Magnets

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  • About 600 to 800 BC the people living in Magnesia in Asia Minor found a stone ( an ore of ironMagnetite Fe2O3) which was capable of attracting iron pieces towards it. They called this stone as magnetite. During the course of time, the name changed to a magnet.
  • Certain substances have a tendency to attract iron filings towards them, such substances are called magnetic substances and the property is known as magnetism. e.g. Iron, Steel, Cobalt, Nickel.
  • The pieces of loadstone which show magnetism are called natural magnets. The natural magnets are of irregular and odd shapes. They don’t have strength.
  • Magnetized iron pieces of iron or other magnetic material are called artificial magnets. They can be made of different shapes.


Shapes of Magnet:

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  • A magnet creates around it what is called as a magnetic field. Thus the region around the magnet in which it exerts a force on other magnets or other magnetic materials is called a magnetic field.
  • The magnetic force is a noncontact force.

Characteristics of Magnet:

Attractive Property:

  • A magnet attracts small pieces of iron. The magnetic field is stronger at the ends of a magnet and thus the ends of the magnet are the centres of attraction. These centres of attraction are called poles of the magnet.

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  • Magnetic lines of force are crowded at the ends of a bar magnet. Actually, the poles are not at the ends of the geometric length of the magnet but they are slightly inside.
  • The points of a magnet where the attraction appears to be maximum are called the poles of the magnet.

Directive Property:

  • When a bar magnet is suspended in air such that it is free to rotate about the transverse axis passing through its centre, then it is found that the bar magnet always aligns itself in a north-south direction.
  • The end of the magnet which is pointing towards the geographical north is called north seeking pole or simply north pole, while the end of the magnet pointing towards the geographical south is called south seeking pole or simply south pole.

Law of Magnetic Poles:

  • Like poles of magnets repel each other and unlike poles attract each other.

Magnets 03

Pair Property:

  • When we cut a magnet into two parts two new magnets are formed. Thus it is impossible to separate the poles of a magnet.
  • Isolated magnetic poles do not exist. i.e. magnetic poles always exist in pairs. Hence the magnets have to be necessarily regarded as dipoles.

Magnets 04

Sure Test of Magnetization:

  • A repulsion is a sure test of attraction because an iron rod is always attracted towards the magnet and unlike poles always attract. Thus if given rod is repelled we can definitely say that the rod is magnetized.


Types of Magnets:

Bar Magnet:

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  • A bar magnet is rectangular parallelepiped body which exhibits magnetic properties.
  • Actually, the poles are not at the ends of the geometric length of the magnet but they are slightly inside.
  • The length of the edge parallel to the magnetic axis is called geometric length of the bar magnet.
  • The line joining the poles of the bar magnet to called an axis of the magnet.
  • The distance between the poles of the bar magnet is called magnetic length.
  • Magnetic length of bar magnet × 1.2 = Geometric length of bar magnet.
  • A vertical plane passing through the magnetic axis of the freely suspended magnet is called magnetic meridian.
  • A vertical plane passing through the magnetic equator of the freely suspended magnet is called equatorial meridian.

Magnetic Needle:

  • It is a magnet tapered towards both ends and pivoted at the centre.
  • It is used to check the direction of magnetic field and to map magnetic lines of force of other magnets.

Horse Shoe Magnet:

  • It is in a shape of a horse shoe.
  • This magnet is usually more powerful than a bar magnet. As both the poles of horseshoe magnet face each other, the attractive power is doubled.
  • The two poles can be made closer than any other type, hence these magnets can be used when a strong magnetic field is required in a small space.
  • They are used in electronic valves of RADAR, electric motors, electric generators and moving coil galvanometers.

Disc Magnet:

  • It is in a shape of a disc.
  • Its poles are located each on its circular faces. i.e. if one face is acting as a north pole, other face will act as a south pole.

Magnetic Keepers:

  • If two magnets are placed side by side there will be mutual repulsion or attraction. This weakens the strength of the magnet. Similarly, a magnet is kept for a long time it loses its magnetic property. It is due to self-inductance and due to Earth’s magnetic field.
  • To prevent this, bar magnets are placed side by side with opposite poles near. A soft iron piece called a keeper is placed across the poles as shown in the figure.

Magnets 06

  • The magnet induces opposite polarity at the ends of the keeper.  Thus this soft iron piece provides a path for the magnetic field lines to form a continuous loop. Thus it helps in preserving the magnetic field.

Advantages of Artificial Magnets:

  • Artificial magnets can be made up of different (convenient) shapes and sizes.
  • Artificial magnets can be made of different strengths.
  • Strong magnetism can be obtained by artificial magnets only.

Uses of Magnets:

  • Its directive property is used to construct magnetic needles and mariner’s compass.
  • Permanent magnets are used in dynamos, electric motors, generators, electrical accelerators, door locks.
  • Electromagnets are widely used in electric bells, electric cranes, tape recorders and speakers.
  • Magnets are used in separating iron particles from solid mixtures using the method of magnetic separation.
  • It is used in pin holder to stock pins and makes easy to pick them when required.
  • They are used as a magnetic lock to keep shutters of doors and cupboards to shut tightly.


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:

Magnets 10

  • 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.


Science > Physics > Magnetism >You are Here
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