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BYPRATIKSHA PRIYAM BARUAHA SEMINAR ON DETERMINING THE VALUE OF HORIZONTAL MAGNETIC FIELD OF EARTH BY MAGNETOMETERS

CONTENTSMagnetTypes of magnetProperties of magnetDefinition connected with magnetEarths magnetic fieldEvidencesTheoriesRelated DefinitionsElements of Earths magnetic fieldExperimentApparatus usedDerivation of working formulaProcedureObservationCalculationError CalculationResultPrecautionEffects of magnetic FieldReferences

MAGNETS A piece of iron or other material which has its component atoms known as magnetic domains so ordered that the material exhibits properties of magnetism, such as attracting other iron-containing objects or aligning itself in an external magnetic field.

It is a material with both attractive and directive properties.

At first the Greeks had knowledge about it at 6th century B.C.

CLASSIFICATION OF MAGNETSDiamagnetismParamagnetismFerromagnetismAnti-ferromagnetismFerrimagnetism

TYPES OF MAGNETBar magnet

Magnetic needle

Horse shoe magnet

Ball ended magnet

PROPERTIES OF MAGNETAttractive property

Directive property

Like poles repel and unlike poles attract

Magnetic poles always exist in pairs

IMPORTANT DEFINITIONS CONNECTED WITH MAGNETISMMagnetic field The space around a magnet within which its influence can be experienced is called its magnetic field.

Magnetic poles Region where magnetic attraction is maximum. It is not at its geometrical ends.

Magnetic axis The line passing through the poles of a magnet.

Magnetic equator The line passing through the center of the magnet and right angles to the magnetic axis.

Magnetic length The distance between the two poles of a magnet. magnetic length = 0.84 x geometric length

EARTHS MAGNETIC FIELD

EVIDENCES IN SUPPORT OF EARTHS MAGNETIC FIELDA freely suspended magnetic needle comes to rest roughly in north south direction.

An iron bar buried in the earth becomes weak magnet after some time.

Existence of neutral points near a bar magnet.

DIFFERENT THEORIES ON ORIGIN OF EARTHS MAGNETIC FIELDIn 1600, William Gilbert suggested that earths magnetism is due to presence of magnetic material at its centre which could be a permanent magnet. However, the core of the earth is so hot that permanent magnet cannot exist there.

Prof. Blackett said that earths magnetism was due to rotation of earth. Every substance is made of charged particles such as protons, neutrons. As these particles rotate along with the earth they cause circulating current which in turn magnetises the earth.

Cosmic rays cause the ionisation of gases in the earths atmosphere. As the earth rotates, strong electric currents are set up due to the movement of the charged ions. Hence currents may be the source of magnetism.

There are large deposits of ferromagnetic materials like iron, nickel etc. In the core of the earth. The core is very hot and molten. The circulating ions in the highly conducting liquid region of the earths core forms current loops and hence produces magnetic field. At recent, this hypothesis seems most probable because our moon, which has no molten core has no magnetic field.

MAGNETIC FIELD OF THE EARTH

Fig.: Magnetic Field of Earth

DEFINITIONS CONNECTED WITH EARTHS MAGNETISMGeographic axis The straight line passing through geographic north and south poles of earth.

Magnetic axis The straight line passing through the magnetic north and south poles of the earth. Magnetic axis of earth makes an angle 20 degree with the geographic axis. At present, the magnetic south pole is located at a point in Arctic Ocean in Northern Canada and magnetic north pole is located at the coast of Antarctica in the direction of Australia towards South pole.

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Magnetic equator It is the great circle on the earth perpendicular to the magnetic axis.

Magnetic meridian The vertical plane passing through magnetic axis of a freely suspended small magnet.

Geographic meridian The vertical plane passing through geographic north and south poles.

ELEMENTS OF EARTHS MAGNETIC FIELDMagnetic declination The angle between the geographical meridian and magnetic meridian.

Angle of dip or magnetic inclination The angle made by the earths total magnetic field B with the horizontal direction in the magnetic meridian.

Horizontal component of earths magnetic field It is the component of Earths total magnetic field in the horizontal direction in the magnetic meridian.

Diagram

Fig.: Elements of Earths Magnetic Field

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APPARATUS USEDA deflection magnetometer

Vibration magnetometer

A rectangular brass bar

A stop watch.

Bar magnet

Position of magnetometerTangent A position

Tangent B position

DERIVATION OF WORKING FORMULAFor the deflection magnetometer :

Magnetic field of a bar magnet at an axial point is B=

where, M = magnetic moment of the magnet

d = distance between the centers of the magnet and the needle. l = half the magnetic length.

Tangent law : If a freely suspended small magnet is acted upon by two uniform mutually perpendicular magnetic fields B1 and B2 simultaneously, then the magnet comes to rest in such a position that the tangent of the angle that the magnet makes with B1 is the ratio of B1 to B2.

tan = When a compass needle is placed on the earths magnetic field, it stays along the horizontal component BH of the field. So, tan = so, the formula becomes

For the vibration magnetometer : Torque experienced by a magnet is given by the formula,

If I is the moment of inertia of magnet, then deflecting torque on the magnet is , and angular accelaration, so, angular velocity,

Then, time period

It gives,

For small angular displacement

WORKING FORMULA 1.

2.

PROCEDURE

The mass (m), length ( L ) and breadth ( b ) of the given magnet are determined.

The moment of inertia (I) of the magnet is then calculated.

All the magnets and magnetic substance are removed from the working table.

The magnetometer is placed on the table with its two arms perpendicular to the magnetic meridian, i.e. perpendicular to the magnetic needle (ns). At this time the point usually reads (OO-OO) of the circular scale.

The magnet (NS) now place on the arm of the magnetometer at the each side of the needle, so that the length of the magnet is parallel to the arm.

The position of the magnet on the arm is adjusted until the position reads about 450, 430 and 470on the circular scale. The reading d1 and d2 corresponding to the two ends of the magnet, are noted from the meter scale fixed on the arm.

With alternate N-pole S-pole and alternate surfaces of the magnet we get altogether 8 readings.

M/BH is calculated.

The magnet is then suspended horizontally in the vibration Magnetometer box. The magnet is deflected by a very small angle by bringing momentarily an auxiliary magnet near to it.

The time taken by the magnet for its 20 oscillations is repeated thrice. Then it is divided by 20 to get the time period T.

The value of I, MBH and T are put into the working formula to get BH.

OBSERVATION

Mass of the magnet = 93.04 g

Length of the magnet, l = 15.2 cm.

Half of the magnetic length, l = 15.2 x 8.5 cm 2 = 7.6 cmBreadth, b = 1.15cm

A) Table for determining the value of

B) Table for determining T and MBH

CALCULATION

Combining the equations, we get

So, the determined value of earths horizontal magnetic field is

ERROR CALCULATION

ResultThe value of BH , the horizontal component of earths magnetic field obtained from the experiment is = (3910-60.094)Tesla.

PRECAUTIONSAll magnetic substances must be removed from the working table.

Arms of the magnetometer must be adjusted properly to bring it perpendicular to the magnetic meridian.

During the oscillation of the magnet the amplitude should be made small

Oscillation of the suspension fibre should be avoided.

The magnetic field helps us find our way around. Using a compass, we can tell which way is north even when there are no familiar landmarks in sight.The magnetic field shields us from much harmful radiation. The sun sends out cosmic rays of high-energy particles known as the solar wind. The charged particles in cosmic rays are deflected by the magnetic field and many are prevented from hitting the atmosphere directly. Some are trapped into two concentric doughnut-like bands around the Earth called the Van Allen Belts

EFFECTS OF MAGNETIC FIELD

AURORASolar wind causes ionisation of atmosphere near the poles which causes displays of colours known as aurora. Also known as Aurora borealis or northern lights in northern lights and aurora australis or southern lights in southern hemisphere.

REFERENCESSimplified Physics; S.L.AroraConcept of Physics; H.C. VermaB.Sc. Practical Physics; C.L. AroraWikipedia

S.NoMean of the scale reading for the two ends of the magnetPosition of the flat surfaceDeflection of the magnet in degree when the magnetMean deflection in degreeMean of the degreeValue of

Mean

On east arm of magnetometerOn west arm of magnetometer

N-pole pointing the needleS-pole pointing the needleN-pole pointing the needleS-pole pointing the needle

End IEnd IIEnd IEnd IIEnd IEnd IIEnd IEnd II

128.15up454545454546454745.2545.259.59X104

9.49X104

down454545454247454745.13

228.14up43414646434245464443.49.06X104

down404145454341454342.8

327.90up474447494747464546.7546.69.84X104

down464548474647474646.5

No. of observationTime of 20 oscillationMean time (t) secPeriod (T)secMoment of inertia (kg m2)MBH(m2tesla)

11401366.831.8X10-41.52X10-4

2134

3136