Magnetism and Matter 1 Flashcards
from where do we derive the word magnet?
What are some commonly known facts about magnets
- The word magnet is derived from
the name of an island in Greece called magnesia where magnetic ore
deposits were found, as early as 600 BC.
-(i) The earth behaves as a magnet with the magnetic field pointing
approximately from the geographic south to the north.
(ii) When a bar magnet is freely suspended, it points in the north-south
direction. The tip which points to the geographic north is called the
north pole and the tip which points to the geographic south is called
the south pole of the magnet.
(iii) There is a repulsive force when north poles ( or south poles ) of two
magnets are brought close together. Conversely, there is an attractive
force between the north pole of one magnet and the south pole of
the other.
(iv) We cannot isolate the north, or south pole of a magnet. If a bar magnet
is broken into two halves, we get two similar bar magnets with
somewhat weaker properties. Unlike electric charges, isolated magnetic
north and south poles known as magnetic monopoles do not exist.
(v) It is possible to make magnets out of iron and its alloys.
describe a bar magnet
I) a bar magnet attracts small pieces of magnetic substaince like Fe,Ni,Co. The attraction is max at the 2 ends, which are c/a poles of the magnet.
ii) The two magnetic poles always exists in a pair. Monopoles do not exist. The 2 poles are of equal strength
y. A similar pattern of iron
filings is observed around a current
carrying solenoid.
Unit of pole strength: Am
magnetic dipole moment
Magnetic dipole consists of 2 unlike poles of equal strength and seperated by a small distance. The distance b/w two poles is called magnetic length.
M= mx2l
Unit: Am^2 or JT-1
Direction: south to north ole
describe magnetic field lines
i) Magnetic field are closed continuous looops such that tangent to the looop at any point gives the direction of field at that point.
ii) Magnetic field lines exit normally from the north pole and enter normally into the south pole, outside the magnet. Inside themagnet magnetic field lines run from the south pole to north pole, indicating the presence of field inside a magnet.
iii) The magnetic field lines contract longitudinally, which shows that 2 unlike poles attract.
iv) The magentic field lines expand laterally, which shows that like poles repel.
v) Two magnetic field lines can never intersect, as if they do, then there would be 2 values of magnetic field at that point, which is not possible.
vi) The greater the density of magnetic field passing through a given area, greater is the magnitude of the field.
vii) Parallel magnetic field lines indicate a uniform field, while if they are non parallel, the field is non uniform.
circular current carrying loop as a magnetic dipole
the magnetic field (at large distances)
due to current in a circular current loop is very similar in behaviour to
the electric field of an electric dipole.
At very large distances from the axis, the current carrying loop behaves like a magnetic dipole.
a planar current loop is equivalent to a magnetic dipole of dipole moment
m = I A, which is the analogue of electric dipole moment p. Note, however,
a fundamental difference: an electric dipole is built up of two elementary
units — the charges (or electric monopoles). In magnetism, a magnetic
dipole (or a current loop) is the most elementary element. The equivalent
of electric charges, i.e., magnetic monopoles, are not known to exist.
This led Ampere to suggest
that all magnetism is due to circulating currents. This seems to be partly
true and no magnetic monopoles have been seen so far. However,
elementary particles such as an electron or a proton also carry an intrinsic
magnetic moment, not accounted by circulating currents.
Bar magnet as an
equivalent solenoid
- The magnetic moment of a bar magnet is thus equal to the magnetic moment of an equivalent solenoid that produces the same
magnetic field.
-
similarities between solenoid and bar magnet
Basis of comparision: Magnetisation, Magnetic Force, Polarity, Demagnetisation, Strength.
Bar Magnet
i) It is permanently magnetised
ii) It produces comparatively weak magnetic force.
iii) Its polarity is fixed and cannot be reversed,
iv) It cannot be demagnetised easily.
v) Strength depends on nature of material used to create it.
Solenoid
i) It is temporarily magnetised.
ii) It produces a strong magnetic force
iii)Its polarity can be reversed by reversing the direction of current in the coil.
iv) It can be demagnetised by switching of current.
v) Depends on quantity of current flow.
What are similarities between bar magnet and solenoid
i) The magnetic field on the axial point is the same for bar magnet and solenoid.
ii) The magnetic moment is same for equivalent solenoid and bar magnet.
iii) Bar magnet and solenoid have both attractive and directive properties,ie, ability to align to the field.
what is gauss law of magnetism
The magnetic flux through any closed surface is 0.
This is because the total field lines entering the surface is equal to the field lines leaving the surface.
This is because, isolated magnetic monopoles do not exist. The smallest unit of existence of magnet is a magnetic dipole , whose field lines form closed continuous loops.
what is a neutral point
It is the point where the magnetic field due to a magnet is equal and opp to the horizontal component of the earths magnetic field.
If the magnet is placed in the magnetic meridian with its north pole facing north, then the neutral points are at the equatorial line.
If the magnet is placed in the magnetic meridian with its north pole facing south, then the neutral points are at axial points.
pole strength
, pole
strength can be defined as the strength of a magnetic
pole to attract magnetic materials towards itself. It is a
scalar quantity and its SI unit is ampere-metre (A-m).
Strength of N-pole and S-pole of a magnet is always equal
and opposite (+m and −m).
ncert similarity b/w magnetix dipole moment of bar magnet and solenid
The magnetic field lines for a bar magnet and a current
carrying solenoid resemble very closely. Therefore, a bar
magnet can be thought as a large number of circulating
currents in analogy with a solenoid. The expression for the
magnetic field due to a solenoid,
B=u/4pi 2m/r^3
where, r is the distance between the centre of solenoid to
the required point.
This expression is same as the expression for magnetic
field of a bar magnet at its axial point. Thus, a bar magnet
and a solenoid produce similar magnetic fields.
