Koj nyob ib puag ncig los ntawm cov hlau nplaum niaj hnub no hauv koj cov neeg hais lus, nyuaj disks, lossis txawm nyob hauv cov cuab yeej tseem ceeb xws li mri tshuab hauv tsev kho mob. Nws tseem ua lub luag haujlwm zoo hauv kev thauj mus los; siv rau hluav taws xob motors thiab lub tsheb ciav hlau Maglev. Nws attracts lossis cov roj ntsha yog qee cov ntaub ntawv nyob ntawm cov hom kev sib nqus uas yog tsim los ntawm cov hlau nplaum. Tab sis koj puas tau mus dhau thiab tau hais kom seb cov hlau nplaum tuaj yeem txo lawv lub zog lossis ua "siv" hauv qee txoj kev? Kev paub txog dab tsi cuam tshuam rau lub zog ntawm cov hlau nplaum yuav muab koj daim duab ntshiab dua ntawm cov hlau nplaum ua haujlwm raws li nws lub neej nruab nrab
Cov hlau nplaum yog dab tsi?
Cov hlau nplaum yog cov khoom uas tsim cov chaw sib nqus, uas koj tuaj yeem siv los ua kom nyiam lossis kho cov khoom, zoo li hlau lossis hlau. Ntawm lub me me qib, cov hlau nplaum yog ua los ntawm cov sib nqus uas cov cheeb tsam sib nqus-me me uas cov moms sib nqus ntawm atoms yog dlhos. Qhov kev sib txuas no yog dab tsi muab cov hlau nplaum lawv lub zog.
Muaj ntau hom hlau nplaum koj yuav ntsib:
Cov Hlau nplaum:Cov no ua rau lawv cov hlauetism dhau sijhawm. Piv txwv li cov hlau hlau neodymium, cov hlau nplaum ferrite, thiab bar hlau nplaum.
Hlau nplaum:Cov no tsuas yog ua raws li cov hlau nplaum thaum muab tso rau hauv qhov muaj zog sib nqus.
Electromagnets:Cov no cia siab rau lub hluav taws xob tam sim no los tsim lawv cov chaw sib nqus.
Magnetism thiab sib nqus
Magnetism los ntawm kev coj cwj pwm ntawm cov tshuab hluav taws xob ntawm qib atomic. Hauv cov hlau nplaum, cov hluav taws xob tsis muaj hluav taws xob tsim cov khoom siv me me. Thaum cov liaj teb no ua kom dlhos, cov khoom siv ua cov hlau nplaum. Ib daim teb sib nqus yog lub zog uas tsis pom koj tuaj yeem siv los ua kom nyiam lossis rov ua cov khoom sib nqus lossis xim hlau, nyob ntawm seb lawv cuam tshuam li cas.
Ua cov hlau nplaum hnav tawm lawm?
Teb luv luv yog tsis muaj; Hlau nplaum tsis hnav ib txwm muaj. Tsis zoo li cov ntaub ntawv feem ntau, cov hlau nplaum ua kom lawv lub zog tsis ua ntej. Txawm li cas los xij, qee yam tseem ceeb tuaj yeem cuam tshuam lawv lub durability thiab ua rau lawv poob lawv cov khoom sib nqus dhau lub sijhawm.
1. Kub
Qhov kub thiab txias lub luag haujlwm tseem ceeb heev hauv cov hlau nplaum ntev. Yog tias cov hlau nplaum tau kub heev, cov atoms sab hauv hauv pib vibrating ntau dua zog, uas tuaj yeem cuam tshuam cov txheej txheem ntawm cov hlau nplaum. Qhov no hu ua thermal demagnetization, thiab nws tuaj yeem ua kom tsis muaj zog lossis ua kom zoo dua koj cov hlau nplaum.
Ntawm Flip sab, ua kom cov hlau nplaum sib luag pab txhawm rau txhawm rau txuag lawv lub zog los ntawm kev txwv atomic txav thiab ua kom cov sib nqus ua ke.
2. Lub cev puas tsuaj
Ceevfaj tsis txhob poob lossis ntaus koj cov hlau nplaum! Kev cuam tshuam ntawm lub cev tuaj yeem tsim cov pob txha me me tuaj yeem tsim cov pob txha me, messing li cov kev sib ncag ntawm cov hlau nplaum thiab tsis muaj zog ntawm cov hlau nplaum lub zog dhau sijhawm.
3. Raug rau lwm daim teb sib nqus
Yog tias koj tso cov hlau nplaum nyob ze ntawm cov hlau nplaum muaj zog sab nraud sib xyaw, nws tuaj yeem poob qee qhov ntawm nws lub zog. Qhov no yog hu ua Demagnetization los ntawm cov teb sab nraud. Piv txwv li, muab cov hlau nplaum uas nyob ze ntawm lub tshuab MRI lossis cov hlau nplaum muaj zog tuaj yeem cuam tshuam nws cov kev sib nciab thiab tsis muaj zog.
4. Piv txwv ntawm cov hlau nplaum ruaj khov
Tsis yog txhua cov hlau nplaum hnav ib txoj kev. Ib txhia yog ua rau kav ntev rau xyoo lawm:
Neodymium hlau nplaum:Cov no yog muaj zog kawg li thiab ntev ntev, ua rau lawv cov kev xaiv zoo tshaj plaws rau cov hlau nplaum tas mus li.
Hlau nplaum cov hlau nplaum:Resistant rau corrosion thiab zoo meej rau ib puag ncig tawv tawv, cov hlau nplaum no yog qhov ruaj khov.
Alnico Hlau:Ua los ntawm txhuas, npib tsib xee, thiab cobalt, cov hlau nplaum no feem ntau siv hauv kev siv cov ntawv thov vim lawv qhov kav ntev.
Los ntawm kev saib xyuas koj cov hlau nplaum thiab tiv thaiv lawv los ntawm cov mob heev, koj tuaj yeem ua kom lawv tuav tau lawv lub zog rau ntau xyoo tom ntej.
Kev cuam tshuam ib puag ncig ntawm lub zog nqus
Lub zog sib nqus tuaj yeem cuam tshuam nrog ib puag ncig, uas tuaj yeem cuam tshuam tau tias koj cov hlau nplaum ua tau zoo npaum li cas thiab ntev npaum li cas lawv ntev. Yam xws li kub, av noo, thiab nthuav tawm cov sib nqus sab nraud tuaj yeem tuaj yeem cuam tshuam txhua yam cuam tshuam rau cov hlau nplaum cov khoom. Piv txwv li, huab cua sov heev tuaj yeem ua rau cov hlau nplaum kom poob lawv lub zog, thaum cov av noo tuaj yeem ua rau muaj koj cov hlau nplaum tsis kam tawm tsam ya raws.
Sab Nraud Daim Teb
Yog tias koj nthuav tawm koj cov hlau nplaum kom muaj zog tawm tsam cov sib nqus ntawm cov duab sib nqus, lawv lub zog tuaj yeem ua rau lub sijhawm tsis muaj zog. Cov teb no cuam tshuam txog kev sib zog ntawm cov sib nqus ntawm cov sib nqus-cov cheeb tsam me me ntawm cov cheeb tsam lub luag haujlwm rau koj cov hlau nplaum tag nrho. Qhov muaj zog thiab ntev dua qhov kev tawm tsam ntau, ntau dua koj cov hlau nplaum yuav qee qhov los yog tag nrho nws cov hlau nplaum.
Kev ua cev tub
Ya dej noo yog lwm qhov kev hem thawj rau koj cov hlau nplaum, tshwj xeeb yog lawv tsis kho lossis coated. Yog tias koj siv cov hlau nplaum neodymium, yuav tsum paub tias lawv muaj peev xwm corrode yooj yim vim lawv cov khoom siv oxidizes thaum raug rau huab cua thiab noo noo. Corrosion kev puas tsuaj lub hlau nplaum tus qauv, txo nws lub zog sib nqus. Cov Khaub Ncaws Tiv Thaiv kom zoo li Nickel lossis Epoxy tuaj yeem tiv thaiv koj cov hlau hlau los ntawm cov teebmeem no thiab pab lawv nyob ruaj khov.
Yuav ua li cas koj tuaj yeem tiv thaiv cov hlau nplaum hnav thiab tswj lub zog
Hlau nplaum tuaj yeem poob lub zog dhau sijhawm, tab sis nrog cov kauj ruam yog, koj tuaj yeem txo qhov cuam tshuam thiab ua kom lawv ua haujlwm tau zoo rau ntev dua.
Tsim nyog cia thiab tuav
Ntawm no yog li cas koj tuaj yeem saib xyuas koj cov hlau nplaum:
1. Khaws lawv rau hauv ib puag ncig qhuav kom tiv thaiv noo noo-cuam tshuam corrosion.
2. Zam kev tshaj tawm lawv mus rau qhov chaw sov dhau los lossis cov sib nqus uas muaj zog uas tuaj yeem ua rau lawv tsis muaj zog.
3. Tswj koj cov hlau nplaum kom zoo kom tsis txhob ua rau lawv cov qauv lossis sib ncag.
Txheej thiab kev tiv thaiv
Zoo li cov hlau nplaum neodymium, ntau lub hlau nplaum tuaj nrog cov kab tiv thaiv xws li Nickel lossis zinc. Cov txheej txheej no pab tiv thaiv corrosion thiab ntxiv cov kav dej ruaj khov. Txheeb xyuas cov txheej tsis tu ncua rau kev puas tsuaj thiab tsis txhob khawb lossis ua rau nws hnav.
Los ntawm kev coj cov kauj ruam yooj yim no, koj tuaj yeem ua kom koj cov hlau nplaum nyob twj ywm hauv kev ua kom sab saum toj thiab tswj lawv lub zog sib nqus kom ntev li ntev tau.
Myths thiab inconceptions txog Magnetism
Muaj ntau cov tswvyim hais txog cov hlau nplaum uas yuav tsis meej pem li cas koj nkag siab lawv cov khoom. Cia peb hais kom meej rau lawv!
1. Cov hlau nplaum nyob tas mus li
Koj yuav xav tias cov hlau nplaum nyob mus ib txhis, tab sis lawv tuaj yeem poob lawv lub zog dhau sijhawm. Thaum tshav kub kub, muaj zog tawm tsam cov sib nqus teb, lossis lub cev puas tsuaj tuaj yeem ua kom tsis muaj zog lossis txawm tias demagnetize lawv.
2. Cov hlau nplaum nyiam tag nrho cov hlau
Nws yooj yim rau assume nyiam txhua cov hlau, tab sis lawv tsuas ua haujlwm nrog cov khoom siv ferromagnetic zoo li hlau, cobalt, npib tsib xee, thiab qee cov alloys. Cov hlau zoo li txhuas, tooj liab, thiab kub? Tsis muaj kev nyiam nyob hauv ib txwm muaj.
3. Magnetism ua haujlwm los ntawm cov khoom siv
Tej zaum koj yuav tau pom tias cov hlau nplaum tuaj yeem ua haujlwm los ntawm ntawv lossis yas. Tab sis lawv lub zog ua kom deb li deb, thiab qee cov ntaub ntawv, xws li cov av hlau tuab, tuaj yeem thaiv lossis kev sib cav sib nqus.
4. Txiav cov hlau nplaum hauv ib nrab tsim ob ncej
Yog tias koj txiav cov hlau nplaum hauv ib nrab, koj yuav tsis tau txais ib daig nrog North Ncej thiab lwm tus nrog South Ncej. Hloov chaw, txhua daim dhau los ua cov hlau nplaum me me nrog ob sab qaum teb thiab qab teb.
5. Cov hlau nplaum tuaj yeem muab tso mus tas li rau ib qho chaw twg
Hlau nplaum tsuas yog lo rau ferrigagnetic cov ntaub ntawv, tsis yog txhua yam koj sim. Thiab txawm tias tom qab ntawd, zoo li cas lawv lo yog nyob ntawm cov khoom thiab cov hlau nplaum lub zog.
6. Txhua tus hlau nplaum raug tsim kom sib npaug
Tsis yog txhua tus hlau nplaum yog tib yam. Neodymium, ceramic, thiab alnico hlau txhua tus muaj cov khoom siv sib txawv, kav ntev, thiab siv. Piv txwv li, neodymium hlau nplaum yog super muaj zog, tab sis lawv xav tau tiv thaiv txheej tiv thaiv kom tsis txhob xeb.
Tam sim no koj npaj tau cais tawm cov mother mysws ntawm kev muaj tiag!
Cov ntawv thov uas cov hlau nplaum ntoo
Hlau nplaum yog qhov tseem ceeb hauv ntau daim ntawv thov kev lag luam thiab txhua hnub, thiab lawv cov kev ua tau zoo cuam tshuam kev ua haujlwm. Nkag siab tias muaj cov hlau hnav cuam tshuam rau koj cov cuab yeej tuaj yeem pab koj ua kom muaj kev cuam tshuam ntev ntev thiab zam kev cuam tshuam.
Kev Siv Khoom Siv
1. Motors thiab Generators
Cov hlau nplaum ua haujlwm li cas rau koj: cov hlau nplaum ruaj khov hauv motors thiab cov tshuab hluav taws xob pab koj hloov kho hluav taws xob hauv lub zog hluav taws xob (thiab lwm yam khoom siv). Lawv tsim cov tshav sib nqus uas xav tau rau cov lus tsa suab lossis hluav taws xob tam sim no.
Vim li cas cov khoom siv hlau nplaum: Yog tias cov hlau nplaum poob lub zog, koj lub tshuab raj ua tsawg dua, ua rau qis zog thiab muaj zog tawm. Cov hlau nplaum tsis muaj zog hauv cov tshuab hluav taws xob tuaj yeem ua rau hluav taws xob tsawg dua, kev siv hluav taws xob ntau dua, lossis txawm tias overheating. Txoj kev hnav no nce ntxiv cov nqi kho thiab cuam tshuam koj txoj haujlwm.
2. Sib Nqus Revelance Duab (MRI)
Cov hlau nplaum ua haujlwm li cas rau koj: hauv MRI MACHINE, muaj zog superconducting cov hlau nplaum uas xav tau cov duab sib nqus ntawm cov qauv sab hauv.
Vim li cas cov hlau nplaum hnav teeb meem: txawm hais tias superconducting hlau nplaum tsis tshua muaj zog, txhua yam poob hauv kev sib txawv ntawm cov duab zoo, ua kom kuaj pom tsis txhim khu kev qha duab. Yog tias koj cia siab rau IRI Technology, kev ntsuas hluav taws xob thiab txij nkawm yog qhov tseem ceeb kom tsis txhob muaj kev cuam tshuam cov neeg mob.
Txhua hnub cuab yeej
Hlau nplaum tseem ua si lub luag haujlwm hauv cov cuab yeej koj siv txhua hnub, pab ua kom yooj yim ua haujlwm thiab txhim kho kev ua haujlwm.
1. Fridge hlau nplaum
Yuav ua li cas lawv ua haujlwm rau koj: fridge hlau nplaum yog qhov yooj yim tab sis key txoj kev los tuav cov ntawv sau thiab nco koj qhov chaw.
Vim li cas cov hlau nplaum hnav teeb meem: dhau sijhawm, cov hlau nplaum tuaj yeem poob lub zog vim muaj zog lossis cov tee ua kom tsis tshua muaj zog. Thaum cov tsaj muaj cov tswg tsawg, nws tseem tuaj yeem ua qhov tsis yooj yim rau koj cov haujlwm ib hnub.
2. Cov khoom hluav taws xob
Yuav ua li cas lawv ua haujlwm rau koj: los ntawm cov neeg hais lus rau lub mloog pob ntseg thiab cov sib zog ua haujlwm, cov hlau nplaum yog qhov tseem ceeb rau cov suab ntau lawm thiab cov ntaub ntawv cia hauv koj cov khoom siv.
Vim li cas cov hlau nplaum hnav teeb meem: tsis muaj zog hlau nplaum hauv lub mloog pob ntseg lossis cov neeg hais lus tuaj yeem cuam tshuam cov suab zoo, cuam tshuam koj txoj kev mloog. Hauv cov tsav nyuaj, hlau nplaum puas yuav ua rau cov ntaub ntawv poob, hem koj cov ntaub ntawv thiab kev ntseeg tau.
3. Sib Nqus Ntsuas
Yuav ua li cas lawv ua haujlwm rau koj: Sib Nqus Xauv Account Koj lub qhov rooj, kev nyab xeeb, lossis chaw los ntawm kev siv cov hlau sib nqus los koom nrog lub tshuab xauv.
Vim li cas cov hlau nplaum hnav teeb meem: tsis muaj zog ntawm kev sib nqus muaj peev xwm ua rau lub xauv kom swb, tso koj kev ruaj ntseg ntawm kev pheej hmoo. Nyob rau hauv cov chaw ruaj ntseg, qhov no tuaj yeem ua rau tsis muaj cai nkag lossis siv tus nqi.
Vim li cas koj yuav tsum tau saib xyuas txog magnet kev tsis zoo
Efficiency thiab ntseeg tau:Woob-tawm cov hlau nplaum tuaj yeem cuam tshuam koj cov haujlwm ua haujlwm, txo txoj kev siv zog, thiab nce kev hnav ris tsho lwm yam. Hauv cov motors thiab cov tshuab hluav taws xob, piv txwv, qhov tsis muaj txiaj ntsig tuaj yeem tsav tsheb lub zog hluav taws xob.
Kev txhawj xeeb txog kev nyab xeeb:Cov hlau nplaum tsis haum xeeb yuav ua rau koj qhov kev nyab xeeb, txawm tias nws yog lub xauv sib nqus lossis ib lub tshuab mri uas tsim cov duab qis dua.
Nyiaj txiag:Kev kho pheej ntau zaus, hloov pauv, lossis rov ntsuas dua vim yog cov khoom siv hlau nplaum tuaj yeem ntxiv sai sai. Los ntawm kev tswj koj cov hlau nplaum thiab hloov lawv raws li xav tau, koj txo cov nqi thiab xyuas kom koj cov cuab yeej ua haujlwm thaum koj xav tau.
Nkag siab tias kev cuam tshuam zoo li cas koj cov cuab yeej thiab cov cuab yeej siv yog qhov tseem ceeb rau nyob ua ntej ntawm cov teeb meem uas muaj peev xwm, nyiaj, thiab kev ntxhov siab.
Thaum twg koj yuav tsum tau hloov cov hlau nplaum?
Hlau nplaum ua si lub luag haujlwm tseem ceeb hauv ntau cov ntawv thov, tab sis lawv cov kev ua tau dej tsis tuaj yeem poob sijhawm vim hnav, ib puag ncig cov lus, lossis tuav tsis raug cai. Kev paub thaum twg los hloov cov hlau nplaum yog qhov tseem ceeb rau kev ua kom koj lub tshuab txuag thiab muaj kev nyab xeeb.
Kos koj cov hlau nplaum yuav poob
1. Koj pom tias muaj peev xwm nqa tau muaj peev xwm nqa khoom lossis nws tsis tuaj yeem tuav cov khoom li nws tau siv rau.
2. Lub sib nqus rub tawm zoo li tsis muaj zog lossis tsawg dua.
3. Nws yog qhov nyuaj rau ua kom haum nrog lwm cov hlau nplaum lossis cov ntaub ntawv ferromagnetic.
4. Muaj kev puas tsuaj rau lub cev, zoo li cov kab nrib pleb, tej daim, lossis deformations.
5. Cov hlau nplaum qhia qhia txog xeb lossis xeb, tshwj xeeb yog nws yog neodymium hlau nplaum.
Yam yuav tsum xav txog thaum hloov cov hlau nplaum
Cuam tshuam rau kev ua haujlwm:Cov hlau nplaum tsis muaj zog cuam tshuam li cas koj lub cuab yeej lossis lub cev ua haujlwm?
Kev Nyab Xeeb Kev Nyab Xeeb:Hauv cov ntawv thov tseem ceeb xws li cov xauv sib nqus lossis cov tshuab muaj kev lag luam, tsis muaj hlau nplaum ua rau muaj kev phom sij.
Nqi-txiaj ntsig:Qee zaum hloov cov hlau nplaum yog qhov pheej yig dua thiab yooj yim dua li sim ua kom rov ua dua nws.
Raug rau ib puag ncig ib puag ncig:Cov hlau nplaum puas tau raug cov cua sov, noo noo, lossis tawm tsam cov khoom sib nqus uas ua rau muaj kev puas tsuaj tas li?
Hloov kho tshiab:Qhov no tuaj yeem yog lub sijhawm zoo hloov mus rau Newer, ntau cov hlau nplaum ruaj khov zoo dua rau koj cov kev xav tau.
Los ntawm kev ceev faj rau cov phiajcim no, koj tuaj yeem ua kom koj cov hlau nplaum ib txwm ua tiav ntawm lawv qhov zoo tshaj plaws.
Tag
Hlau nplaum yog incredibly ruaj, tab sis lawv tuaj yeem hnav tawm hauv qee qhov xwm txheej. Yam xws li kub, lub cev kev ntxhov siab, thiab nthuav tawm cov xim sib nqus sab nraud tuaj yeem ua rau lawv xiam lub zog dhau sijhawm. Txawm li cas los xij, yog tias koj ua raws cov kev cia thiab tuav cov kev coj ua, koj tuaj yeem khaws koj cov hlau nplaum tau zoo rau xyoo lawm. Txawm hais tias koj ua haujlwm nrog cov hlau nplaum bar, cov hlau nplaum haib, los yog kev nkag siab txog kev ua haujlwm magnetism no yuav pab koj kom tau txais ntau tshaj plaws ntawm cov cuab yeej fasclinating.
Cov Lus Nug
1. Vim li cas neodymium cov hlau nplaum tau txais tsis muaj zog dhau sijhawm?
Neodymium hlau nplaum tuaj yeem poob lub zog qeeb dhau sijhawm vim muaj kev cuam tshuam rau cov magnet lub crystal qauv. Cov xim sib nqus, lub zog hluav taws xob hloov pauv, lossis raug rau qhov kub siab (zoo li tus taw qhia kev sib txig) tuaj yeem ua rau lawv tsis muaj zog. Txawm li cas los xij, cov txheej txheem no yog qeeb. Yog tias koj khaws cov hlau nplaum hauv cov xwm txheej zoo, nws tuaj yeem siv ntau pua xyoo 700 xyoo rau koj kom pom tias muaj peev xwm ntsuas tau.
2. Puas tuaj yeem neodymium cov hlau nplaum demagnetize yog tias raug cua sov?
Yog lawm, lawv tuaj yeem ua tau. Yog tias koj nthuav tawm cov hlau nplaum neodymium kom kub dhau nws lub peev xwm curie, nws yuav poob nws lub peev xwm los ua ib qho kev sib nqus. Ntawm qhov kub tseem ceeb no, lub zog tsis pub dhau cov hlau nplaum destabilizes, thiab nws cov sib nqus uas nres tsis sib haum. Txhawm rau tswj hwm nws lub zog, koj yuav tsum zam dhau cov hlau nplaum kom muaj cua sov.
3. Dab tsi yuav cuam tshuam tsis zoo lub zog ntawm neodymium hlau nplaum?
Ntau yam muaj peev xwm ua rau koj cov hlau nplaum tsis muaj zog, xws li kis tau cov hlau nplaum sib nqus, zoo li kev puas tsuaj lub cev (xws li ntaus nws nrog rab rauj), lossis kub saum toj ntawm qhov rooj certie. Khaws koj cov hlau nplaum tsis raug lossis ze rau lwm qhov chaw muaj zog cuam tshuam cov khoom sib nqus kuj tseem tuaj yeem ua rau demagnetization. Nco ntsoov tias koj tuav thiab khaws koj cov hlau nplaum kom zoo kom lawv muaj zog.
4. Yuav ua li cas koj tuaj yeem Remagnetize ib tug neodymium hlau nplaum yog tias nws ua tsis muaj zog?
Yog tias koj cov hlau nplaum tsis muaj zog, koj tuaj yeem nco qab nws los ntawm kev ua rau nws muaj zog sab nraud sib nqus. Tso nws cov hlau nyob hauv ib daim teb muaj zog tuaj yeem ua lub sib nqus ntawm cov hlau nplaum, pab txhawb nws lub zog. Txawm li cas los xij, yog tias nws cov qauv siv lead ua puas tsuaj, tej zaum koj yuav tsis muaj peev xwm rov qab ua tiav.
5. Puas yog neodymium hlau nplaum poob lub zog yog tias tsis siv?
Tsis loj. Yog tias koj tawm ntawm cov hlau nplaum tsis siv nyob rau hauv ib txwm muaj mob, nws yuav khaws feem ntau ntawm nws lub zog vim tias nws cov sib nqus ua ke nyob ruaj khov. Txawm li cas los xij, qhov tseeb zoo li cov kev hloov pauv lossis cov sib nqus uas quav muag tuaj yeem ua rau nws tsis muaj sijhawm. Tsuav koj khaws nws kom yog, koj cov hlau nplaum yuav nyob ruaj khov rau ntev dua nws cov kev siv ncua.