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Learning objectives
state properties of magnets
describe induced magnetism
distinguish between magnetic and non-magnetic materials
describe electrical methods of magnetization and demagnetization
describe the plotting of magnetic field lines with a compass
draw the magnetic pattern around a bar magnet and between the poles of two bar magnets
distinguish between the magnetic properties and uses of temporary magnets (e.g. iron) and permanent magnets (e.g. steel)
Properties of magnets
Properties of magnets
The two regions where the magnetic forces are the strongest in the magnet are called the poles of the magnet.
The pole of a freely suspended magnet which points to the north is called the north-seeking pole or north pole.
The pole of a freely suspended magnet which points to the south is called the south-seeking pole or south pole.
Law of magnetism :- Like poles repel, unlike poles attract
Testing a magnet
Attraction can confirm whether a specimen is a magnetic material, it does not tell us whether a specimen has been magnetized.
Only repulsion can confirm that a magnetic material is already magnetized.
Magnetic induction
Induced magnetism is the temporary magnetisation of an unmagnetised ferromagnetic material when the material is brought near a magnet.
Induced magnets have the same magnetic properties like ordinary magnets.
The strength of an induced magnet is increases if
* a stronger magnet is used.
* the distance between the magnet and the induced magnet is decreased.
* the induced magnet is made of a soft magnetic material.
Applications and consequences of induced magnetism
* In hospitals, strong magnets can be used to induced magnetism in a steel splinter and remove it from a patient's eye.
* Steel tools and girders in bridges are often slightly magnetised by magnetic induction from the earth's magnetism.
Therory of magnetism
According to the theory of magnetism, each electrons acts as a tiny magnet as it spins and orbit around the nucleus of an atom.
In non magnetic materials, electron motions are such that the magnetic effects cancel each other out. However, in magnetic materials, the magnetic atoms arrange themselves in groups each with all its atomic magnets pointing in a certain direction. These groups are called magnetic domains and each acts as a magnet.
In the unmagnetised state, the domains in a magnetic material all point in different directions and their magnetic effects cancel each other.
In the magnetised state, the domains all point in a particular direction and their magnetic strength becomes maximum.
Consequences of the theory of magnetism
* Storage of magnets using keepers:- To prevent weakening of the magnets
* Magnetic saturation:- Each magnet has a possible strength
* Demagnetisation:- By heating and hammering
* Breaking of a magnet into smaller pieces :- The smaller pieces becomes magnet. There are no mono-pole magnet.
Methods of magnetisation
Methods of magnetisation
By heating and hammering
By heating and hammering
A steel bar , lying in a north-south direction, is heated to a high temperature and then hammered softly during cooling.
By stroking
By stroking
One or two permanent magnets can be used to magnetise a steel bar by stroking using the single or double touch method.
Using a direct current flowing through a solenoid
Using a direct current flowing through a solenoid
When a direct current flows through the solenoid, the steel bar in it will be magnetised with polarities depending on the direction in which the current flows.
Methods of demagnetisation
Methods of demagnetisation
By hammering
By hammering
A magnet, lying in a east-west direction, is hammered vigorously during cooling.
By heating
By heating
When a magnet is heated very strongly, it loses its magnetism.
Using alternating current flowing through a solenoid aligned in the east-West direction
Using alternating current flowing through a solenoid aligned in the east-West direction
When an alternating current flows through the solenoid, the magnet in it will be demagnetised as the magnet is slowly withdrawn further and further away from the solenoid.
Note that high frequency alternating current can also be used to magnetised a magnetic substance.
Magnetic fields
Magnetic fields
A magnetic field is a region where an magnetic material experiences a magnetic force.
Plotting of magnetic lines of force
Magnetic field patterns
A strong magnet has a strong magnetic field, i.e. it produces more lines of force and the lines are closer to each other.
The magnetic lines of force can never cross each other.
The magnetic lines of force points from the North pole to the South pole.
http://www.magnet.fsu.edu/education/tutorials/slideshows/fieldlines/index.html
A neutral point is a place where the resultant magnetic field strength is zero.
Earth's magnetic field
Earth's magnetic field
Animals and Magnetic field
Find out about animals making use of magnetic field to migrate and find their way from the following websites.
Magnetic shielding, outside the magnet. Magnetic lines of force are made to pass through magnetic materials.
Soft magnetic and hard magnetic materials
Soft magnetic and hard magnetic materials
Iron and steel are common examples of soft and hard magnetic materials respectively.
Uses of permanent magnets and electromagnets
Uses of permanent magnets and electromagnets
Other interesting applications of magnetism
New drug delivery system uses magnetism
http://news.cnet.com/8301-27083_3-10356754-247.html
Magnetic Therapy
http://science.discovery.com/video/skepticalinquirer/magnettherapy.html
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