Nimeona ni vizuri tujikumbushe kidogo kuhusu Mtanzania huyu dadis aliyefungua fikra za wanataaluma wa sayansi. Ni ndefu lakini nafikiri ni muhimu.
Je watu kama Tanzania tuwaenzije?
Kwa waliosoma somo la fizikia, jina la Newton haliwezi kuwa geni hata kidogo kwao, huyu ndiye mvumbuzi wa kanuni ya joto inayojulikana kama ‘Newton’s law of cooling’ (kanuni ya upoaji).
Kanuni hiyo inaeleza kuwa, kiasi cha joto cha kitu fulani kinachopotezwa katika muda fulani, kinategemea na utofauti wa joto ridi kati ya hicho kitu na mazingira yaliyokizunguka kitu hicho kitu, endapo tu kama utofauti wa joto ridi lao ni dogo.
Kwa kanuni hii ya Newton, watu wote waliosoma fizikia duniani kote, walilazimishwa kuamini kuwa kitu chenye joto dogo kitapoteza joto lake kwa haraka zaidi kuliko kile chenye joto la juu.
Mwaka 1963, Mtanzania aitwaye Erasto Mpemba alifanikiwa kufanya ugunduzi ambao moja kwa moja uliweza kupingana na ile kanuni ya Newton.
Mpemba alibaini kuwa, maji yaliyochemka yakiwekwa kwenye jokofu pamoja na maji yenye ubaridi wa kawaida, yale yaliyochemka yanaganda haraka zaidi na kuwa barafu kuliko yale yenye ubaridi wa kawaida.
Tangu Mtanzania huyu kugundua kanuni hiyo iliyopewa jina la Mpemba effect, mpaka leo ni miaka 50 lakini hatambuliki kabisa na taasisi za kisayansi wala zile za kitaaluma kama vyuo hapa nchini. Jambo la kushangaza, Mpemba ni maarufu sana kwenye taasisi za kimataifa, wanayansi wamehangaisha vichwa kujua sababu ya maji ya moto kuganda haraka zaidi kuwa barafu kuliko yale ya baridi.
Mwaka 2013, The Royal Society of Chemistry, baada ya kuwatangazia wanafunzi wote wa sayansi duniani wanasoma shahada ya uzamili kuwa wafanye tafiti za kugundua sababu za suala hilo.
Ugunduzi wa Mpemba
Kama ambavyo wagunduzi wengi waligundua vitu mbalimbali wakiwa kwenye shughuli zao za kimaisha za kila siku, ndivyo naye Mpemba alivyogundua suala hilo akiwa kwenye shughuli zake za maisha za kila siku.
Akiwa ni Mwanafunzi wa kidato cha tatu katika Shule ya Sekondari ya Magamba iliyopo Lushoto, Mpemba ndipo aligundua suala hilo wakati akitengeneza barafu, shughuli iliyokuwa ikifanywa na wanafunzi wa shule hiyo.
Kwenye andiko la kitaaluma, lililoandikwa na Erasto Mpemba pamoja na Dk Denis Osborne mwaka 1969 na kupewa jina la Cool? Mtanzania huyo ameeleza namna alivyoweza kugundua suala hilo.
Kwenye andiko hilo ambalo Mpemba alianza kwa kujitambulisha, alisema ugunduzi wake umetokana na matumizi mabovu ya jokofu.
Bila shaka, kauli hiyo ya Mpemba imetokana na ukweli kuwa, kitaalamu inashauriwa kutoweka kitu cha moto kwenye jokofu, haswa yale yanayotumika majumbani.
Akiwa na umri unaokadiriwa kuwa miaka 13, Mpemba alikuwa mwanafunzi wa kidato cha tatu katika shule hiyo ambayo sasa inajulikana kama Sebastian Kolowa Memorial University (Sekomu). Wanafunzi walikuwa na utaratibu wa kununua maziwa kutoka kwa wananchi na kuyatumia kutengeza barafu.
Akizungumza na gazeti hili nyumbani kwake jijini Dar es Salaam hivi karibuni, Mpemba anaanza kwa kueleza historia ya Shule ya Magamba anayosema kuwa, mwanzoni ilikuwa hoteli, ambayo wakoloni walikuwa wanaitumia sana wakati wa mapumziko.
Anasema kuwa, baadaye ilinunuliwa na kanisa na ikawa shule ambapo Mpemba ni kati ya wanafunzi 30 wa mwanzo walioanzisha shule hiyo mwaka 1961.
“Tulisoma vizuri sana, shule ile ilikuwa na kila kitu, majokofu yalikuwa mengi, mifumo wa joto ya kukabiliana na baridi ilikuwa kila mahali, kwa kweli tulisoma shule nzuri na walimu wetu karibu wote walikuwa wazungu isipokuwa mmoja tu wa Kiswahili ndiye alikuwa Mtanzania,” anasema Mpemba.
Anasema kwamba, kutokana na kuwa na majokofu hayo, wanafunzi walikuwa wananunua maziwa kutoka kwa wananchi na kutengeneza barafu.
“Mimi nilikuwa mtunza stoo, wenzangu walikuwa wanakuja wananiomba sukari nawapa tunatengeneza barafu, tukitoka darasani jioni tunaenda kwenye michezo na baada ya michezo tunakuja kula zile barafu,” anasema.
Anasema kuwa, kwenye somo la baiolojia walikuwa wamefundishwa kuwa maziwa yanatakiwa yachemshwe kwanza kabla ya kuyanywa, hivyo hatua ya kwanza baada ya kununua kutoka kwa wanakijiji ilikuwa ni kuyachemsha kisha kuyachanganya na sukari kabla ya kuyaweka kwenye jokofu.
“Nikiwa kidato cha tatu, siku moja tulinunua maziwa kama kawaida yetu, mimi nikaja nikawapa wapishi wanisaidie kuyachemsha, wakati wakiendelea na zoezi hilo mwenzangu mmoja akaja akaweka maziwa yake bila hata kuyachemsha.
“Nikamuuliza vipi bwana mbona unaweka maziwa kwenye vyombo vya kugandishia bila hata kuyachemsha? Akaniambia kwani ndama huwa anachemsha maziwa anaponyonya?” anasema Mpemba.
Anaeleza kuwa, kijana huyo aliyeweka maziwa bila kuchemsha alifanya hivyo kwa kuwa chombo cha kugandishia barafu kilikuwa kimebaki kimoja tu na hivyo akataka akiwahi kabla ya Mpemba.
“Kwa bahati nzuri kulikuwa na mwanafunzi mwingine ambaye alikuwa ni mgonjwa, yeye alishaweka maziwa yake mapema na yalikuwa yameshageuka kuwa barafu, hivyo akampa Mpemba chombo alichokuwa amegandishia barafu zake.
Anasema kuwa alichukua kile chombo akaweka maziwa yake na kisha kuweka trei hilo kwenye jokofu kisha akaenda darasani.
Mpemba anaeleza kuwa walipotoka darasani kuja kuangalia barafu zao, wakakuta yule aliyekuta ameweka maziwa ya baridi chombo cheke kimetengeza barafu sehemu ya juu tu lakini ndani kuna maji lakini zile za Mpemba ambazo aliweka maji yake yakiwa ya moto barafu zake zimekuwa ngumu kabisa.
“Yule mwenzangu hakuamini kabisa, akaniambia kuwa nimebadilishia chombo chake na kuwa zile ambazo zilikuwa zimetengeneza barafu ndizo zake, yule aliyenipa trei akajaribu kueleza kuwa zile zilizoganda ni zangu lakini wapi.
Ugomvi ulikuwa mkubwa sana mpaka yule mwanafunzi akaenda kunisemea kwa mwalimu wetu wa fizikia, yule yule mwalimu ndiye aliyempa yule bwana kichwa, kwa kumwambia haiwezekani maziwa ya moto yagande mapema kuliko ya baridi, sasa wewe utasema nini bwana wakati mzungu ameshasema,”anaeleza Mpemba.
Anasema kwamba, kwa kuwa sukari haikuwa shida, kesho yake alinunua chupa nne za maziwa ambazo moja ilikuwa ikiuzwa senti 10, akayagawa sehemu mbili chombo kimoja akaweka maziwa ya baridi na kingine akaweka maziwa ya moto na vyote akaviweka kwenye jokofu.
“Nilitaka kuonyesha watu kuwa sikubadilisha matrei lakini pia mwenyewe nilikuwa najiuliza ni kwa namna gani ile hali ilitokea hivyo nilitaka kujihakikishia mwenyewe,” anasema.
Anasema kuwa, majibu ya jaribio lake yalikuwa kama yale ya awali kwani baadaye ilikuja kuonekana kuwa, yale maziwa yaliyokuwa ya baridi hayajaganda lakini yale ya moto yametengeza barafu ngumu kabisa.
Hata baada ya kufanya majaribio hayo, anasema bado baadhi hawakumwamini, hivyo Mpemba akaendelea na maisha yake kama kawaida.
Kwenye andiko lake Mpemba anasema kuwa, wakati wa likizo alikutana na rafiki yake ambaye alikuwa anafanya kazi ya upishi Tanga mijini, ambaye pia alikuwa akitengeneza barafu na kuziuza.
Aliniambia kuwa kwa siku hutengeneza faida ya Sh20, kwa kuuza barafu tu, nilimuuliza ni vipi anatengeneza barafu zake.
“Jibu alilonipa ni kuwa, ‘Nachemsha maziwa na kuyaweka kwenye jokofu yakiwa ya moto, na barafu zinakuwa tayari ndani ya muda mfupi,” Mpemba alimnukuu rafiki yake huyo.
“Nilimuuliza ni nani alikwambia kuwa maziwa ya moto yanaganda haraka zaidi, na yeye aliniambia kuwa kaka yake aliyekuwa anatengeneza barafu kwa zaidi ya miaka mitano ndiye alimwambia kuwa maziwa ya moto yanaganda mapema zaidi kuliko ya baridi,” anasema.
Anasema kuwa, alilinganisha kilicho mtokea yeye akiwa anatengeneza barafu shuleni, na majibu aliyopata kwa rafiki yake, hata hivyo hakuendelea kufanya majaribio ya suala hilo.
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The Mpemba effect is the observation that, in some specific, fairly common circumstances, hotter water freezes faster than colder water.
The effect is named for the Tanzanian high-school student Erasto B. Mpemba. Mpemba first encountered the phenomenon in the classroom of Eugene Marschall at Mkwawa Secondary (formerly High) School, Iringa, Tanzania, where Mpemba was a student. Eugene Marschall, a member of the Teachers for East Africa/TEA program, taught chemistry and physics at this school from 1965 to 1967. Mpemba first noticed the effect in 1963 after his account of the freezing of hot ice cream mix in cookery classes, and went on to publish experimental results with Dr. Denis G. Osborne in 1969.
At first sight, the behaviour seems contrary to thermodynamics. However, most thermodynamicists believe that each observation of the Mpemba effect can be explained with standard physical theory. Many effects can contribute to the observation, depending on the experimental set-up:
Different definition of freezing (Is it the physical definition of the point at which water forms a visible surface layer of ice, or the point at which the entire volume of water becomes a solid block of ice?)
Evaporation, reducing the volume to be frozen. Evaporation is endothermic.
Convection, accelerating heat transfers. Reduction of water density below 4°C tends to suppress the convection currents cooling the lower part of the liquid mass; the lower density of hot water would reduce this effect, perhaps sustaining the more rapid initial cooling.
The insulating effects of frost
The effect of boiling on dissolved gases
Supercooling. It is hypothesized that cold water, when placed in a freezing environment, supercools more than hot water in the same environment, thus solidifying slower than hot water. However, supercooling tends not to be significant where there are particles, which act as nuclei for ice crystals, thus precipitating rapid freezing.
The effect of solutes such as calcium and magnesium carbonate.
According to an article by Monwhea Jeng, there is no unique explanation yet for why, in some specific circumstances, hotter water freezes faster than colder water. The article makes no attempt to focus on the relevant transport phenomena concepts such as temperature and fluid flow fields. Indeed when temperature fields are introduced, the author states:
Analysis of the situation is now quite complex, since we are no longer considering a single parameter, but a scalar function, and computational fluid dynamics (CFD) is notoriously difficult. The greatest difficulties with CFD are with turbulent flow; with laminar (streamline) flow, as we have in this case, there is much better consistency between independent analyses, and a CFD analysis could be expected to be illuminating.
This effect is a heat transfer problem, therefore well suited to be studied from a transport phenomena viewpoint, based on continuum mechanics. When heat transfer is analyzed in terms of partial differential equations, whose solutions depend on a number of conditions, it becomes clear that measuring only a few lumped parameters, such as the water average temperature is generally insufficient to define the system behaviour, since conditions such as geometry, fluid properties and temperature and flow fields play an important role. The counterintuitiveness of the effect, if analyzed only in terms of simplified thermodynamics illustrates the need to include all the relevant variables and use the best available theoretical tools when approaching a physical problem.
Similar behavior may have been observed by ancient scientists such as Aristotle, and Early Modern scientists such as Francis Bacon and René Descartes. Aristotle's explanation involved a physical property he called antiperistasis, defined as "the supposed increase in the intensity of a quality as a result of being surrounded by its contrary quality". He used the concept of antiperistasis to provide evidence for his conjecture that human bodies and bodies of water were hotter in the winter than in the summer, a theory that was later disproved by Medieval and Renaissance observations.
