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Lesson_4.1 - Lesson 4.1 Magnets and magnetic field...

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Lesson 4.1 Magnets and magnetic field Magnetism is a kind of force that exists between certain kind of materials. The first such magnetic force was detected in a material called magnetite or lodstone. It is said that the phenomenon was first noticed by a shepherd when the iron-tipped end of his crook got pulled by a certain rock. Sailors used lodestone to help them navigate. They had found that when a piece of magnetite was suspended from a thread, it came to rest in a North-South direction. Every magnet has two poles. This is where most of its magnetic strength is concentrated. These poles are called north (N) and south (S) or north-seeking and south seeking poles. When the north pole of one magnet is placed near the north pole of another magnet, the poles are repelled. When the south poles of two magnets are placed near one another, they also are repelled from one another. When the north and south poles of two magnets are placed near one another, they are attracted to one another. A freely suspended magnet comes to rest in the north-south direction. A material that is attracted to a magnet is called a magnetic material. Iron, cobalt and nickel are examples of magnetic materials. In the presence of a magnet, a magnetic material turns into a magnet with a north pole and a south pole. The resulting attraction is the result of interaction between the two magnets. We will represent the strength of a magnet by the term pole strength (m). The force between two magnets is directly proportional to their pole strengths and inversely proportional to the square of the distance between the poles. This is analogous to the Coulomb’s law for electric charges. The striking difference between electric charges and magnetic poles is that a north pole cannot exist independent of a south pole. When a magnet is broken into small pieces, each piece behaves like a magnet having a north pole and a south pole. Isolated single magnetic poles (monopoles) have not been observed so far.
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Magnetic Field Just like the electric field that surrounds an electric charge, there is a magnetic filed that surrounds a magnet. Magnetic field is a three dimensional space around a magnet where a magnet exerts force on other magnets. Just like the electric field lines that describe an electric field, a magnetic field is defined by magnetic filed lines. If a single north pole can be isolated, then a magnetic field line can be defined as the path this unit north pole will follow in the field. This means that a magnetic field line will originate at the north pole and end at the south pole. The field lines are crowded near the poles indicating that the field is stronger there. The field strength is uniform where the field lines are parallel. Magnetic field B is measured in a unit called tesla (T) which is the same as NA -1 m -1 .
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