IF YOU WANT best notes class 12 short notes physics chapter5 magnetics and matter as per cbse
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class 12 chapter magnetics and matter short notes as per cbse best notes for class 12 physics it is from a very well known platform physics wallah it is very helpful for mre also when i was in calss 12
Parishram (2024)
FORMULA SHEET_(MAGNETISM & Matter)
➢ The term magnetism usually refers to the property by virtue of which a piece of iron or steel is attracted.
➢ A natural magnet is an ore of iron (Fe3O4) which attracts small pieces of iron, cobalt and nickel towards it.
➢ Lode stone is a natural magnet.
➢ The magnets which are prepared artificially are called artificial magnets. e.g., a bar magnet, a magnetic needle,
electromagnet, a horse-shoe magnet etc.
➢ According to molecular theory, every molecule of a magnetic substance (whether magnetised or not) is a
complete magnet itself.
➢ The poles of a magnet are the two points near but within the ends of the magnet, at which the entire magnetism
can be assumed to be concentrated.
➢ The poles always occur in pairs and they are of equal strength. Like poles repel and unlike poles attract.
➢ When a magnet is suspended freely, it comes to rest along north-south direction. The end point towards
geographic north is called north pole and the end point towards geographic south is called south pole.
➢ Coulomb's law: The force between any two magnetic poles is directly proportional to the product of their pole
strengths and inversely proportional to the square of the distance between them.
➢ The force between two point poles of strength m1 and m2 at a distance d apart is given by
m1m2
F= , where is called the absolute permeability of the medium.
4d 2
➢ Also, = 0r, where 0 = 4 × 10–7 henry/metre is the permeability of free space and r is the relative
permeability of the medium. 0 is also expressed as TmA–1 (T = tesla).
➢ Unit pole is defined as that pole which when placed in vacuum (or in air) at a distance of one meter from an equal
and similar pole, repels it with a force equal to 10–7 newton.
➢ An arrangement of two unlike poles of equal strength and separated by a small distance is called magnetic dipole.
The distance 2l between the two magnetic pole is called the magnetic length of the magnetic dipole is denoted
by (2l ) , a vector from south to north pole of the magnetic dipole.
➢ Magnetic dipole moment: The product of the pole strength of either magnetic pole and the magnetic length of
the magnetic dipole is called its magnetic dipole moment. It is denoted by M = m(2l )
Here, m is pole strength of the magnetic dipole. S.I. unit of magnetic dipole moment is (Am2).
➢ Magnetic lines of force: Magnetic lines of force are imaginary curves.
➢ These are continuous and closed curves. Inside the magnet, they travel from south pole to north pole. Outside
the magnet, they travel from north pole to south pole. They have neither a beginning nor an end.
, 2
➢ They are crowded near the poles indicating a stronger magnetic field near the poles.
➢ Tangent drawn to the curve at any point denotes the direction of magnetic field at that point.
➢ Lines of force are drawn equidistant and parallel to each other to indicate a uniform magnetic field.
➢ No line of force passes through a neutral point situated in the magnetic field of a magnet.
➢ Lines of force never intersect each other. At a point, the field has one direction only.
➢ Magnetic field due to a bar magnet: The magnetic field due to a bar magnet of length 2l and having magnetic
dipole moment M at a distance r from its centre
(i) on its axial line is given by
0 2Mr 2M
B= . 2 2 2 0 . 3 (r l )
4 ( r − l ) 4 r
(ii) on its equatorial line is given by
0 2 Mr M
B= . 0 . (r l )
4 ( r 2 + l 2 ) 2 4 r 3
➢ Torque on a magnetic dipole in a magnetic field: When a magnetic dipole of magnetic dipole moment M is
placed in a uniform magnetic field of strength B making an angle with the direction of magnetic field, it
experiences a torque, which is given by
| |=| M B | MB sin
➢ Magnetic dipole moment can be defined as the torque acting on a magnetic dipole placed normal to a uniform
magnetic field of unit strength.
➢ Potential energy stored in a magnetic dipole on rotating inside a magnetic field: The work done in rotating
a magnetic dipole against the torque acting on it, when placed in magnetic field is stored inside it in the form of
potential energy. When magnetic dipole is rotated from initial position = 1 to final position = 2 then,
potential energy stored is given by
U = –MB (cos 2 – cos 1)
➢ A current carrying loop behaves as a magnet i.e. magnetic dipole. Thus, magnetic dipole moment of a current
loop i.e. M = IA, where A = area of the loop.
➢ In an atom, electrons revolve around the nucleus in circular orbits. The movement of the electron in circular orbit
around the nucleus in anticlockwise direction is equivalent to the flow of current in the orbit in clockwise
direction. Thus, the orbit of electron is considered as tiny current loop.
eL
M =
2m
eL
➢ Bohr Magneton (B) =
4m
➢ A current carrying straight solenoid behaves like a bar magnet.
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