Magnets. Permanent vs. Temporary Magnets. Characteristics. Name originates from the region of Magnesia, a part of Greece

Transcription

1 Magnets Name originates from the region of Magnesia, a part of Greece Location of the first found naturally occurring magnetic rocks lodestone Magnet: a material that produces an invisible magnetic field Characteristics Magnetic Poles: ends of a magnet that have opposing magnetic forces Opposite poles attract Like poles repel Permanent vs. Temporary Magnets Permanent Magnets: have a magnetic field all the time Temporary Magnets: have a magnetic field for only a certain amount of time, before it is lost 1

2 We can create magnets by rubbing a magnet over a piece of magnetic material Magnets can attract other magnets or magnetic materials Magnetic Field: The area of magnetic force surrounding a magnet. Strongest at the poles of a magnet, but exists around the entire magnet. The number of field lines in any given region indicates the relative strength of the field If you were to place a magnetic material, such as iron, near the magnet it would be most attracted to either the north or south pole Iron would also be attracted by the magnetic field around the magnet 2

3 Magnetic fields Magnetic Materials The electrons of all atoms spin as they move about the nucleus A spinning electron produces a magnetic field with both a north and south pole In most materials, the magnetic fields of individual atoms cancel each other, so the materials aren t magnetic The poles line up in the same direction in microscopic magnetic regions, called magnetic domains When all the domains are arranged with their poles in the same direction, the iron bar becomes a permanent magnet When the domains are arranged randomly, the iron bar is not magnetized 3

4 Earth As A Magnet If you hang a magnet by a string, the north seeking pole will always point north because the earth itself is a huge magnet An instrument that takes advantage of the earths magnetic field is the compass A compass has a magnetized needle in it that turns freely The north and south pole of the earth s axis are referred to as geographic north pole sometimes called true north However, geographic north pole is equivalent to the south pole of a magnet, and geographic south pole is equivalent to the north pole of a magnet (see pg 568) Evidence suggests that the earths magnetic field is caused by the movement of molten metals near the earths core Measurements show that the earths magnetic poles change position over time Changes in the flow of the molten metals inside the earth may cause the magnetic poles to move Electromagnetism In 1820, Christian Oersted, a Danish physicist made an observation that when a compass was brought near electric current, the compass needle no longer pointed north. It turned 90 degrees. The compass needle turned in the opposite direction when he reversed the current. 4

5 Electromagnets He hypothesized that when an electric current flowed through it, the wire acted like a magnet. Somehow electricity could produce magnetism. Oersted s discovery is responsible for the invention of new tools based on the principles of electromagnetism. Electromagnet: a magnet made of a soft-iron core surrounded by a coil of wire through which an electric current passed. The strength depends on the number of turns in the coil, the amount of current, and the size of the iron core. The greater number of turns a coil has, the stronger the magnetic field can produce. The greater the size of the soft-iron core, the stronger the magnet is. When a magnet is turned on an electric current flows through the wire coil, creating a magnetic field around the coil. The magnetic domains in the soft-iron core align with the magnetic field of the coil. 5

6 The soft-iron core becomes magnetized. One end of the soft-iron core is a north pole, and the other end is the south pole. The magnetic field of the magnetized soft-iron core combines with the magnetic field of the wire coil. The combined magnetic fields create a very strong magnet. Pure iron is referred to as soft iron. An electromagnet exerts a magnetic force that can make things move. Generators Devices for converting mechanical energy to electrical energy. Spin a coil of wire through a magnetic field Will make a current flow through wire Make alternating current as it moves past the different poles of the magnet. 6