To Study The Earth

Transcript

 To  To study the earth’s magnetic feld   Contents • • • • • • • • Aim Requirements  The Tangent Tangent galvanometer galvanometer  Theory and Working Working  Procedure Observations and calculations Result Bibliograhy Aim  To  To study the earth’s magnetic feld using a tangent galvanometer Requirements • • • • • • • • • • !ntroduction A tangent galvanometer A commutator An ammeter A battery A lug key A rheostat A sirit level Connecting "ires A iece o# sand aer !ntroduction Although historically ancient travelers made abundant use o# the earth’s magnetic feld #or the e$loration o# the earth% they "ere ignorant o# its origin& !n many resects the earth’s magnetic feld e$hibits characteristics similar to those o# a bar magnet' nonetheless% the mechanisms resonsible #or generating each are a re vastly di(erent&& )agnetic feld lines aear to originate near the south geograhic ole% i&e& magnetic north ole% and terminate near the north geograhic ole% i&e& magnetic *outh Pole& Earth's magnetic also kno"n feld% as the geo magnetic feld% is the mag netic feld tha t e$tends #rom the +arth,s interior to "here it meets the solar "ind% a stream o# charg ed articles emanating #rom the *un& !ts magnitude at the +arth,s sur#ace ranges #rom -. to /. microteslas 01&-. to 1&/. gauss2&  Roughly seaking it is the feld o# a magnetic diole currently currently tilted at an angle o# about 31 3 1 degrees "ith resect to +arth,s rotational a$is% as i# there "ere a bar magnet laced at that angle at the center o# the +arth& 4nlike 4nlik e a bar magnet% ho"ever% +arth,s magnetic feld changes over time because it is generated by a geodynamo 0in +arth,s case% the motion o# molten iron alloys in its outer core2&  The 5orth and *outh magnetic oles oles "ander "idely% but su6ciently slo"ly #or ordinary comasses to remain use#ul #or navigation& 7o"ever% at irregular intervals averaging several hundred thousand years% the +arth,s feld reverses and the 5orth and *outh )agnetic Poles relatively abrutly s"itch laces&  These reversals o# the geomagnetic oles leave a record in rocks that are o# value toaleomagnetists in calculating geomagnetic felds in the ast& *uch in#ormation in turn is hel#ul in studying the motions o# continents and ocean 8oors in the rocess o# late tectonics& A Tangent galvanometer A tangent galvanometer is an early measuring instrument used #or the measurement o# electric current& current& !t "orks by using a comass needle to comare a magnetic feld generated by the unkno"n current to the magnetic feld o# the +arth& !t gets its name #rom its oerating rincile% the tangent la" o# magnetism% "hich states that the tangent o# the angle a comass needle makes is roortional to the ratio o# the strengths o# the t"o erendicular magnetic felds& !t "as frst described by Claude Pouillet in 39:;&  The tangent galvanometer A tangent galvanometer consists o# a coil o# insulated coer "ire "ound on a circular non to ?1>& A long thin aluminum ointer is attached to the needle at its centre and at right angle to it& To avoid errors due to aralla$% a lane mirror is mounted belo" the comass needle& !n oeration% the instrument is frst rotated until the magnetic feld o# the +arth% indicated by the comass needle% is arallel "ith the lane o# the coil& Then the unkno"n current is alied to the coil& This creates a second magnetic feld on the a$is o# the coil% erendicular to the +arth,s magnetic feld&  The comass needle resonds to the vector sum o# the t"o felds% and de8ects to an angle equal to the tangent o# the ratio o# the t"o felds& @rom the angle read #rom the comass,s scale% the current could be #ound #rom a table& -  The current suly "ires "ires have to be "ound in a small heli$% like a ig,s tail% other"ise the feld due to the "ire "ill a(ect the comass needle and an incorrect reading reading "ill be obtained&  Theory and Working Working As deicted in @igure @igure 0A2 the earth’s magnetic feld B e can be decomosed into a comonent B h "hich is arallel to the lane o# the hori=on and a comonent B v "hich is erendicular to the lane o# the hori=on& Thus% B e and Bh are related by Be = Bh /cos (θi) eq-1 @ig 0A2  Where i is the angle o# inclination& !# a comass needle is subDected to a kno"n e$ternal e$ternal magnetic feld B $ "hich acts erendicularly to B h% the comass needle "ill de8ect through an angle  $ a"ay #rom magnetic south 0*ee @igure 0B22  Consequently% B h is related to B $ by Bh = Bx/tan(θx) eq-2  Thus% +q’s - and 3 relate the earth’s magnetic feld% "hich is unkno"n% "ith the magnetic feld B$% "hich is kno"n k no"n in rincile& The tangent galvanometer is the rimary iece o# equiment used in er#orming the e$eriment& & The magnitude o# the magnetic feld B$% in units o# o# microtesla% at the center o# the coil is Bx = N µ!/ 2a " 1# = N ($% " 1&1 )!/    "here µ E FG H 31 I; % N is the number o# turns "hich the coil comrises% 2a is the diameter o# the coil measured in meters% and current though the coil measured mea sured in ams&  ! is the current !#  is the de#l de#le ection tion o# the needl eedle e% then then acc accordi ordin ng to tangent la"% Jet I is the current assing through the coil o#   radius a "ith n turns% then the feld generated by the current carrying circular coil is% +quating 032 and 0-2% "e get%  The le#t hand side o# equation 0F2 is a constant and is called the reduction #actor 0K2 o# the given tangent galvanometer& 5o" #rom equation 0:2 L 0.2% the hori=ontal intensity o# earth’s magnetic feld Bh is% Procedure alc*lating the hori+ontal com,onent 3& Connect the galvanometer 05 E .2% ammeter and o"er suly in series& -& Align the galvanometer such that it creates a magnetic feld feld er eren endi dicu cula larr to that that o# +art +arth’ h’s s feld feld 0the 0the com comass ass needle should be arallel to the "ire loo2& Mo not move the galvanometer "hile taking data& :& Turn on the o"er suly to 8o" current through the galvanometer F& Record the inclinations corresonding to each value o# i 0current2 by varying the current in the table& .& Record the inclinations #or each value o# current in both direct and reverse current by changing the commutator& /& Record Record the values #or about ; di(erent values o# i& Observations calculationsN and 5umber o# turns in the coil E . Circum#erence Circum#erence o# the coil% 2 Radius o# the coil% a E -. π  πa  E .1 cm E .1$ 31 <- m  E ;&?/ cm E ;&?/ $ 31 <- m 1 .o determine the hori+ontal com,onent o earth0s magnetic feld (Bh)  The 7ori=ontal comonent o# earth,s magnetic feld 0B h2 can be calculated using the #ormula% 2 .o .o determine det ermine the t he red*ction red*c tion actor  actor o . . Mir Mirect ect curr urrent ent Revers verse e current 3n *rrent θ 1 θ 2 θ 1 θ 2 4ean ( θ o ) i/tan θ (5) 3 1&/ :. :. :1 :1 :-&. 1&?F3 - 1&9 F1 F1 :; :; :9&. 3&11/ : 1&? F. F. F1 F1 F-&. 1&?9- F 3 .1 .1 FF FF F; 1&?:- . 3&3 .1 .1 F- F- F/ 3&1/- / 3&- .. .. F/ F/ .1&. 1&?9? ; 3&F /1 /1 .1 .1 .. 1&?91 Mean (k) = 0.986 Note .a6e de7ection 8et9een : and # degrees 5o" Bh can be calculated by A#ter A#ter Calculation "e obtain B h E :&9?3F; $ 31<. T : ;rom gra,h Reduction #actor K o# the tang tangen entt galv galvan anom omet eter er be determined& @rom the grah dra"n as% can can  Result  The reduction reduction #actor o# the given tangent galvanometer% K E 1&?9/ A 7ori=ontal comonent o# earth’s magnetic feld% Bh = :<1 x 1-> .  To  To study the earth’s magnetic feld   Contents