On how much X-ray and UV radiation processes are coupled in accretion disks AGN case

a r X i v :a s t r o -p h /0507152v 1 6 J u l 2005On how much X-ray and UV radiation processes are coupled in

accretion disks:AGN case

Daniel Proga,1

1Princeton University Observatory,Peyton Hall,Princeton,NJ 08544.e-mail:

dproga@https://www.sodocs.net/doc/796486388.html,

ABSTRACT

Within the standard accretion disk theory for active galactic nuclei (AGN),the observed X-rays are often modeled by Compton up-scattering of ultraviolet (UV)disk photons inside a hot disk corona.Here,we point out that for many AGN,radiation pressure due to the very same UV disk photons can drive a ?ow from the disk into the corona and couple the processes producing X-rays and UV photons.This coupling could lead to quenching of the disk corona because the regions above the UV disk will be too dense,too opaque,and consequently too cold.We discuss various consequences of this new type of the X-ray/UV coupling on the dynamical and radiative properties of AGN.Subject headings:accretion,accretion disks –galaxies:active:nuclei –methods:numerical –radiation mechanisms 1.Introduction High X-ray ?uxes observed in active galactic nuclei (AGN)are a serious challenge for

the standard accretion disk theory.They are also a serious challenge for any AGN out?ow model because no matter what is the physics of X-ray production,one has to deal with the so-called overionization problem:how out?ows absorbing the ultraviolet (UV)radiation avoid full photoionization due to strong X-ray radiation.

In most pictures depicting the centers of AGN,the X-rays are produced in a small region,referred to as the central engine,whereas the out?ows are produced outside of the central engine.Therefore,the X-ray production mechanisms are usually considered to op-erate separately from the out?ow production mechanisms.In this paper,we discuss new dynamical and radiative consequences of UV photons being emitted into the central engine.In particular,we argue that radiation pressure due to UV lines (line force)couples the X-ray and UV radiation processes by driving the disk material into the regime above the disk where X-rays are to be produced.

2.Disk Corona and Disk Out?ow:“a tale of two merging cities”

The spectral energy distribution(SED)of active galactic nuclei(AGN)is very broad. It spans the wavelength range from radio to hard X-rays.Most of the AGN luminosity, L is in the optical–UV–IR regime but some signi?cant fraction is in the X-ray band.It is commonly accepted that AGN are powered by accretion of matter onto a supermassive black hole(BH).AGN are typically very luminous(0.001L Edd<～L<～1L Edd,where L Edd is the Eddington limit).For most AGN,the accretion?ow is thought to form an optically thick,geometrically thin Keplerian disk.In the standard picture,this disk radiates thermally mostly in the optical–UV regime(Shakura&Sunyaev1973).

2.1.Disk Corona

Generally,the optically thick disk model can account for the optical–UV radiation but it does not account for the spectral shape and high?ux observed in X-rays.The most commonly accepted model for the production of X-rays is multiple Compton up-scattering (Comptonization)of UV photons(e.g.,Sunyaev&Titarchuck1980)o?hot electrons in a disk corona(e.g.,Walter&Courvoisier1992;Haardt&Maraschi1991,1993;Sobolewska et al.2004).The derived X-ray spectrum depends on the temperature and optical depth of the scattering electrons.Despite recent advances in observations and modeling of X-rays,the geometry and radiation processes responsible for X-ray emission remain poorly constrained. Two types of geometries are being considered for an accretion disk and Comptonizing corona: (1)“slab”or“sandwich”geometries(e.g.,the top and bottom panels of Fig.1in Reynolds &Nowak2003,hereafter RN)and(2)“sphere+disk geometries”(e.g.,the middle two panels of Fig.1in RN).

Here we focus on exploring the?rst class of geometries where the hot corona is thought to be located immediately above the disk.The disk emits soft thermal photons which provide the main source of cooling for the hot electrons in the corona.It is possible that at the same time,the hard photons produced by Comptonization are an important source of heating for the re?ecting matter which reprocesses them into soft photons.Thus there is a radiative coupling between the X-ray and UV radiation.Many models account for this coupling and assume that a signi?cant fraction of the gravitational power is dissipated in the hot tenuous layers(e.g.,Haardt&Maraschi1993;Svensson&Zdziarski1994).Although the basic idea behind a hot disk corona is straightforward,the physical model of the corona remains one of the biggest challenges in the?eld.Phenomenologically,one can imagine that the corona is heated by dissipation of the accretion power via magnetic processes(e.g.,Galeev et al. 1979;Field&Rogers1993).

Direct studies of these complex multidimensional,time-dependent processes by means of numerical magnetohydrodynamical(MHD)simulations have begun.For example,Miller &Stone(2000)found that in a strati?ed disk,a MHD turbulence driven by the magnetoro-tational instability(MRI,Balbus&Hawley1998)is capable of driving magnetogravitational modes of the Parker instability.However,it has not been demonstrated yet that magnetic buoyancy can supply the disk corona with su?cient power to explain the observed X-ray emission.In fact,several numerical simulations indicate that local dissipation not magnetic buoyancy is the primary saturation mechanism of the MRI(e.g.,Brandenburg et al.1995; Stone et al.1996;Miller&Stone2000,Turner2004).

2.2.Disk Wind

Viable models for AGN must also account for mass out?ows which is another important aspect of activity in galactic nuclei.The most relevant to this paper are the out?ows that can be inferred from spectral features observed in the UV and X-ray bands(i.e.,we do not discuss AGN jets).Broad absorption lines(BALs)in QSOs are the best example of spectral features revealing the existence of such out?ows.These lines are almost always blueshifted relative to the emission-line rest frame,indicating the presence of out?ows from the active nucleus,with velocities as large as0.2c(e.g.,Turnshek1998).The apparent X-ray properties can be a?ected by out?ows,too.For example,the relative strength of the soft X-ray?ux anti-correlates with the C IV absorption equivalent width for QSOs(e.g.,Brandt,Laor& Wills2000).

There has been considerable time and e?ort spent to understand AGN out?ows(e.g., Arav,Shlosman&Weymann1997;Crenshaw,Kraemer&George2002,and references therein).In particular,many theoretical models have been proposed to explain out?ows in AGNs(e.g.,Crenshaw et al.2002).One of most plausible scenarios for AGN out?ows is a wind from an accretion disk around a black hole where the line force drives a wind from a disk by the local disk radiation at radii where the disk radiation is mostly in the UV(e.g., Murray et al.1995,MCGV hereafter;Proga,Stone,&Kallman2000,PSK hereafter;Proga &Kallman2004,PK hereafter).In this scenario,AGN out?ows are a natural consequence of the standard accretion disk theory because the theory predicts high enough radiative?ux and gas opacity in the UV regime,for the line force to drive an out?ow.

Numerical simulations of the radiation driven disk winds illustrate why and how a wind from an accretion disk can account for AGN out?ows.In particular,the simulations have been essential in studying the robustness of the radiation launching and acceleration of the wind(e.g.,PSK,PK).For example,PK considered relatively unfavorable conditions for line

driving(LD)as they took into account the central engine radiation as a source of ionizing photons but neglected its contribution to the radiation force.Additionally,they accounted for the attenuation of the X-ray radiation by computing the X-ray optical depth in the radial direction assuming that only electron scattering contributes to the opacity.The main result of the simulations is that the disk atmosphere can’shield’itself from external X-rays so that the local disk radiation can launch gas o?the disk photosphere.

2.3.The Role of the Failed Wind

LD can change the?ow near the accretion disk in many ways.A powerful wind is just the most dramatic of them for which fairly strict requirements must be https://www.sodocs.net/doc/796486388.html,ing some physical arguments as well as numerical simulations one can show that line-driven disk winds are produced only when the e?ective luminosity of the disk(i.e.the luminosity of the disk, L D times the total line opacity in the optically thin case,M max)exceeds the Eddington limit (e.g.,Proga,Stone&Drew1998,PSD hereafter).For the BH mass M BH=108M⊙of a typical quasar,PK found that for L D>0.3L Edd a strong disk wind develops whereas for L D<～0.3L Edd there is no disk wind.For a less luminous disk or stronger ionizing radiation that reduces M max,or both,the line force can still lift material o?the disk but it fails to accelerate the?ow to escape velocity.Such a failed disk wind has been found in simulations with and without X-ray ionization(see PSD98for no X-ray cases,runs1and6there,and PSK and PK for X-ray cases).

For X-ray cases,a large fraction of the failed wind is not fully ionized and its temperature is comparable the disk e?ective temperature,T D(see Fig.1).LD can then change the vertical structure of an accretion disk by dynamically increasing the disk scale height.Therefore,a failed wind solution can be referred to as a pu?ed-up disk.

The disk wind solution is very sensitive to M BH:for a?xed ratio,L D/L Edd(e.g.,0.5) it is easier to produce a wind for M BH>～107M⊙than for M BH<～107M⊙(PK).Thus less dramatic but very important changes in the density distribution above the disk occur for a broad range of AGN luminosities and BH masses(e.g.,L D M max

To illustrate properties of the pu?ed-up disk/failed wind,we show results from one of PK’s simulations for an AGN with M BH=108M⊙(we refer a reader to PK for a description of the calculations).PK present detail results from only one of the simulations

where M BH=108M⊙and L D=0.5L Edd.Here,we present and discuss results for the same M BH but for L D=0.3L Edd.For these parameters,the disk radiation launches the?ow o?the disk but fails to accelerate it to escape velocity(Fig.1).

The top panel of Fig.1shows very clearly a high density?ow above the disk,i.e.,the line force can maintain gas with the density as high as10?14g cm?3as high as40%of radius along the equator.We note that in the radiation dominated regime,the standard disk theory predicts the disk scale height,H D=3L/L Edd r?=0.9r?(where r?≡6GM BH/c2 is the inner disk radius and unit of the length scale:r′=r/r?and z′=z/r?).The density distribution in the vertical direction is much broader compared not only to the standard disk model but also compared to the X-ray heated disk corona.For example,for the gas temperature T=8×108K,the scale height of a disk corona in hydrostatic equilibrium (HSE),H C/r?=[kT r3/(μm p GM BH)]1/2=0.7at r′=20.The pu?ed-up disk is far from HSE and displays unsteady behavior:the?ow is complex with a few?laments and various knots and clumps of gas moving both upwards and downwards.The direction and speed of motion at any one position is apt to change unpredictably with time.There are two main reasons for this behavior:(i)the gravity and driving?ux di?erently scale with z′(e.g.,PSD) and(ii)the overionization of the innermost?ow by the central engine radiation(compare the left and right panels).

PK’s line-driven wind model takes into account X-ray radiation from a point source located at the origin of the coordinate system.Thus it does not account for the X-ray emission as envisioned in the disk corona scenario described in§2.1.However,we can estimate some of the e?ects of X-rays emitted immediately above the UV disk.For example, we can estimate the photoionization parameter,ξ≡4πF x/n inside the pu?ed-up disk(F x is the X-ray?ux and n is the number density).We assume that at a give point(r′,z′) above the disk,the ionizing?ux is comparable to the?ux emitted by the disk at(r′,0), i.e.,F x(r′,z′)≈σT4D(r′)(where T D is the e?ective disk temperature at r′).For r′=20this yields T D=20,000K and F X=2×1013erg s?1cm?2s?1.For the density of10?13g cm?3, typical for the region above the disk at z′≈4,ξ=5000.This highξimplies that at this location,the gas would be fully ionized.However,for a few of reasons,this does not mean that the energy released above the disk must suppress formation of the failed wind.

Firstly,the failed wind is opaque as illustrated in the middle panel of Fig.1which plots an estimate of the electron scattering optical depth as a function of position over the unit length scale,τ≡ρr?σe.Thus the size of a region fully ionized by locally dissipated energy will be small compared to the size of the failed wind.In other words,the failed wind can likely shield itself from locally produced X-rays as it does from external X-rays. Secondly,as we eluded to above,the presence of dense and cold gas above the disk creates

unfavorable conditions for transport and liberation of the energy above the disk in the?rst place.Contrary to the standard disk corona scenario where the magnetized bubble carrying energy expands almost freely once outside the disk,here the bubble must rise through an extended,high density,dynamic region.Generally,in a case of a disk with a failed disk wind, the strength of a magnetic?eld needed to dominate over the gas and radiation pressure outside the disk must be orders of magnitude higher than in a case of a bare disk.Finally, the critical assumption about the X-ray production is the fact that the hot gas is tenuous so that the main cooling mechanism is inverse Compton emission.However,for the gas density in the failed wind of>10?14g cm?3the bremsstrahlung losses will exceed the Compton losses(e.g.,eq.1in Haardt&Maraschi1993).Therefore,even if the hot electrons were produced above the disk,they will cool very e?ciently by bremsstrahlung instead of inverse Compton emission and the coronal X-ray?ux would be suppressed.

3.Summary and Discussion

In the standard accretion disk theory for AGN,strong X-ray radiation is accounted for by allowing the gravitational energy to be dissipated above the disk.This energy dissipation is to lead to formation of a hot disk corona located immediately above the disk and inside this corona,UV disk photons are to be up-scattered to X-rays.We point out here,that for a broad range of AGN properties,radiation pressure due to the very same UV disk photons can drive a?ow from the disk into the corona.

LD as a mechanism producing disk winds has been studied extensively,but LD as a mechanism changing the density distribution above and inside the disk has not been given its due.Therefore,we comment mostly on some qualitative e?ects that would be very important and relevant to AGN and other accretion disk systems such as X-ray binaries(XBs).In particular,we use some physical arguments,quote and present results from simulations of line-driven disk winds to explore possible coupling between the X-ray and UV production processes.

The base for our discussion are simulations of line-driven?ows from accretion disks studied by PK.The simulations show that the?ows are opaque and can shield themselves not only from external X-rays but also from X-rays produced locally(i.e.,in the region just above the disk where the?ows are driven into).These opaque?ows can lead to a suppression of energy dissipation above the disk.If so,the corona and X-rays would have to be produced interior to the UV emitting part of the disk.This means that the UV and X-ray emitting regions do not overlap and argues against a two-phase accretion disk model(i.e.,against the slab/sandwich type of geometries).If this is true then we have an inverse overionization

problem:UV driven?ow suppresses X-ray production.If however,line-driven?ows were suppressed by locally emitted X-rays,then the AGN wind would have to be launched from the disk exterior to the UV disk where coronal activity is negligible.It is possible that the intermediate situation occurs where X-rays are produced above the disk and the disk material is launched o?the disk at di?erent location/time.Such a double activity above the disk would lead to formation of a two-phase corona where high temperature plasma bubbles coexist with cooler and denser clumps.

The wind quenching of the hot corona raises many questions about the geometry,dy-namics and energetics of AGN.For example,if the X-rays are quenched by the failed wind, what happens to the energy pumped into the gas via magnetic?elds?Could this energy contribute to the UV continuum?Will the resulted UV continuum drive a?ow di?erent than that predicted by current models?

To address these and other issues,one would need to perform global simulations of a turbulent MHD accretion disk with line driven?ows in the radiation-dominated regime.We anticipate that this will be possible soon as local simulations of these type without LD have been already preformed(e.g.,Turner2004;Hirose et al.in preparation).

There are many observational implications of the fact that the radiation driven wind or the failed wind can quench the X-ray production.For example,one would expect an inverse correlation between X-ray?uxes and winds.Perhaps,the observed anti-correlation between the relative strength of the soft X-ray?ux and the C IV absorption equivalent width for QSOs(e.g.,Brandt et al.2000)is due to the wind quenching rather than obscuration.The wind quenching can also play a role in solving the overionization problem in QSOs.The expected inverse correlation between X-ray?uxes and winds is consistent with the fact that BALs are not observed in X-ray sources such as Seyfert galaxies.

The wind quenching can also a?ect the disk re?ected spectrum,big blue bump and iron lines.The observed spectra are complex convolutions of the primary and reprocessed photons because some fraction of the X-rays are always intercepted by the optically thick matter and reprocessed before escaping.Thus,to understand those spectra,it is necessary to compute very carefully the e?ects of reprocessing as many physical processes play a role (e.g.,Ross&Fabian1993;˙Zycki et al.1994;Nayakshin et al.2000).Future calculations of X-ray spectra should include the e?ects of line-driven?ows because the?ows change the key parameters determining the spectra,i.e.,the temperature and optical depth of the scattering electrons.These e?ects have at their core a coupling between X-rays and UV photons and therefore can help to better constrain the AGN models.

We note that the X-ray/UV coupling is unlikely to operate in the solar corona.For this

reasons among others,solar and AGN coronal activities may signi?cantly di?er.However, this coupling or its variant may operate in XBs.For AGN and XBs,the same two types of disk/corona geometries are being considered(e.g.,Zdziarski&Gierli′n ski2004and Fig. 14there).Except for smallest radii,the bound-free and line opacities and radiation?ux are likely high enough for radiation pressure to lift dense material o?the disk in some spectral/luminosity states of XBs.Therefore,one can expect the sphere-disk geometry in these systems because of quenching of the disk corona.In fact,this geometry is inferred from analysis of the photon index of Comptonization spectrum and frequencies of quasi periodic oscillations observed in some XBs(e.g.,Titarchuk&Fiorito2004;Titarchuk& Shaposhnikov2005).It appears then that we identi?ed a physical mechanism that can determine a geometry of an accretion disk and corona in a wide range of accreting systems.

We thank T.Kallman,G.Richards,A.R′o˙z a′n ska,A.Siemiginowska,and J.Stone for useful discussions.We acknowledge support from NASA under LTSA grant NAG5-11736and support provided by NASA through grants HST-AR-09947.01-A and HST-AR-10305.05-A from the Space Telescope Science Institute,which is operated by the Association of Univer-sities for Research in Astronomy,Inc.,under NASA contract NAS5-26555.

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Fig.1.—From top to bottom:Maps of logarithmic density(top panel),optical depth(middle panel),and gas temperature(bottom panel) of the AGN failed disk wind,described in the text.The density and temperature maps are overplotted with the direction of the poloidal velocity ?eld.In making this?gure,we used the density and optical depth?oors of10?20g cm?3and10?2,respectively.In all three panels,the disk rotational axis is along the left hand vertical frame,while the disk photosphere is along the lower horizontal frame.Note the di?erence in the range along the r′and′z′axises.

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义。“有效”是指教师的教学方式能促使学生较好地达到三维目标，在整个过程中起到事半功倍的作用。有效教学注重学生的进步和发展，教师必须确立学生的主体地位，树立“一切为了学生的发展”的思想。“有趣”是指如何使用游戏教学、故事教学、TPR的教学等新的教学方法，来调动学生参与课堂活动的积极性？“有用”是指学习的材料，使学生感到有价值。它需要教师在课堂教学中使学生感受到所学知识能够协助他们解决生活中的实际问题。有效教学需要教师具备一种反思的意识，经常实行行动研究。要求每一位教师持续地反思自己的日常教学行为，持续地改进自己的教学。 有效教学强调教学效果，注重学生通过一段时间在教师引导下努力学习所获得的进步与发展。有效教学应该包括有效果和有效率两个层面。我们知道，小学英语教学的主要途径是课堂教学，而课堂教学又是在教学活动中得以体现的。所以，有效的课堂教学活动是顺利达到教学目标的可靠保障。作者认为提升小学英语课堂教学的有效性，教师应从以下几方面考虑： 一、课堂教学活动要目标明确 新课程理念下，基础教育阶段英语课程的总体目标是：培养学生的综合语言使用水平，学生能够用英语做事情是课堂教学目标达成的主要体现。在小学英语课堂教学中，教师广泛采用贴近学生生活的，易于学生参与体验的任务型教学活动。例如，在教学 PEP 四年级上册 Unit 5“What would you like？”Part A的教学内容时，

PEP人教版小学英语六年级下册教材解析(精校版)

吉安县中小学教师说教材活动 全局把握温故知新 ——浅谈我对六年级英语下册教材的理解 吉安县实验小学 XXX 在我们的日常教学活动中，明确目标，吃透教材，选择合适的策略是非常重要的，在我们小学英语教学中也是一样。今年我继续担任我校六年级英语的教学工作，在此，说一说我对人教版小学六年级上册英语教材的理解，与各位同仁探讨。 一、紧扣课程标准，了解教材特点。 1.《英语课程标准》对小学阶段应达到的目标作了明确的阐述，我们小学六年级应该达到的应该是二级，其要求是： 对英语学习有持续的兴趣和爱好。能用简单的英语互致问候、交换有关个人、家庭和朋友的简单信息。能根据所学内容表演小对话或歌谣。能在图片的帮助下听懂、读懂并讲述简单的故事。能根据图片或提示写简单的句子。在学习中乐于参与、积极合作、主动请教。乐于了解异国文化、习俗。 我们的这一册教材就是人民教育出版社根据国家教育部颁布的《英语课程标准》编订的以小学三年级为起始年级的小学英语教材的最后一册课本。通过完成此册课本的学习，学生的英语水平要达到《英语课程标准》规定的基础教育阶段英语课程的二级目标。因此，本册教材的教学对学生在小学毕业时能否达到教学目标规定的要求有举足轻重的作用。 2.本套教材的编写特点是： *强调语言运用（如let’s do、let’s sing等） *注重能力培养（Group work、Let’s check等）

*突出兴趣出发（卡通插图、图文并茂） *重视双向交流（融合中西方文化知识） *融合学科内容（贯穿自然科学、社会文化、语言艺术领域） *重视灵活扩展（选学C部分） *实现整体设计（三至六年级系统编排） 3.在编写上，本套教材是这样安排的： （1）、全册共有4个单元，两个recycle是针对小学所有教学内容的总体复习单元。 （2）、新的教学内容较少，12周的时间完成新授单元的教学，2周的时间对这四个单元进行复习巩固，阶段复习； （3）、语言点较为集中，Unit One 是比较级，Unit 3 , Unit 4 是过去时，是本册的难点。 在每一个单元中，分12页，编写体例如下：

小学英语教学有效性的策略研究

小学英语教学有效性的策 略研究 Revised by Hanlin on 10 January 2021

《小学英语教学有效性的策略研究》 研究课题鉴定成果概况 课题名称：小学英语教学有效性的策略研究 课题类别:甘肃省教育科学“十二五”规划重点课题 课题批准号:GS（2010）Z045 课题鉴定证书号：GSGB【2013】J005 课题负责人：张旭东 所在单位：金昌市金川区第一小学 课题组参与人员:姚兴瑞刘婷娟杨兆光王文君李其霞张莉杨永涛武 功颜张海生顾秀文 鉴定等级：优秀 为了适应教育发展的趋势，让教师真正走进新课程，构建新课堂，我们结合本市区小学实际，根据自身条件，在充分酝酿的基础上，经多方论证，精心选题，确定《新课改理念下的小学英语有效学习策略研究》上报为省级课题，2010年10月被省教育科研组批复立项为甘肃省教育科学“十一五”重点课题。 三年多来，经过课题组成员不断的实践、反思、总结，我们对新课改理念下的小学英语有效学习的策略、教师的教学手段、有效学习的组织形式、课堂教学的模式、合作学习的有效策略等，进行了具体的研究，下面将我校的课题研究工作总结如下： 一、课题提出的背景 1.新一轮课程改革的需要 《英语课程标准》“倡导学生主动参与合作，交流与探究等多种学习活动，改进学习方式，促进学生相互帮助，体验集体荣誉感和成就感，发展合作精神，使学生真正成为学习的主人。”它要求把学生的交往活动即合作学习纳入语言学习活动。但是随着社会经济的不断发展,很多农民工进城打工创业,他们也渴望自

己的孩子拥有城里孩子所能得到的优质教育,这就造成了城市学校班额太大,再加上学生层次不齐,给我们教育教学提出了严峻的挑战,为了很快适应和扭转这种局面,不断提高教育教学质量,我们提出了在《新课改理念下的小学英语有效学习策略研究》。小组合作学习是一种教学策略，既能充分调动学生学习英语的兴趣和积极性，又能提高学习效果，在大班额环境下实施小组合作学习策略尤为重要。 2.我市教育现状的需要 截至目前，全市原有的162所完全小学已调整为142所；20所独立初中调整为16所，撤并了中小学24所。虽然通过集中力量办较大规模的寄宿制农村中小学，农村中小学教育资源进行有效整合后，改善了办学条件，使农村中小学生接受了良好的学校教育，对提高办学效益和教学质量，缩小城乡教育水平差距起到了推动作用，但是教育资源有效整合的力度还是不大，跟不上时代发展的需要，跟不上广大农村家长对高质量学校的需求。望子成龙、望女成凤的愿望使他们不惜一切代价将孩子送往城市学校读书。目前我市农村学校生源越来越少，班额越来越小，老师过剩。相反，城市学校生源越来越多，班额越来越大。如何解决大班额环境下的教学质量问题，就显得尤为重要。 3.我校实际情况的需要 我校在金昌市范围内是一所比较特殊的城市小学，说特殊是因为我校属城市小学，但是区属职工的子女少，而来自金川区宁远和双湾两镇和外来进城打工的子女多，虽学校各年级都增加了班级，还是满足不了源源不断增加的学生，每个班级学生都在60人左右，这样学校班额大、学生多，班级人数远远超过了正常班级人数的要求。一个专职的小学英语教师要负责的有近三百五十个学生，甚至更多。在英语课堂上，一方面教师感到力不从心；另一方面如果还依然采用“一刀切”的办法进行教学，在长期“齐步走”“一言堂”的课堂教学环境下，小组合作学习也只是简单的分组进行个别的合作学习而已，教师在有限的课堂40分钟内难以真正做到面向全体学生，让每个学生真正参与到活动中，鉴于此我们提出了“大班额环境下小学英语教学小组合作学习有效性”的课题，结合本校的实际，力争以其作为深化并打造我校英语特色学校的着力点，借他人肩膀，加

最新小学英语 时态公式

现在进行时：（主+be+V.ing）三要素（只有一个公式） 肯定句：主语+be动词+V.ing +其他. 否定句：主语+be动词+not+V.ing +其他. 一疑：Be动词+主语+ V.ing +其他？ 肯回：Yes,主语+be动词 否回：No,主语+be动词+not 特疑：特词+ be动词+主语+ V.ing+其他? 一般现在时：（主+be+其）（主+V.+其）（主+V.+其）（三个公式）肯定句：主语+be动词+其他. 否定句：主语+be动词+not+其他. 一疑： Be动词+主语+其他? 肯回：Yes,主语+be动词 否回：No,主语+be动词+not 特疑：特词+ be动词+主语+其他? 肯定句：(非三单)主语+V.（原型）+其他. 否定句：(非三单)主语+don’t + V.（原型）+其他. 一疑：Do+(非三单)主语+ V.（原型）+其他 ? 肯回：Yes,主语+do 否回：No,主语+don’t 特疑：特词+ do+(非三单)主语+ V.（原型）+其他 ? 肯定句：(三单)主语+V.（三单）+其他. 否定句：(三单)主语+doesn’t + V.（原型）+其他. 一疑：Does+(三单)主语+ V.（原型）+其他 ? 肯回：Yes,主语+does 否回：No,主语+does’t 特疑：特词+ does+(三单)主语+ V.（原型）+其他 ?

一般过去时：（主+be+其）（主+V.+其）（两个公式） 肯定句：主语+be动词（过去式）+其他. 否定句：主语+be动词（过去式）+not+其他. 一疑： Be动词（过去式）+主语+其他? 肯回：Yes,主语+be动词（过去式） 否回：No,主语+be动词（过去式）+not 特疑：特词+ be动词（过去式）+主语+其他? 肯定句：主语+V.（过去式）+其他. 否定句：主语+didn’t + V.（原型）+其他. 一疑：Did+主语+ V.（原型）+其他 ? 肯回：Yes,主语+did 否回：No,主语+didn’t 特疑：特词+ did+主语+ V.（原型）+其他 ? 新人教版八年级物理上、下册知识点 八年级上册物理复习提纲 第一章机械运动 一、长度和时间的测量 1、测量某个物理量时用来进行比较的标准量叫做单位。为方便交流，国际计量组织制定了一套国际统一的单位， 叫国际单位制（简称SI）。 2、长度的单位：在国际单位制中，长度的基本单位是米(m)，其他单位有：千米(km)、分米(dm)、厘米(cm)、毫 米(mm)、微米(μm)、纳米(nm)。1km=1 000m；1dm=0.1m；1cm=0.01m；1mm=0.001m；1μm=0.000 001m； 1nm=0.000 000 001m。测量长度的常用工具：刻度尺。刻度尺的使用方法：①注意刻度标尺的零刻度线、最小分度值和量程；②测量时刻度尺的刻度线要紧贴被测物体，位置要放正，不得歪斜，零刻度线应对准所测物体的一端；③读数时视线要垂直于尺面，并且对正观测点，不能仰视或者俯视。 3、国际单位制中，时间的基本单位是秒(s)。时间的单位还有小时(h)、分(min)。1h=60min 1min=60s。 4、测量值和真实值之间的差异叫做误差，我们不能消灭误差，但应尽量减小误差。误差的产生与测量仪器、测量 方法、测量的人有关。减少误差方法：多次测量求平均值、选用精密测量工具、改进测量方法。误差与错误区别：误差不是错误，错误不该发生能够避免，误差永远存在不能避免。 二、运动的描述 1、运动是宇宙中最普遍的现象，物理学里把物体位置变化叫做机械运动。 2、在研究物体的运动时，选作标准的物体叫做参照物。参照物的选择：任何物体都可做参照物，应根据需要选择 合适的参照物（不能选被研究的物体作参照物）。研究地面上物体的运动情况时，通常选地面为参照物。选择不同的参照物来观察同一个物体结论可能不同。同一个物体是运动还是静止取决于所选的参照物，这就是运动和静止的相对性。 三、运动的快慢 1、物体运动的快慢用速度表示。在相同时间内，物体经过的路程越长，它的速度就越快；物体经过相同的路程， 所花的时间越短，速度越快。在匀速直线运动中，速度等于运动物体在单位时间内通过的路程。在物理学中，为了比较物体运动的快慢，采用“相同时间比较路程”的方法，也就是将物体运动的路程除以所用时间。这样，在比较不同运动物体的快慢时，可以保证时间相同。

小学英语课堂教学有效性开题报告(正)

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天水市教育科学“十二五”规划课题 开题报告书 尊敬的各位领导，各位专家，各位老师： 大家好！我校由老师负责申报的课题《小学英语课堂教学有效性研究》于2012年12月30日经天水市教育科学研究所批准立项，课题批准号为：TSJY[2012]T12。现就对本课题的课题研究的背景、课题概念的界定、课题研究的理论依据、课题的研究内容、课题研究的目标、课题研究的方法、课题研究的步骤等方面做如下说明，敬请大家提出宝贵意见： 一、课题研究的背景 随着基础教育课程改革的不断深入，我国的小学英语教育发生了根本性的变化。但由于种种现实因素，要把课改理念转化为教师自觉和具体的课堂教学行为，转化为有效的课堂教学技艺，还有很长一段路程需要跋涉和跨越。 当前，小学英语课堂教学有以下几个特点：一是学生从小受周围环境和家庭、学校教育的影响，养成了许多不良的行为和学习习惯。如：自我管理能力差，学习效率不高，作业潦草等。二是由于种种原因，教学行为不拘小节，课堂教学随意性大，对学生的学习习惯和行为习惯造成很大的负面影响；教学时为了省事，不尊重学生的个体差异，采取一刀切的教学方式。三是教学方法落后。有的老师观念比较陈旧，抓住应试教育方法不放，一味追求学习成绩，以成绩作为唯一的评价标准；组织课堂教学模式化，只重视预设，不重视生成，不愿捕捉课堂互动过程中有价

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