TNPSC Free Notes Physics In English

இந்தக் கட்டுரையில், TNPSC குரூப் 1, குரூப் 2, குரூப் 2A, குரூப் 4 மாநிலப் போட்டித் தேர்வுகளான TNUSRB, TRB, TET, TNEB போன்றவற்றுக்கான முறைகள் இலவசக் குறிப்புகளைப் பெறுவீர்கள்.தேர்வுக்கு தயாராவோர் இங்குள்ள பாடக்குறிப்புகளை படித்து பயன்பெற வாழ்த்துகிறோம்.

Magnetisim

Introduction:
 The branch of physics which deals with the property of a magnet is also called
magnetism.
 The earliest evidence for magnets are found in a region of Asia Minor called Magnesia.
 It is believed that the Chinese had known the property of magnet even before 200 B.C.
 They used a magnetic compass for navigation in 1200 A.D.
 William Gilbert laid the foundation for magnetism and suggested that the Earth has a
giant bar magnet.

 He was the first man who performed the systematic research on the properties of the
lodestone (magnetic iron ore) and published his findings in the influential ‘De Magnete’
(The Magnet).
Classification of magnets:
Natural Magnets:
 Magnets found in the nature are called natural magnets. They are permanent magnets i.e.,
they will never lose their magnetic power.
 Lodestone called magnetite (Iron oxide) which is the ore of iron is the strongest natural
magnet. Minerals like Pyrrhotite (Iron Sulphide), Ferrite and Coulumbite are also natural
magnets.
Artificial Magnets:
 Magnets that are made by people in the laboratory or factory are called artificial magnets.
These are also known as manmade magnets, which are stronger than the natural magnets.
 Bar magnets, U-shaped magnets, horseshoe magnets, cylindrical magnets, disc magnets,
ring magnets and electromagnets are some examples of artificial magnets.
 Artificial magnets are usually made up of iron, nickel, cobalt, steel, etc. Alloy of the
metals Neodynium and Samarium are also used to make artificial magnets.
Magnetic Properties:
 Attractive property
 Reflective property
 Directive property
Attractive property:
Unlike poles of a magnet attract each other. i.e., the north pole and the south pole of a magnet
attract each other.
Reflective property:
Like poles repel each other i.e., a north pole repels another north pole and a south pole repels
another south pole.
Directive Property:
A freely suspended bar magnet always aligns itself in the geographic north-south direction. The
property of a magnet, by which it aligns itself along the geographic north-south direction, when
it is freely suspended, is known as the directive property of a magnet.

Based on their behaviour in a magnetic field they can be classified as
 Diamagnetic materials
 Paramagnetic materials
 Ferromagnetic materials
Diamagnetic materials:
 When suspended in an external uniform magnetic field they will align themselves
perpendicular to the direction of the magnetic field.
 They have a tendency to move away from the stronger part to the weaker part when
suspended in a non-uniform magnetic field.
 They get magnetised in a direction opposite to the magnetic field.
 Examples for diamagnetic substances are bismuth, copper, mercury, gold, water, alcohol,
air and hydrogen.
 Magnetic character of these substances is not affected by the external temperature.
Paramagnetic materials:
 When suspended in an external uniform magnetic field they will align themselves parallel
to the direction of the magnetic field.
 They have a tendency to move from the weaker part to the stronger part when suspended
in a non-uniform magnetic field.
 They get magnetised in the direction of the field. Examples for paramagnetic substances
are aluminium, platinum, chromium, oxygen, manganese, solutions of salts of nickel and
iron.
 Magnetic character of these substances is affected by the external temperature.
Ferromagnetic materials:
 When suspended in an external uniform magnetic field they will align themselves parallel
to the direction of the magnetic field.
 It has a tendency to move quickly from the weaker part to the stronger part when
suspended in a non-uniform magnetic field.
 They get strongly magnetised in the direction of the field.
 Examples for ferromagnetic substances are iron, cobalt, nickel, steel and their alloys.
 Magnetic character of these substances is affected by the external temperature. When
they are heated they become para magnetic.

Magnetisation:
Magnetisation is a process in which a substance is made a permanent or temporary magnet by
exposing it to an external magnetic field. This is one of the methods to produce artificial
magnets.
Artificial magnets:
 Artificial magnets are produced from magnetic materials. These are generally made by
magnetizing iron or steel alloys electrically.
 Depending on their ability to retain their magnetic property, artificial magnets are
classified as permanent magnets or temporary magnets.
Temporary magnets:
Temporary magnets are produced with the help of an external magnetic field. They lose their
magnetic property as soon as the external magnetic field is removed.
Permanent Magnets:
 Permanent magnets are artificial magnets that retain their magnetic property even in the
absence of an external magnetic field. These magnets are produced from substances like
hardened steel and some alloys. The most commonly used permanent magnets are made
of ALNICO (An alloy of aluminium, nickel and cobalt)
 Neodymium magnets are the strongest and the most powerful magnets on the Earth.
 The most powerful magnet in the universe is actually a neutron star called magnetar
The magnetic properties of a magnet will be removed from it by the following ways.
 Placing the magnet idle for a long time.
 Continuous hammering of the magnetic substance.
 Dropping the magnet from a height.
 Heating a magnet to a high temperature.
 Passing a variable current in a coil that encloses the magnet.
 Improper storage of the magnet

Earth Magnetisim:

 The magnetic axis andthe geographical axis (axis of rotation) do not coincide with each
other.
 The magnetic axis of the Earth is inclined at an angle of about 10° to 15° with the
geographical axis.
Cause the Earth’s magnetism, are as follows.
 Masses of magnetic substances in the Earth
 Radiations from the Sun
 Action of the Moon
Magnetic Properties:
Magnetic field:
 An invisible field all around magnets which attracts magnetic materials. In this space we
can feel the force of attraction or repulsion due to the magnet.
 Symbol : B
 Unit : tesla

Earth magnetic field:
 Protection of Ozone
 Navigation
Magnetic flux:
 Magnetic flux is the number of magnetic field lines passing through a given area.
 Symbol : ϕ
 Unit : weber (Wb).
Magnetic flux density:
 The number of magnetic field lines crossing unit area kept normal to the direction of field
lines is called magnetic flux density.
 Unit: Wb/m 2

Properties Of Magnetic Lines Of Forces:
 Closed, continuous curves, extending through the body of the magnet.
 Start from the North Pole and end at the South Pole.
 Never intersect.
 Maximum at the poles than at the equator.
 The tangent drawn at any point on the curved line gives the direction of magnetic field
Magnetic effect of current:
Discovered by Hans Christian Oersted.
Right Hand Thumb rule:
H.A.Lorentz found that a charge moving in a magnetic field, in a direction other than the
direction of magnetic field, experiences a force. It is called the magnetic Lorentz force.

Fleming’s Left Hand Rule:

 The law states that while stretching the three fingers of left hand in perpendicular manner
with each other, if the direction of the current is denoted by the middle finger of the left
hand and the second finger is for direction of the magnetic field, then the thumb of the
left hand denotes the direction of the force or movement of the conductor.
 Example – Electric motor

Fleming’s Right Hand Rule:

 Stretch the thumb, fore finger and middle finger of your right hand
mutuallyperpendicular to each other. If the fore finger indicates the direction of magnetic
field andthe thumb indicates the direction of motion of the conductor, then the middle
finger willindicate the direction of induced current. Fleming’s Right hand rule is also
called ‘generator rule’.
 Example – Electric Generator
Applications Of Magnet:
 Speaker
 Magnetic Levitation Trains
 Medical System (Magnetic resonance imaging)

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