1.0 MAGNETISM

1.1 Introduction:

• Magnetism is the force that a magnetic object exerts, through its magnetic field, on another

object. The two objects do not have to physically touch each other for the force to be exerted.

Object 2 feels the magnetic force from Object 1 because of Object 1’s surrounding magnetic

field.

• Humans have known about magnetism for many thousands of years. For example, lodestone is

a magnetized form of the iron oxide mineral magnetite. It has the property of attracting iron

objects. 

• The root of the English word magnet is from the Greek word magnes, probably

from Magnesia in Asia Minor, once an important source of lodestone.

1.2 MAGNETS

A magnet is a substance which can identify North-south directions. The substance has north-south pole.

Types of Magnets.

1.       Bar magnets.

2.       Horseshoe magnets.

3.    Compass needle.

        The end pointing North is North seeking pole.

The end pointing South is South seeking pole.

PROPERTIES OF MAGNETS.

1.      They can attract metals like iron, steel, cobalt and nickel.

2.      The attractive forces are concentrated at the two ends.

3.      When similar magnetic poles are brought near each other, they repel.

4.      When different magnetic poles are brought near each other, they attract.

1.3 THE FUNDAMENTAL LAW OF MAGNETISM.

• Like magnetic poles repel and unlike magnetic poles attract. 
• This means that two N poles or two S poles will push away from each other while a N pole and a S pole will be drawn towards each other.

1.4 MAGNETIC AND NON-MAGNETIC MATERIALS.

The materials/substances which can be attracted towards magnets are called MAGNETIC SUBSTANCES. The materials are themselves not magnets because they don’t attract each other e.g. iron, steel, cobalt, and nickel.

The materials which can’t be attracted towards a magnet are NON-MAGNETIC MATERIALS. E.g. wood, plastic, glass, zinc, carbon and rubber.

1.5 MAGNETIZATION.

Is the process of changing a magnetic material like steel, iron, cobalt to a magnet.

There are 3 main methods:

1.      STROKING METHOD.

Stroke the piece of steel with the north pole. Start at end A of the magnet and lift the magnet clear at end B of each stroke. Do this about 10 times in the same direction. Finally end A is magnetized. For double touch, use two opposite poles to stroke at the same time.

a.   Double touch 

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

2.    INDUCTION METHOD.

A magnetic material is magnetized by bringing it into contact with a magnet.

Disadvantages:

1.    Produces weak magnets not strong magnets.

2.    Tend to loose magnetism very fast as soon as they are separated from the magnets.

3.    ELECTRICAL METHOD.

Wind about 1 metre of insulated copper wire around a large nail AB, then connect the ends of the wire to the battery.

These magnets are called electromagnets.

ELECTROMAGNETS.

These are used in making devices such as electric bells, relay doors etc.

1.6 DEMAGNETIZATION

Demagnetization is the process of removing magnetism from a material.

Methods of demagnetization

1.    HAMMERING AND HEATING.

Place the magnet in East-West direction then heat it up and hammer it for some time.

2.    ALTERNATING CURRENT METHOD.

Coil a wire round the magnet and then connect the two ends of the wire to the two terminals of the battery. Then the connections to the battery terminal are quickly interchanged continuously for several minutes.

1.7 THE DOMAIN THEORY OF MAGNETISM

• The small portions of a magnetic material which behave like a magnet. These small portions are called DOMAINS. For a magnetic material, domains have North and south magnetic poles aligning in different directions.

• For this material to be magnetized, its domains have to be aligned in such a way that their North poles face one common direction and south poles face another common direction.
• Magnetic field: The region around a magnet where it has a magnetic effect.
• In any magnet there are several invisible lines that extend from north to south pole through the outside of the magnet and then move back to the north pole through the inside of the magnet forming closed loops. These imaginary lines are called magnetic lines of force which form magnetic field of a magnet.
• NB – When two similar poles are placed close to each other, there is a point somewhere between them at which the magnetic field lines do not pass. This point is called neutral point. OR When a pair of S-poles are kept close to each other.
• NB – A neutral point occurs because the magnetic lines of forces from similar poles do not join each other but repel and avoid each other.

• A neutral point can’t form between two unlike poles because the magnetic field lines from one pole join the lines from the other pole.

1.8 THE EARTH’S MAGNETISM.

• The earth has a magnetic field that extends from deep below earth’s surface to the sky. The magnetic north pole of the earth corresponds to real magnetic south pole and the magnetic south pole corresponds to real north pole.

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

-The vertical plane containing the earth’s axis of rotation at any place is called Geographic meridian.

-The angle between direction of alignment of the magnet and the geographic axis is magnetic declination.

-The horizontal direction in the magnetic meridian is magnetic dip-angle of dip.

1.9 APPLICATION OF EARTH’S MAGNETIC FIELD.

1.    The map readers use earth’s magnetic field to find locations of places.

2.    Map readers with a compass can locate north pole using the compass.

3.    It is used to detect minerals or rocks magnetized by earth’s field during solidification.

4.    Used to fix satellites with magnetometers to transmit the information about earth’s field high above earth’s surface.

1.9.1 USES OF MAGNETS.

1.    Magnetic recording media-tapes and cassettes.

2.    Making credit cards – ATM cards.

3.    Making speakers and microphones.

4.    In computers and televisions. – floppy and hard disks.

5.    In transformers.

6.    In electric motors.

7.    In generators.

8.    Used in hospitals when dealing with injuries caused by iron or steel.

9.    Used in steel works.


Comprehensive Physics notes prepared by Peter Lam Thuol