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TOPIC 3: MAGNETISM | PHYSICS FORM 1

October 2, 2020

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Concept of Magnetism

The Origin of Magnetism

Explain the origin of magnetism

Chinese were the first to use the metal magnetite called lodestone.
A Lodestone was capable of attracting small iron pieces and it was used
as a crude navigation compass by Greeks. Lodestones were the earliest
magnets.

Iron, nickel and cobalt
are the only naturally occurring magnetic materials. Magnet has two
ends known as magnetic poles in which the greatest attraction power is
concentrated.

Magnetic and Non-magnetic Materials/Substances

Identify magnetic and non-Magnetic material/substances

Magnetic substances

These are substances which have a property of being attracted by a magnet.E g; iron,steel,cobalt and nickel.

Pole strength refers to the ability of a magnet to attract objects.
Ferromagnetic substances
have very high magnetic susceptibility (easily magnetized).Eg; iron,
nickel and cobalt.
Electromagnet is the substance which requires electric current to attain magnetism.
Permanent magnet is a substance which is already a magnet and it doesn’t require electric current to attain magnetism.

Non-magnetic substances

These
are substances which are not attracted by a magnet.Eg; copper, brass,
aluminium,glass,plastic and wood.These substances have very weak
magnetic properties.

The Properties of Magnets

State the properties of magnets

Magnets have a tendency of attracting magnetic substances and have no action on non-magnetic materials.

Types of magnetic materials

There are three types of magnetic materials:

Diagmatic materials:
Are substances which have a tendency to repel from a stronger to weaker
magnetic field. Eg; bismuth, water, gold,air,hydrogen,common
salt,diamond,silver and copper.
Paramagnetic materials:
Are substances which become weakly magnetized when placed in magnetic
field. Eg; aluminium,platinum,chromium,oxygen and manganese.
Ferromagnetic materials:
Are substances which becomes magnetized when placed in magnetic field.
Their magnetic domain become aligned in one direction when they are
placed in magnetic field. Eg; iron, cobalt and nickel.

Magnetic domain
refers to the molecular magnets lined up with each other which
constitute ferromagnetic materials.

The direction of magnetic poles varies from one domain to another if the magnet is unmagnified

Types of Magnets

Identify types of Magnets

Magnets may also be classified according to their shapes. This includes:

Horse shoe magnet
Rod magnet
Field magnet
Bar magnet

Application of Magnets

Identify application of magnets

Magnets are used in:

women handbags closing
picking up heavy loads
electrical appliances like meter and receivers
sound and video recording equipment
computer memory and disks
electrical trains

Magnetisation and Demagnetisation

The Concept of Magnetisation and Demagnetisation

Explain the concept of magnetisation and demagnetisation

Magnetisation:Is a process of making a magnet from a magnetic substance.

Demagnetisation:Is the process by which a magnet is made to lose its magnetism.

Demonstrating Magnetisation and Demagnetisation

Demonstrate magnetisation and demagnetisation

Methods of magnetisation

There are various ways, including;

Induction
Stroking
Electrical method

Induction method.

This is done by placing a piece of unmagnetised steel bar near or in contact with a pole of a magnet and then removing it.

In this case an iron nail placed near a bar magnet will be induced with magnetism.

Stroking method

This is done by stroking a bar magnet into an unmagnetised steel bar.There are two stroking methods, namely;

Single touch:
A magnetised bar magnet is formed by a single stroke.-A steel bar is
stroked repeatedly by a very strong bar magnetism the same direction
with the north pole i.e from A to B. The bar magnet is lifted at B and
then returned at A. After several strokes the steel bar will be
magnetised with north pole at A and south pole at B.
Double touch:
Two bar magnets are used to magnetise a single steel bar. The steel bar
is magnetised by two bar magnets from its center to its ends using left
and right hands simultaneously for several times. Between each stroke
the two bar magnets are lifted up high and returned to the center for
another stroking.

In this case the steel bar will be magnetized with south pole at A and north pole at B.

Note:

In both single and double touch methods, the magnetising magnet none of their strengths.
Between
successive strokes, the pole is lifted high above bar, otherwise the
magnetism already induced in it will tend to be weakened.3.Consequent
poles will be formed at the center of the steel bar when the two bar
magnets placed at the center are of like poles. Same poles will be
obtained on both ends.

Electrical method

A
cylindrical coil wound with many of insulated copper wire is connected
in series with a battery. A steel bar is placed inside the solenoid and
the current switched on and off.When the steel bar is removed and
tested, it is found to be magnetised.

Note:If
the current is switched off for so long, the bar will not be
magnetized.The poles of the bar magnet depends on the direction of flow
of current. The end at which the direction of the current is in
clockwise direction will be south pole and if anticlockwise it will be
north pole.

Methods of demagnetization

Electrical method

The
magnet is placed inside a solenoid through which an a. c is flowing.The
magnet is withdrawn from the solenoid while the current is flowing
pointing in the W-E direction. When the magnet is held in W-E direction,
it doesn’t remain with residue magnetism due to induction from earths
magnetic field.

Other methods of demagnetisation include;

Heating a magnet.
Hammering while pointing E-W direction.

Methods of Storing Magnets

Design methods of storing magnets

Magnets are stored in magnetic keepers.They
are stored in pairs,with unlike poles together and with pieces of
iron(magnetic keepers) across both ends.The keepers are magnetised by
induction.

Magnetic Fields of a Magnet

The Concept of Magnetic Fields of a Magnet

Explain the concept of magnetic fields of a magnet

Magnetic field:Is
a space surrounding a magnet in which a magnetic force is exerted or
experienced. It consists of magnetic field lines which are imaginary
lines of force around a magnet from North Pole to South Pole.

The Magnetic Lines of Force around a Magnet using Iron Fillings or Compass Needle

Illustrate the magnetic lines of force around a magnet using iron fillings or compass needle

Experiment.

Aim:To study the properties of magnetic field lines around a bar magnet.

Materials: Iron fillings, bar magnets and a piece of paper.

Procedures

Place a sheet of plane paper over a bar magnet.
Sprinkle iron fillings on the sheet of paper.
Gently tap the sheet of paper.

Observation

Iron fillings will form a pattern which depends on the magnetic lines of force of the magnet.
The
lines of equal magnetic strength are seen flowing between magnetic
poles .The lines are referred to as lines of magnetic flux or field
lines. The pattern magnetic force is called a magnetic field.
When investigating a magnetic field with iron fillings the field is strongest where the fillings are crowded.
By
investigating a magnetic field lines and a bar magnet using a small
compass needle, the magnetic flux(lines) runs from north pole to south
pole.

Neutral point is a point at which the resultant magnetic flux density is zero.

this
area, X, the two magnetic field s cancel or neutralize each other.It
happens when two like magnetic poles are brought together.

Therefore neutral point is;

an area in a magnetic field where the resultant magnetic field strength is zero.
a point which exists where two magnetic field neutralize each other.

The Methods of Magnetic Shielding

Explain the methods of magnetic shielding

Magnetic shielding:Is a screen made from high permeability material used to isolate any material from unwanted magnetic fields

Example;
The electron beam in the cathode ray tubes of TV sets, and very
delicate measuring instruments are shielded(protected)from magnetic
influence by placing them in soft iron cases with thick wells.

Here the object is shielded from the strong magnetic fields by a soft iron ring around it.

Earth’s Magnetic Field

The Phenomenon of Earth’s Magnetism

Explain the phenomenon of earth’s magnetism

Earth
is imagined as a single big magnet, with its south seeking pole near
the geographic north called a north magnetic pole and the north seeking
pole is near geographic south pole called south magnetic pole.

The
earth behaves as if at its center there is a short piece of a bar
magnet inclined at a small angle to its rotation (spinning) axis. When a
bar magnet is hanged horizontally with a string, it will oscillates for
a short time and then comes to rest with its poles pointing in the N-S
direction due to the earth’s magnetic field.

This
gives a notion that the earths north pole is in the southern hemisphere
where any magnets north pole will always point.The two earth’s magnetic
poles are joined by a line called magnetic meridian.The geographic
meridian joins the true north and true south.

Direction of Earth’s Magnetic Field

Determine direction of earth’s magnetic field

Compass needle:Is
a thin magnet balanced on a point, usually at the center of gravity
used to identify N-S direction.Spinning a compass needle, it will
eventually come to rest with its poles pointing to the N-S
direction.This gives direction at any point on the earth’s surface.

The Earth’s Magnetic Lines of Force about a Bar Magnet

Locate the earth’ magnetic lines of force about a bar magnet

The
magnetic field lines around a bar magnet can be mapped with the help of
a magnetic compass. The magnetic field lines due to a bar magnet are
closed loops. They leave at the north pole and enter at the south pole.
When you plot the magnetic field lines around a bar magnet, the
horizontal component of the earth’s magnetic field, B₀,
influences the magnetic field induction, B, due to the bar magnet. At
points close to the bar magnet, magnetic induction B due to the bar
magnet is very high as compared to the horizontal component of the
earth’s magnetic field, B₀. Thus, B₀is negligible.

According
to the inverse square law of magnetism, magnetic field induction B due
to the bar magnet decreases as we move away from it. At certain points
around the bar magnet, B and B₀are equal in magnitude and
opposite in direction. Therefore, they cancel each other out, and the
resultant magnetic field is zero.

Thus,
the point where magnetic field induction B due to a bar magnet is equal
in magnitude and opposite in direction to the horizontal component of
the earth’s magnetic field induction, B₀, is called a neutral point.

The neutral points around a bar magnet can be located in two different cases

When the north pole of the bar magnet points towards the earth’s north pole
when the south pole of the bar magnet points towards the earth’s north pole.

When the north pole of a bar magnet points towards the north pole of the earth.

Fix a sheet of white paper on a drawing board with brass pins.
Take a compass needle, place it at the centre of the paper, and mark the north and south directions.
Draw a straight line along the paper connecting the two points. This represents the magnetic meridian of the earth.
Represent the geographical directions at the corner of the paper.
Draw
an arrow from the geographical south to the geographical north on the
right side of the paper to indicate the direction of the horizontal
component of the earth’s magnetic field, B₀.
Take a
bar magnet and place it at the centre of the paper such that the north
pole of the bar magnet points towards the north pole of the earth.
Now place the compass needle at the north pole of the bar magnet and mark a point where the north pole of the compass needle is.
Shift the compass such that the south pole of the compass needle is at the point you just marked.
Mark
another point at the north of the compass needle, and then shift the
compass, as done earlier.Repeat the procedure till the compass needle
reaches the other end of the bar magnet.
Join all the points to get a continuous smooth curve, which represents a magnetic field line.
Repeat the procedure from the north pole of the magnet, but from different points, and draw the magnetic field lines.

The
two points on either side of the bar magnet at equal distances from its
centre, where the compass needle does not show any specific direction.
At these points, the magnetic field induction B due to the bar magnet
and the horizontal component of the earth’s magnetic field induction, B₀are
equal in magnitude and opposite in direction. The resultant magnetic
field is zero. These points represent the neutral points denoted by N₁and N₂. These two points fall on the equatorial line of the bar magnet.

Thus,
when the north pole of a bar magnet points towards the geographical
north pole of the earth, the two neutral points lie on the equatorial
line of the bar magnet such that they are equidistant from the centre of
the bar magnet.

The Angle of Inclination (dip) and Angles of Declination

Measure the angle of inclination (dip) and angles of declination

Angle of dip is the angle between the direction of the earth’s magnetic flux and the horizontal.

Measurement of angle of dip

The angle of dip is not constant over the earth’s surface; it varies from place to place.

This
difference is brought about by the earth’s magnetic field direction at a
point on the earth’s surface. The easiest way to measure the angle of
dip is using a thin iron rod.The rod is suspended by length of a thread
such that it balances horizontally.

Without
disturbing the position of attachment, the rod is magnetised by
bringing close a bar magnet and touching. On re-suspending the rod will
dip with its north pole pointing downwards as shown in the diagram
above. Using a protractor, the angle between the axis of the rod and the
horizontal can be measured.

Angle of declination

Is
the angle between the magnetic meridian and geographic meridian. The
angle of declination varies from place to place all over the world. Maps
shows declination at different points of the world.

Application of Earth’s Magnetic Field

State the application of earth’s magnetic field

The earth’s magnetic field is used to: