Name –
Naman Rawat
Class – XII th A
Subject – Physic
Teacher’s Signature
ACKNOWLEDGEMENT
I wish to express my deep gratitude and sincere
thanks to the
Principal,
Mrs. Lata
,
Sachdeva Millennium School, Agra
For her encouragement and for all the facilities
that she provided for this project work. I sincerely
appreciate this magnanimity by taking me into her
fold for which I shall remain indebted to her. I
extend my hearty thanks to
Mr. Vishal sir
who guided me to the successful completion of
this project. I take this opportunity to express my
deep sense of gratitude for his invaluable
guidance, content encouragement, constructive
comments, sympathetic attitude and immense
motivation, which has sustained my efforts at all
stages of this project work
AIM
The aim of the project is to study
the
Earth’s magnetic field and find its
value (BH) using a tangent
galvanometer.
Tangent galvanometerTop view
of a
Tangent galvanometer
INTRODUCTION
Earth's magnetic field, also known as the geomagnetic field,
is the magnetic field that extends from the 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 25 to 65 microtears (0.25 to0.65 gauss).Roughly
speaking it is the field of a magnetic dipole currently tilted at
an angle of about 10 degrees with respect to Earth's
rotationalaxis, as if there were a bar magnet placed at that
angle at the center of theEarth. Unlike a bar magnet,
however, Earth's magnetic field changes overtime because it
is generated by a geodynamic (in Earth's case, the motionof
molten iron alloys in its outer core).The North and South
magnetic poles wander widely, but sufficientlyslowly for
ordinary compasses to remain useful for navigation.
However,at irregular intervals averaging several hundred
thousand years, the Earth'sfield reverses and the North and
South Magnetic Poles relatively abruptlyswitch places. These
reversals of the geomagnetic poles leave a record inrocks
that are of value to paleomagnetists in calculating
geomagneticfields in the past. Such information in turn is
helpful in studying themotions of continents and ocean floors
in the process of plate tectonics.The magnetosphere is the
region above the ionosphere and extends severaltens of
thousands of kilometers into space, protecting the Earth
from thecharged particles of the solar wind and cosmic rays
that would otherwisestrip away the upper atmosphere,
including the ozone layer that protectsthe Earth from
harmful ultraviolet radiation.Earth's magnetic field serves to
deflect most of the solar wind, whosecharged particles would
otherwise strip away the ozone layer that protectsthe Earth
from harmful ultraviolet radiation. One stripping mechanism
isfor gas to be caught in bubbles of magnetic field, which are
ripped off bysolar winds.The intensity of the field is often
measured in gauss (G), but is generallyreported in nanoteslas
(nT), with 1 G = 100,000 nT. A nanotesla is also
referred to as a gamma (γ).The tesla is the SI unit of the
Magnetic field, B. The field ranges between
approximately 25,000 and 65,000 nT (0.25
0.65G). Near the surface of the Earth, its magnetic field
can be closelyapproximated by the field of a magnetic
dipole positioned at the center ofthe Earth and tilted at
an angle of about 10° with respect to the rotationalaxis
of the Earth. The dipole is roughly equivalent to a
powerful barmagnet, with its South Pole pointing
towards the geomagnetic North Pole.The north pole of
a magnet is so defined because, if allowed to
rotatefreely, it points roughly northward (in the
geographic sense). Since thenorth pole of a magnet
attracts the south poles of other magnets and
repelsthe north poles, it must be attracted to the South
Pole.
TANGENT GALVANOMETER
Principle
The tangent galvanometer works on the principle of
tangent law.
Tangent law of Magnetism
The tangent law of magnetism states that the tangent
of the angle ofa compass needle which is due to the
movement under the influenceof magnetic field is
directly proportional to the ratio of
strengths
oftwo perpendicular magnetic fields.
In simpler words, the tangent of the angle made by the
movingneedle under the magnetic field directlindicates
the strength of the perpendicular magnetic fields.
Definition
• Tangent galvanometer is the device which was used
to measuresmall amounts of electric current.
Construction
• The working of tangent galvanometer is based on the
principle oftangent law of magnetism.
• It consists of a coil of insulated copper wire wound on
a circularnon-magnetic frame.
• It is utmost necessary that the coil wound is done in
helicalarrangement otherwise, the field due to the wire
will affect thecompass needle, thus inducing an error in
the reading.
• This frame is mounted vertically on a horizontal base
for support.
• The coil of insulated copper wire is usually rotated on
a vertical axis passing through its centre.
• A small sized magnetic compass with a powerful
magnetic needle ismade
to pivote at the centre
of this coil, such that it
is free to rotatein a
horizontal plane.
•The circular scale is used to read the movement of this
magneticneedle which is divided into four quadrants,
each ranging from 0° to90°.
•A pointer is attached to this needle at right angles,
usually made upof thin alluminium as alluminium is
lighter in mass.
•The usual way of discarding possibilities of parallax is
also used i.e placing of a plane mirror below the
compass needle.
Working
• The instrument needle starts moving firstly under the
influence ofEarth's magnetic field.
• Movement continues untill the magnetic field of
earth is parallelwith the plane of coil.
• Then, on application of an uknown current, a second
magnetic fieldon the axis of the coil which is
perpendicular to the Earth's magneticfield is created.
• Hence the compass needle responds to the vector
sum of the twofields.
•This deflection angle is equal to the tangent of the
ratio of those twofields.
APPLICATIONS
1. T.G. can be used to measure the magnitude of the
horizontal component of the geomagnetic field
2. The principle can be used to compare the
galvanometer constants.
3. For calibration of secondary instruments.
APPARATUS AND MATERIALS REQUIRED
➢Tangent Galvanometer (TG),
➢Commutator (C),
➢Rheostat (R),
➢Battery (E),
➢Ammeter (A),
➢Key (K), Plug Key
THEORY
Tangent galvanometer is an early measuring
instrument for small electriccurrents. It consists of a
coil of insulated copper wire wound on a circularnonmagnetic frame. Its working is based on the principle of
the tangentlaw of magnetism. When a current is
passed through the circular coil, amagnetic field (B) is
produced at the center of the coil in a
direction perpendicular to the plane of the coil. The wo
rking of tangentgalvanometer is based on the tangent
law. It is stated as when a magnet issuspended freely in
magnetic field F and H, the magnet comes to rest
making an angle θ with
the direction H such that,
Eq 1:
F = H tan θ
When a bar magnet is suspended in two Magnetic
fields B and Bh,it comes to rest making an angle
θ
with the direction of Bh.Let a current I be passed
through the coil of radius R, having turns N.Then
magnetic field produced at the centre of coil is
,
Eq 2
4π
R
Let H is the horizontal component of earth’s
14
magnetic field and the magnetic needle comes to
rest at angle
with thedirection of H, then according Eq. (1),
=
2
IN
4
R
= 10
−
7
2π
IN
Eq 3 :
=
2π
×10
−
7
IN
by substituting the value of current I, from eq. (3),
Eq 4:
=
02
4
RH
radius of coil of galvanometer R, deflection
and N, the value of H can be calculated.
PROCEDURE
Connections are made as shown in the figure given
below, where K is thekey, E the battery, A the
ammeter, R the rheostat, C the commutator, andT.G
the tangent galvanometer. The commutator can
reverse the currentthrough the T.G coil without
changing the current in the rest of the circuit.Taking
the average of the resulting two readings for deflection
averagesout, any small error in positioning the T.
G coil relative to the earth’s
magnetic field H.
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