ store information Also done with tapes, videos, CDs, DVDs and disks Magnetic levitation trains float above the tracks using strong magnets, so that there is no friction with the tracks to slow the trains down Nuclear Magnetic Resonance Imaging (MRI) imaging in hospitals Magnets contain lots of tiny domains (like tiny magnets), when they are not lined up the metal is not magnetic HOW TO DEMAGNETIZE A MAGNET A magnet can be demagnetized by: 1. Hitting it repeatedly 2. Placing it in a solenoid and reducing the current down to zero 3. Heat the magnet The magnetic field around a bar magnet. The force point to the South pole and away from the North pole HOW TO FIND THE MAGNETIC FIELD 1. You can sprinkle Iron filings onto a piece of paper above the magnet and tap the page. The filings line up along the magnetic lines of force 2. Use compasses to plot the lines of force around the magnet (this allows you to work out the North and South poles) Magnetic Levitation Train Magnetic levitation trains levitate above the track by means of a magnetic suspension system, thus reducing or eliminating vibration, friction, and noise. Magnetic levitation trains can reach extremely high speeds; this experimental train in Germany, above, reaches 435 km/h (270 mph).