Neutral point of a Magnet Download

Pages attached
This is to certify that Umang Gupta of class XII-B has
performed the experiment entitled
" To determine the location of Neutral points and
calculate the Dipole Moment of Bar Magnet given".
Under the supervision of Ma'am Rakhi Thapar and lab
assistant Naveen Sir
Mrs. Rakhi Thapar
(Physics Department)
Montfort School
I, Umang Gupta, of class XII-B sincerely acknowledge
my Physics teacher Rakhi Thapar for her invaluable
support in selecting this extremely important project. I
express my gratitude for her knowledgeable suggestions,
comments , advice and guidance throughout.
I would also like to thank the lab assistant Mr. Naveen,
for providing me with the necessary information and
Umang Gupta
Roll No: 41
I chose to perform this experiment , to locate the neutral
point, as an extension to the theory we have already learnt
in the class.
We have been taught that a point in the magnetic field
where the fields due to the bar magnet is equal and
opposite to the horizontal intensity of Earth's magnetic
I feel the observations made in the experiment and the
conclusion drawn from them complement the concept by
verifying it sufficiently.
Magnetism is a class of physical phenomena that are
mediated by magnetic fields. Electric currents and the
magnetic moments of elementary particles give rise to a
magnetic field, which acts on other currents and magnetic
moments. Every material is influenced to some extent by
a magnetic field. The most familiar effect is on permanent
magnets, which have persistent magnetic moments caused
by ferromagnetism. The prefix Ferro- refers to iron,
because permanent magnetism was first observed in a
form of natural iron ore called magnetite, Fe3O4. Most
materials do not have permanent moments. Some are
attracted to a magnetic field (paramagnetic); others are
repulsed by a magnetic field (diamagnetism); others have
a more complex relationship with an applied magnetic
field (spin glass behavior and antiferromagnetism).
Substances that are negligibly affected by magnetic fields
are known as non-magnetic substances. These include
copper, aluminium, gases, and plastic. Pure oxygen
exhibits magnetic properties when cooled to a liquid state.
The magnetic state (or magnetic phase) of a material
depends on temperature and other variables such as
pressure and the applied magnetic field.
Magnetism is a force of nature, which causes special
kinds of objects to attract to each other. Magnetism is all
around you. You can find it in the most common places
like the magnet on your refrigerator. The force of
magnetism flows from one pole to another. A pole is the
point where the force is pointed. The force of magnetism
causes material to point along the direction the magnetic
force points. This means that the force has direction.
The force is represented by lines, which point from the
positive pole to the negative pole of the magnet. It forces
small pieces of iron to live up in the direction the magnet
force points. The lines represent what is called the
magnetic field of the magnet. The magnetic field is
strongest where the lines of force come together, and is
weakest when the lines of force are far apart.
A magnet is surrounded by a magnetic field. This field
has both a North side, and a South side. This field is
strongest where the lines of force came together and
weakest when they are far apart.
Flux lines are the lines that determine how current flows
throughout a magnet. They are circular lines, which run
around the magnet. It is determined which direction a
current is flowing by using a series of left-hand rules.
Each magnet has its own flux shape, and rule.
Magnetism and electricity are very closely related. When
speaking of them together they are spoken of as an
electromagnetic force. There are three main factors that
must be thought of when realizing that magnetism and
electricity are related. These things are a) moving electric
charges produce magnetic fields, b) magnetic fields exert
forces on moving electric charges, c) when you change
magnetic fields in the presence of electric charges it
causes a current to flow.
Magnetism is present throughout the world. It is used in
electronics, medicine, generators, and in so many other
things. Magnetism affects all aspects of our lives.
To determine the location of neutral points and calculate
the dipole moment of bar magnet given
 A Drawing Board
 A Compass Needle
 A Bar Magnet
 A plain sheet of paper
 Fixing pins
 Meter Scale
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The Magnetic Field : When the
charged particle move through a magnetic
field B, it feels Lorentz force F given by the
cross product
F = Q (V x B)
where Q is the electric charge of the particle
and V is the velocity vector of the particle
Because this is a cross product, the force is
perpendicular to both the motion of the
particle and the magnetic field. It follows that
the magnetic force does not work on the
particle, it may change the direction of the
particle's movement , but it cannot cause it to
speed up or slow down. The magnitude of the
force is
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F= QVBsin Ɵ
where Ɵ is the angle between the V and B vectors
The magnetic field lines: The magnetic field
lines are a visual and intuitive realization of the magnetic
field. Their properties are
 The magnetic field lines of a magnet(or a solenoid)
gorm continuous closed loops. This is unlike the
electric dipole where these fileds lines begin from a
positive charge and end on a negative charge or
escape to infinity.
 The tangent to the field line at a given point
represents the direction of the net magentic field B at
that point
Lines of force : Lines of force is a closed
imaginary curve starting from the North Pole and
ending in the South pole in the magnetic field such
that the tangent drawn at any point o the curve gives
the direction of resultant magentic field at that point.
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The Dipole : A very common source of magentic
field shown in the nature is dipole, with a "south
pole" and a "north pole", terms dating back to the use
of magnets as compass, interacting with the Earth's
magnetic field to indicate the North and South Pole.
Since the opposite ends of the magnets are attracted ,
the North Pole of a magnet attracted to the south pole
of another magnet. The Earth's North Magnetic Pole
(currently in the Arctic Ocean, north of Canada) is
physically a south pole as it attracts the North Pole of
a compass. A magnetic field contains energy, and
physical systems move around configurations with
lower energy. When diamagnetic material is placed
in a magnetic field, a magnetic dipole tends to align
itself in opposed polarity to that filed, thereby
lowering the net filed strength.
When ferromagnetic material is placed within a
magnetic field , the dipoles align to the applied force,
thus expanding the domain walls of the magnetic
The magnetic Dipole Moment: The
magnetic moment of a magnet is a quantity that
determines the torque it will experience in an external
magnetic field. A loop of electric current , a bar
magnet, an electron , a molecule , and a planet all
have magnetic moments. The magnetic moment may
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be considered to be a vector having magnitude and
direction . The direction of magnetic moment points
from south to the North Pole of the magnet. The
magnetic field produced by the magnet is
proportional to its magnetic moment. More precisely,
the term magnetic moment normally refers to a
system's magnetic dipole moment, which produce the
first term in the multiple expansion of a general
magnetic field. The dipole component of an object's
magnetic field is symmetric about the direction of its
magnetic dipole moment, and decreases as the
inverse cube of the distance from the object.
Elements of the Earth's Magnetic
field: Earth's magnetic field, also known as the
geomagnetic field, is the magnetic field that extends
from Earth's interior to where it meets the solar wind,
a stream of charged particles emanating from the sun.
Its magnitude at the Earth's surface ranges from 2565 microstela(0.25-.0.65 gauss). It is approximately
the field of a magnetic dipole titled at an angle of 10
degrees with respect to Earth's rotational axis, as if
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there were a bar magnet placed at that angle at the
Center of the Earth, unlike a bar magnet , however.
Earth's magnetic field changes over time because it is
generated by a geodynamo(in the Earth's case, the
molten iron alloy in its outer core)
The North Magnetic Pole wander sufficiently slowly
to keep ordinary compasses useful for navigation. At
random intervals, however, are averaging around
several hundred thousand years, the Earth's field
reverses and the North and South Magnetic poles
switch places. These reversals of the geomagnetic
poles leave a record in rocks that allow
paleomagnetists to calculate past motions of
continents and ocean floors as a result of
The magnetosphere is a region above the ionosphere
and extends several tens of thousands kilometers into
space, protecting the Earth from cosmic rays that
would otherwise strip away the upper atmosphere,
including the ozone layer that protects the Earth from
harmful ultraviolet radiations.
The magnetosphere is the region above the
ionosphere and extends several tens of thousands of
kilometers into space, protecting the Earth from
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cosmic rays that would otherwise strip away the
upper atmosphere, including the ozone layer that
protects the Earth from harmful ultraviolet radiations.
Dipolar Approximations : Near the surface
of the Earth, its magnetic field can be closely
approximated by the field of a magnetic Dipole
positioned at the center of the Earth and tilted at an
angle of about 10 degree with respect to the rotational
axis of the Earth. The dipole is roughly equivalent to
a powerful bar magnet, with its south pole pointing
towards the geomagnetic North Pole. This may seem
surprising, but the North Pole of a magnet is so
defined because, if allowed to rotate freely, it points
roughly northwards(in the geographic sense). Since
the north pole of a magnet attracts the south poles of
other magnet and repels the North Pole, it must be
attracted to the south pole of the Earth's Magnet. The
dipolar field accounts for 80% to 90% of the field in
most locations.
Intensity : The intensity of the field is often
measured in gauss(G),but is generally reported in
nanostela(nT) , with 1 G = 100,000 nT. A nanastela is
also referred to as gamma (ɣ) . The telsa is the SI unit of
the magnetic field, B. The field ranges between (25,000
to 65,000 nT) (0.,25-0.65 G). By comparison, a strong
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refrigerator magnet has a field of about 100 gauss(0.010
As a result of two magnetic fields acting at the same
place, the resultant field has a special feature. At a
particular point, if the compass needle does not point in
any particular direction, then there is no net magnetic
field at that point. Such a point is called Neutral Point or
the Null point.
A Neutral point is a point where the resultant magnetic
field is zero.
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 Stretch sheet of paper over the drawing board. Fix it
with pins
 Place the compass needle over the plain sheet of
paper so as to determine the geographic North and
South. This can be done by rotating the drawing
 Point at which the Red Cross mark and the meddles
of the compass coincide is the geographic North and
South. Then the corresponding North and South poles
with geographic North and South poles
 Then remove the compass and draw lines along these
points, these pass the axis
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 Place the bar magnet along the North of the
geographic North. Keeping it in mind that white dot
signifies the North Pole if the bar magnet
 Now draw the outline of the bar magnet
 Place the compass needle at different points around
the magnet and trace the points
 Place the needle at the subsequent position such that
one end of it coincides with dots already dotted.
Continue the process till the number of dots are
obtained. After drawing several lines of forces, we
get the final plot.
 Then find the neutral points( the points at which there
is no deflection of needle).
So , there are two neutral points. When we remove
the bar magnets, we get the final plot of magnetic
lines of forces.
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Let ,
 B be the horizontal component of the Earth's
Magnetic field in Delhi
 M be the magnitude of the magnetic field
 d be the distance of neutral points from the
centre of the Bar Magnet
 2l be the length of the Bar Magnet
 m is the pole strength of the Bar Magnet
The observation of the sheet is attached on next page
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The horizontal component of earth's magnetic field in
Delhi (B ʜ) = 3.2 x 10 (given)
According to the observation sheet:
 Distance of neutral point from centre of the bar
magnet (2d) = ---------------m
 Length of the Bar Magnet (2l) = -----------------m
We know that,
μ 2Md
4π (d² - l² ) ²
At netural point , B = B ʜ = 0.33 X 10^-4 T
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B ʜ = μ 2Md
4π (d² - l² ) ²
M= Bʜ
4π (d² - l² ) ²
2μ d
μ = 4π x 10^-7
M = m x 2l
m = M /2l
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 The neutral points are at a distance of -------------M
from the centre of the Bar Magnet with its North Pole
pointing to the geographic North.
 The Magnitude of the magnetic field is------------Am ²
 The pole strength of the Bar Magnet is ------------Am
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 All the point must be traced very accurately and
neatly using a compass
 The North Pole of the bar magnet must be placed
along the direction of geographic North
 A sharp pencil must be used to draw all the magnetic
lines of forces
 Distance of neutral points must be measured from the
centre of Bar Magnet
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The following books assisted and inspired me
in my Endeavour of completing this
experiment as my practical work
Lastly, I would like to mention about the
inputs drawn by me from various sources
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