Magnets Part – II

Magnetic Compass OR Mariner’s Compass:

Magnetic compass

  • A magnetic compass consists of a small magnetic needle pivoted at the centre of a small brass box which has a glass top. Generally, the north end is painted in red.
  • It works on the principle that when a magnet is free to rotate about a transverse axis then under the action of Earth’s magnetic field, the needle aligns itself in North-South direction. Thus using Compass the North and South direction can be located and thus other directions can be obtained.
  • Uses of the magnetic compass are.
    • To decide North-South directions.
    • To find the direction of magnetic field at a place.
    • To plot or draw magnetic lines of force
    • To test the polarity of a magnet.
    • It is very useful for travelers, mariners a, d navigators to find direction when they sail through the unknown location.

Pin Holder:

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  • Pin holder is used on writing tables to hold pins. It consists of a thin round magnet fitted at its mouth.
  • When the pin holder is turned upside down, the pins at the bottom of the holder stick to inside of the mouth of the holder. Now they can be easily picked out and be used.

Magnetic Locks for Shutters of Cupboards:

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  • The magnetic lock is fitted on the frame of the cupboard, while a thin iron strip is fixed on the shutter, exactly opposite to the magnetic lock.
  • When the shutter is brought near the frame, the magnetic attraction between the iron strip and magnet click shut the shutter tightly to the frame.

Types of Material:

Magnetic Materials:

  • Materials which are attracted by magnets are called magnetic material.
  • e.g.  Iron, cobalt, nickel

Nonmagnetic Materials:

  • Materials which are not attracted by magnets are called non magnetic material.
  • e.g. glass, plastic, rubber.

Experiment to Find Magnetic Material:

  • Bring a magnet near the material to be categorized.
  • If the material is getting attracted towards the magnet, then the material is categorized as magnetic material.
  • If the material is non getting attracted towards the magnet, then the material is categorized as nonmagnetic material.

Experiment to Locate Poles of a Magnet:

  • Suspend a bar magnet with twistless thread to a wooden stand such that it is capable of rotating about a transverse axis passing through its centre. Thus the magnet is horizontal.
  • Let the magnet be come to rest. When it comes to rest, the end pointing towards geographical north is called north pole and the end  pointing towards geographical south is called south pole.

Experiment to Show That the Strength of Magnet is Located at the Poles:

  • Take some iron filings in a dish. Place a bar magnet in it.
  • We observe that the iron filings stick to the magnet but cluster around the poles rather than the middle portion of the magnet.
  • This shows that the strength of the magnet is located at the poles.


Experiment to Show That the Like Poles Repel and Unlike Poles Attract:

  • Suspend a bar magnet with twistless thread to a wooden stand such that it is capable of rotating about a transverse axis passing through its centre. Thus the magnet is horizontal.
  • Let the magnet be come to rest. When it comes to rest, the end pointing towards geographical north is called north pole and the end  pointing towards geographical south is called south pole.
  • Now bring north pole of another magnet near north pole of suspended magnet. We observe that the north pole of the suspended magnet moves away from the north pole of the other magnet. This phenomenon is called magnetic repulsion.
  • Now take away the other magnet and allow the suspended magnet to come to rest. Now  bring south pole of another magnet near north pole of suspended magnet. We observe that the north pole of the suspended magnet moves towards the south pole of the other magnet. This phenomenon is called magnetic attraction.
  • Thus we can conclude that like poles repel and unlike poles attract.
  • In this experiment if you interchange the poles, the result will be the same.

Note:

  • If iron rod is suspended in case of the magnet, then in both the cases the rod will get attracted towards  the other magnet.

Experiment to Show That the Poles of Magnet Cannot Be Separated:

  • Take a thin bar magnet which can be cut by scissors. Mark its north pole and south pole. Cut this magnet into two halves at the centre.
  • Put these pieces in iron filings. Iron filings get attracted to both pieces at the ends.
  • Suspend these pieces with twistless thread to a wooden stand such that it is capable of rotating about a transverse axis passing through its centre. Thus the magnet is horizontal. We observe that both the pieces come to rest in north south direction.
  • This shows that no matter how small you cut the magnet, each piece will have both north pole and south pole. Thus the two poles of a magnet cannot be separated from each other.

A horizontally suspended magnet always comes to rest in north south direction:

  • The earth itself is a giant magnet. Its magnetic North pole is near geographical South pole. Its magnetic South pole is near geographical North pole.
  • Now, unlike poles of magnet always attract each other and like poles of magnet always repel each other. Thus the north pole of the suspended magnet gets attracted towards the magnetic south pole of the earth (geographical north). Similarly the south pole of the suspended magnet gets attracted towards the magnetic north pole of the earth (geographical south).
  • Thus the magnet when suspended in air such that it is free to rotate about a transverse axis passing through its centre, it always comes to rest in north south direction.

 



If a bar magnet is suspended vertically, it does not hang in the north south direction:

  • When a bar magnet is suspended vertically it is acted upon by two forces. Magnetic force due to earth’s magnetic field and gravitational force due to earth’s gravitational field.
  • The magnetic force tries to align the magnet in the north south direction, while gravitational force tries to move the magnet downward.
  • The gravitational force acting on the magnet is much more stronger than the magnetic force acting on the magnet.
  • Hence, a bar magnet when suspended vertically, it does not hang in the north south direction.

Repulsion rather than attraction is the test for identifying magnet:

  • Unlike poles of magnet always attract each other and like poles of magnet always repel each other. Thus there are two phenomena involved. attraction and repulsion.
  • If a material is brought near a north pole of a magnet and is getting attracted. Then there are two possibilities
  • The material is magnetic and itself is not a magnet and is getting attracted towards the magnet.
  • The material is magnet and its south pole is getting attracted towards the north pole of the magnet.
  • Thus attraction gives two sub-possibilities.
  • If a material is brought near a north pole of a magnet and is getting repelled. Then it means that the material is magnet and its north pole is brought near the north pole of the magnet.
  • If there is neither attraction or repulsion, the material is non magnetic.
  • Thus, repulsion rather than attraction is the test for identifying magnet.

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