PHY4: MAGNETISM

This unit is about Magnetism as studied in Ordinary level Physics.

MAGNETISM

Magnetic materials;

These are material that can be attracted by magnetic examples include: magnetic alloys such as iron, nickel, alnico etc.

Non magnetic materials

These are materials that cannot be attracted by magnets. They include: wood, copper, brass, plastics etc.

Ferro magnetic substances:

These are materials which acquires weak magnetism e.g. iron.

Para magnetic substances

These are materials which acquires weak magnetism e.g. steal.

Pole of a magnet

This is a point or an area on a magnet where it’s attractive force is strongest or where the lines of force are closest.

When a freely suspended magnet is allowed resting, it will point in the north south direction. The pole facing the north is the North Pole and the pole facing the south is the South Pole.

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Magnetic axis

This is a line joining the poles of magnet.

Magnetic meridian

It is a vertical plane containing the magnetic axis of a freely suspended magnet under the action of the earth’s magnetic field.

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Types of magnets 

  • Bar magnet

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  • Ring magnet

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Any magnet has the following properties

1 -Two poles: a north pole and a south pole

2 -Like poles (N-N and S-S) repel each other and unlike poles (N-S) attract each other .This is the law of magnetism.

3 -When a magnet is freely suspended it swings and comes to rest in the north to south direction.

4 -The pole of a magnet that freely points to the earth’s geographical south pole is called the south seeking pole (south pole) which the pole that point to the north pole of the earth is called the north seeking pole (north pole).

TESTING THE POLARITY OF A MAGNET

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The magnet whose polarity is to be determined is brought near the poles of a freely suspended.

If repulsion occurs the two poles are of the same polarity (since like poles repel) if attraction occurs no specific conclusion can be drawn or reached because attraction could be;

  1. Between a magnet and an unmagnified material
  2. Between two unlike poles of two magnet.

Therefore the only sure test for polarity of magnet is repulsion.

MAKING A MAGNET

Electrical method

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A few coils of un insulated copper wire are wound onto a card board or paper forming a solenoid leaving the ends to be (coil). Current is switched on for a short time then switched off.

When the nail is removed from the coil (solenoid), it is found able to at track small pieces of magnetic materials for a short time .The polarity of the formed magnet depends on the flow of current.

If the current of a given end flow in the clock wise direction of current flow looking into the solenoid that end becomes the South Pole.

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Single touch method

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An unmagnified material can be magnetized by single touch method to produce poles at its ends.

The steel bar to be magnetized is placed on a bench .One pole of a bar magnet is dragged along the length of the steel bar and brought back to the same end where the dragging started. This is repeated several times.

This method tended to concentrate one end and causes the other pole to spread out along the bar on the direction of stroking.

This serves as a disadvantage as it produces magnets  whose poles are concentrated at one end therefore it’s better to use the method of double stroking.

Double stroke (touch) method

The steel bar to be magnetized is laid on a table. Two opposite pole of two magnets are dragged from the centre simultaneously in opposite directions. The polarity of the end where the stroking finishes is opposite to that of the stroking magnet.

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Induction method

When a piece of un magnetized steel is placed in contact with a pole of a magnet and then removed, it is found to be magnetized.

This is called induced magnetism. Tests shows that the induced pole nearest to the magnet is of opposite sign to that at the inducing pole. It can be induced magnetism can be used to form a magnetic chain.

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Induced magnets are generally weak magnets that quickly lose their magnetism as soon the inducing magnet is taken away.

THE VIDEO DESCRIBES MORE ABOUT MAGNETISM

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DE – MAGNETISING A MAGNET

This is the process by which a magnet loses its magnetism.

Methods of demagnetizing a magnet

Heating

A magnet is heated until it becomes red hot and it is allowed to cool or rest in the east –west direction such a magnet on testing it is found to have lost its magnetism

Hammering

A magnet  hammered while lying in the east – west direction will lose its magnetism.

Electrical method

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A magnet to be demagnetized is placed in a solenoid with its axis in the east west direction.

An alternating current is passed through the solenoid; the magnet is then slowly with drawn to several meters in the east – west direction .The a.c takes the magnet through a series of alternate magnetic cycles until it is completely demagnetized.

Note;

The magnet is held in the east –west direction so that it does not remain with residual magnetism owing to induction by the earth magnetic field.

Breaking of a magnet

If a magnet is broken into two parts each path of a north pole and a south pole in it but the magnetic strength of the magnetic strength of each piece is reduced to half the magnetic strength of the original piece.

Further divisions continuously produce north and south poles with reduced magnetic strength.

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NB

Every molecule of a magnet is itself a magnet.

Domain theory

A magnetic material consists of several groups containing atomic magnets facing in random direction when in an unmagnified state.

The molecular of a magnet is in particular order and their motion is in a particular ordered direction .The groups of molecules are called domains (domain theory.)

PROPERTIES OF IRON AND STEEL

To distinguish between the magnetic properties of iron and steel a strip of steel and a strip of the dimension initially un magnetized are placed side by side in contact with poles of a magnetized.

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Two strips one of iron and the other of steel are attached to magnet as shown .Both strips became magnetized by induction and on dipping their end into iron filings cling to iron than steel.

When the magnet is taken away and the strips are held in the fungus it is noticed that all the fillings fall off from iron while a few but those on steel remain.

This shows that iron can easily be magnetized and it easily losses its magnetism while steel is not easily magnetized but once magnetized it does not lose its magnetism. Substances that are easily magnetized and easily lose their magnetism are known as magnetically soft materials e.g Iron.  where as those which retain their magnetism once magnetized are known as magnetically hard materials e.g. steel.

Properties of soft and hard magnetic substances

Magnetically soft materialsMagnetically  hard materials
1. Easily magnetized for a given current.1. Not easily magnetized by the same current.
2. Does not retain its magnetism for a long time.2. Retain its magnetism for long.

Due to their different properties soft and hard substance are used for different purposes.

Soft magnetic materials

These are used for making electromagnets where strong magnetism is required for a short time e.g. tape recorders, electric bells e.t.c.

Hard magnetic materials

These are used for making permanents e.g. those used in electric motors, compass, needle, bar magnets.

MAGNETIC FIELDS

A magnetic field is a region surrounding a magnet where a magnetic force is experienced or felt .The magnetic field lines start at the North Pole and end at the South Pole and are represented with lines with arrows.

PLOTTING MAGNETIC FIELD PATTERNS

  1. Using iron filings

A magnet where field is to be studied is placed under a sheet of stiff white paper over a thin layer of fillings is sprinkled from a caster. On tapping the paper gently with a pencil, the fillings from it chains along the magnet revealing the flux pattern.

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The principle of the method is as follows;

Each filling becomes magnetized by induction on tapping the paper, the fillings vibrate and consequently turn into the magnetic field.

The method fails for weak fields which are not able to magnetize the flux filling.

Using a plotting compass.

The magnetic flux can be traced out by means of a plotting compass. A bar magnet is placed on a white sheet of paper.

Starting near one pole of the magnet, the position of the ends, N, S of the needle is market by pencil dots, A, B and C as shown.

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The compass is now moved until the end N is exactly over the dot B and the new position of S is now marked by a third dot C.

This process is repeated and a series of dots obtained which are joined thus giving the magnetic field line which repeats the direction of the magnetic field line are dotted in the some way and every line is labeled with an arrow to indicate its direction.

Examples of magnetic field patterns

Bar magnet

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ASSIGNMENT : MAGNETISM assignment MARKS : 10  DURATION : 1 week, 3 days

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