Sunday, May 20, 2012

Facts about Magnetism

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Magnetite-chemically having the formula Fe3O4 

Brief history about magnets

 As early as 600 B.C. the Greeks knew that a certain form of iron ore, now known as magnetite or lodestone, had the property of attracting small pieces of iron.

Later, during the Middle Ages, crude navigational compasses were made by attaching pieces of lodestone to wooden splints floating on bowls of water. These splints always come rest pointing in a N-S direction, and were the forerunners of the modern aircraft and ship compasses.

The word lodestone is derived from an old English word meaning way, and refers to the directional property of the stone.

Chemically, it consists of iron oxide having the formula Fe3O4. The word magnetism is derived from Magnesia, the place where magnetic iron ore was first discovered.

Magnetic poles

If a piece of lodestone is dipped into iron filings it is noticed that the filings cling in tufts, usually at two places in particular.

When the experiment is performed with a bar magnet the filings are seen to cling in tufts near the ends. Few, if any; filings are attracted to the middle of the bar.

The places in a magnet where the resultant attractive force appears to be concentrated are called the poles.

Magnetic and non-magnetic substances

 Apart from iron, the only other elements which are attracted strongly by a magnet are cobalt and nickel. These, together with certain strongly magnetic alloys are described as ferromagnetic.

Substances such as copper, brass, wood and glass are not attracted by a magnet, and are commonly described as non-magnetic.

Nevertheless, experiments with very powerful magnets have shown that even the so-called non-magnetic substances have very feeble magnetic properties. (That will be discussed more in the next article).

Suspending a magnet. Magnetic axis. Magnetic Meridian

When a magnet is freely suspended so that it can swing in a horizontal place it oscillates to and fro for a short time and then comes to rest in an approximate N-S direction.

magnetic meridian

The magnet may be regarded as having a magnetic axis about which its magnetism is symmetrical, and it comes to rest with this axis in a vertical plane called the magnetic meridian.

 In other words:

The magnetic meridian is a vertical plane containing the magnetic axis of freely suspended magnet at rest under the action of the earth’s magnetic field.

The pole which points towards the north is called the north-seeking or simply the N pole; the other is called the south-seeking or S pole.

The first law of magnetism

first law of magnetism

If the N pole of a magnet is brought near the N pole of a suspended magnet it is noticed the repulsion occurs.

Similarly, repulsion is observed between two S poles.

On the other hand, a N and a S pole always attract one another.

These results may be summed up in the law,

like poles repel, unlike poles attract.

The above statement is sometimes called the first law of magnetism.

Test for polarity of a magnet

 The polarity of any magnet may be tested by bringing both its poles, in turn, near to the known poles of a suspended magnet.

Repulsion will indicate similar polarity.

If attraction occurs, no firm conclusion can be drawn, since attraction would be obtained between either:

(a) two unlike poles, or

(b) a pole and a piece of unmagnetized material.

   Repulsion is, therefore, the only sure test for polarity.

To magnetize a steel bar by an electrical method

 The best method of making magnets is to use magnetic effect of an electric current.

Magnetization by an electric method

A cylindrical coil wound with 500 or more turns of insulated copper wire is connected in series with a 6 or 12 Volts electric battery and switch 'as shown in the image'.

A coil of this type is called a solenoid.

A steel bar is placed inside the coil and the current switched on and off.

On removing and testing the bar it is found to be magnetized. It is unnecessary to leave the current on for any
length of time, as the bar will not become magnetized any more strongly and the coil may be damaged through overheating.

The polarity of the magnet depends on the direction of flow of the current. If, on looking at the end of the bar, the current is flowing in a clockwise direction, that end will be a S pole; if anticlockwise, it will be a N pole. This rule may easily be remembered by the two symbols shown in the above image.

Commercially, C- and U-shaped magnets are made by linking them with one very thick copper conductor through which an enormous surge of current is passed for a fraction of a second.

Another methods of magnetization: Single and divided touch

Before the magnetic effect of an electric current was discovered in the early nineteenth century magnets were made by stroking steel bars with a lodestone or with another magnet.

There are two ways in which this may be done, called the methods of single and divided touch respectively.

1- In single touch a steel bar is stroked from end to end several times in the same direction with one pole of a magnet.

Between successive strokes the pole is lifted high above the bar, otherwise the magnetism already induced in it will tend to be weakened.

The disadvantage of this method is that it produces magnets in which one pole is nearer the end of the bar than the other.

2- It is better to use the method of divided touch, in which the steel bar is stroked from the center outwards with unlike poles of two magnets simultaneously.

In both of these methods it is to be noted that the polarity produced at that end of the bar where the stroking finishes is of opposite kind to that of the stroking pole.

Steel knitting needles or pieces of clock spring may be magnetized by both methods and the polarity tested by obtaining repulsion with a magnetic needle.

The magnets so made should also be dipped into iron filings, when the distribution of the filings will reveal the superiority of the method of divided touch.

Older methods of making magnets

Another obsolete method of making magnets is described in a treatise of magnetism written by Dr. Gilbert in the region of Elizabeth I. In this method red hot steel bars are hammered while allowing them to cool when lying in a N-S direction. (More illustration will be said about this method in the next post).

Consequent poles

Making a magnet with consequent poles

 One should never assume, without prior test, that a bar magnet always has opposite poles at its ends. If a steel bar is magnetized by divided touch using two S poles we obtain a N pole at both ends of the bar and a double S pole in the center. In this condition the bar is said to possess a consequent poles.

Induced magnetism

Induced magnetism- Chain of nails experiment

When a piece of unmagnetized steel is placed either near to or in contact with a pole of a magnet and then removed it is found to be magnetized. This is called induced magnetism.

Tests with a compass needle show that the induced pole nearest the magnet is of opposite sign to that of the inducing pole.

 Chain of nails experiment

   Induced magnetism can be used to form a ‘magnetic chain’, as you can see in the image. Each nail added to the chain magnetizes the next one by induction, and attraction occurs between their adjacent unlike poles.

Incidentally, the attraction between a magnet and unmagnetized piece of material is always preceded by induction.

Demagnetizing a magnet by use of alternating current

The best way of demagnetizing a magnet is to place it inside a solenoid through which an alternating current is flowing.

The current may be obtained from a 12 or 14 V mains transformer. While the current is still flowing, the magnet is withdrawn slowly to a distance of several meters from the solenoid in an E-W (East-West) direction.

Demgnetization by ac

The alternating current takes the magnet through a series of everdimishing magnetic cycles, 50 times a second, until no magnetism is left in it.

The magnet is held in an E-W direction so that it will not be left with some residual magnetism owing to induction in the earth’s magnetic field.

Keep your watch away from magnets

Unless with care, the above method is useful for demagnetizing a watch. Watches should always be kept a way from magnets, as the balance-spring, if made of steel, is liable to become magnetized. When this happens the watch no longer keeps good time.

Another method of destroying magnetism is to heat the magnet to redness and then to allow it to cool while it is lying in an E-W direction.

This is not recommended as a practical method, since teat treatment will spoil the steel.

Finally, it should be noted that any vibration or rough treatment, such as dropping or hammering, particularly when the magnet is lying E-W, will cause weakening of the magnetism.

To study the magnetic properties of iron and steel

    It is important to distinguish between the magnetic properties of iron and steel. The term 'soft' as applied to iron means reasonably pure iron. It is otherwise known as wrought iron, and is soft in the sense that it bends easily and can be readily hammered into any required shape when red hot.

Steel, which consists of iron combined with a small percentage of carbon, is a much harder and stronger material.

A strip of soft iron and a strip of the same dimensions, both initially unmagnetized, are placed side by side in contact with a pole of magnet. Both strips becomes magnetized by induction, and on dipping their free ends into iron filings it is noticed that slightly more cling to the iron than to the steel.

From this we conclude that the induced magnetism in the iron is slightly greater than that in the steel when both are subjected to the same magnetizing force.

If both strips are held firmly in the fingers while the magnet is removed it is noticed that practically all the filings fall from the iron, while few, if any, fall from the steel.

The magnetic chain experiment illustrated above may also be used to show the difference between iron and steel. If the topmost nail is held between finger and thumb while the magnet is removed the chain immediately collapse, showing that the induced magnetism in the iron is only temporary.

When, however, the experiment is carried out using steel pen nibs or short pieces of clock-spring, the chain remains intact, showing that the magnetism induced in the steel is permanent.

Uses of hard and soft magnetic materials

   Magnetic materials used in the electrical industry are classified as hard or soft according as to whether they retain their magnetism or lost it easily. Both types are equally important.

  Until the early years of the 20th century magnets were made of hard carbon steel 'iron containing 1-1.5 per cent carbon'. Subsequently it was found that the addition of small proportions of tungsten, chromium and cobalt greatly improved the magnetic properties of the steel.

Research along these lines has led to the discovery of special alloys for making powerful permanent magnets.

Outstanding examples are Alcomax, Alnico and Ticonal, which contain iron, nickel, cobalt and aluminum in various proportions.

Now, magnets in great range of types and sizes are used in the construction of electric motors, dynamos and current-measuring instruments.

Several magnets may be found in the ordinary house-hold, for example, in the electricity meter, radio loudspeaker and telephone earpiece.

In contrast with the above there are many types of electrical equipment in which rapid change or reversal of magnetism is required. Into this class come electric bells, relays, electromagnets, transformer cores and dynamo and motor armatures.

In the construction of these, soft magnetic alloys such as Mumetal '73% nickel, 22% iron, 5% copper' and Stalloy '96% iron, 4% silicon' are used as well as soft iron.

Sintered materials

Much research has gone into the development of sintered magnetic materials. Sintering is the name given to the conversion of powder into solid blocks by the application of heat and great pressure.

By using various metallic powders, either very soft or very hard magnetic materials may be made by this method. Many of the best permanent magnets for general use are of this type.