a r X i v :c o n d -m a t /9803392v 1 [c o n d -m a t .s t a t -m e c h ] 31 M a r 1998Driven Di?usive Systems:An Introduction and Recent Developments B.Schmittmann and R.K.P.Zia Center for Stochastic Processes in Science and Engineering Physics Department Virginia Polytechnic Institute and State University Blacksburg,VA,24061-0435USA 1Introduction In nature,there are no true equilibrium phenomena,since all of these require in?nite times and in?nite thermal reservoirs or perfect insulations.Neverthe-less,for a large class of systems,it is possible to set up conditions under which predictions from equilibrium statistical mechanics provide excellent approxi-mations,as many of the inventions of the industrial revolution can attest to.By contrast,non-equilibrium phenomena are not only ubiquitous,but often elude the powers of the Boltzmann-Gibbs framework.Unfortunately,to date,the theoretical development of non-equilibrium statistical mechanics is at a stage comparable to that of its equilibrium counterpart in the days before
Maxwell and https://www.doczj.com/doc/c714629803.html,ing the intuition developed by studying equilib-rium statistical mechanics,we are often “surprised”by the behavior displayed by systems far from equilibrium,even if they appear to be in time-independent states.For example,the well honed arguments,based on the competition be-tween energy and entropy,frequently fail dramatically.In these lectures,we will present some explorations into the intriguing realm of non-equilibrium steady states ,focusing only on a small class,namely,driven di?usive systems.
Motivated by both the theoretical interest in non-equilibrium steady states and the physics of fast ionic conductors[1],Katz,Lebowitz and Spohn[2]intro-duced a deceptively minor modi?cation to a well-known system in equilibrium statistical mechanics:the Ising[3]model with nearest-neighbor interactions. In lattice gas language[4],the time evolution of this model can be speci?ed by particles hopping to nearest vacant sites,with rates which simulate cou-pling to a thermal bath as well as an external?eld,such as gravity.If“brick wall”boundary conditions are imposed in the direction associated with grav-ity(particles re?ected at the boundary,comparable to a?oor or a ceiling), and if the rates obey detailed balance,then this system will eventually settle into an equilibrium state,similar to that of gas molecules in a typical room on earth.Although there is a local bias in the hopping rates(due to gravity), thermal equilibrium is established by an inhomogeneous particle density,at all T<∞.However,if periodic boundary conditions are imposed,then trans-lational invariance is completely restored so that,in the?nal steady state, the particle density is homogeneous,for all T above some?nite critical T c, while a current will be present.Clearly,for gravity,such boundary conditions can be imposed only in art,as by M.C.Escher[5].In physics,it is possible to establish such a situation with an electric?eld,e.g.,by placing the d=2 lattice on the surface of a cylinder and applying a linearly increasing magnetic ?eld down the cylinder xis.If the particles are charged,they will experience a local bias everywhere on the cylinder and a current will persist in the steady state.Echoing the physics of fast ionic conductors,we will therefore use the term“electric”?eld to describe the external drive and imagine our particles to be“charged”,in their response to this drive.This is the prototype of a “driven di?usive system.”Now,such a system constantly gains(loses)en-ergy from(to)the external?eld(thermal bath),so that the time independent state is by no means an equilibrium state,`a la Boltzmann-Gibbs.Instead,it is a non-equilibrium steady state,with an unknown distribution in general.As discovered in the last decade,modifying the Ising model to include a simple local bias leads to a large variety of far-from-simple behavior.The scope of these lectures necessarily limits us to only a bird’s eye view.Since a more extensive review has been published recently[6],we will restrict these notes to a brief“introduction”to this subject.Instead,we choose to include some developments since that review.
For completeness,we summarize,in Section II,the lattice model introduced by KLS and the Langevin equation believed to capture its essence in the long-time large-distance regime.The next section(III)is devoted to some of the surprising behavior displayed by this system,at temperatures far above,near, and well below T c.A collection of recent developments is presented in Section IV.In the?nal section(V),we conclude with a brief summary.
2The Microscopic Model and a Continuum Field Approach
On a square lattice,with L x×L y sites and toroidal boundary conditions,a particle or a hole may occupy each site.A con?guration is speci?ed by the occupation numbers{n i},where i is a site label and n is either1or0.Oc-casionally,we also use spin language,de?ning s≡2n?1=±1.To access the critical point,half-?lled lattices are generally used in Monte Carlo simu-lations: i n i=L x L y/2.The particles are endowed with nearest-neighbor at-traction(ferromagnetic,in spin language),modeled in the usual way through the Hamiltonian:H=?4J
To understand collective behavior in the long-time and large-scale limit,we often rely on continuum descriptions,which are hopefully universal to some extent.Successful examples include hydrodynamics and Landau-Ginzburg the-ories.Certainly,the?4theory,enhanced by renormalization group techniques, o?ers excellent predictions for both the statics and the dynamics near equi-librium of the Ising model.Following these lines,we formulate a continuum
theory for the KLS model,in arbitrary dimension d.In principle,such a de-scription can be obtained by coarse-graining the microscopic dynamics[8]but we will pursue a more phenomenological approach here.Seeking a theory in the long-time,large wavelength limit,we?rst identify the slow variables of the theory.These are typically
?ordering?elds which experience critical slowing down near T c,and ?any conserved densities.
The KLS model is particularly simple since it involves a single ordering?eld, namely the local“magnetization”or excess particle density,?(x,t),which is also the only conserved quantity.Here,x stands for(x1,x2,...,x d?1,x d=y). The last entry denotes the one-dimensional“parallel”subspace selected by E.Thus,we begin with a continuity equation,?t?+?j=0,and postulate an appropriate form for the current j.In the absence of E,it simply takes its Model B[9]form,j(x,t)=?λ?δH
2(??)2+τ
4!
?4}.As usual,τ∝T?T c and u>
0.While the?rst contribution to j is a deterministic term,re?ecting local chemical potential gradients,the second term,η,models the thermal noise. The noise is Gaussian distributed,with zero mean and positive second moment
proportional to the unit matrix,i.e.,<ηiηj>=2σδijδ(x?x′)δ(t?t′),i,j= 1,...,d.In the presence of E,we should expect at least two modi?cations to j,namely?rst,an additional contribution j E modeling the nonvanishing
mass transport through the system and second,the generation of anisotropies, since E singles out a speci?c direction.By virtue of the excluded volume
constraint,the“Ohmic”current j E vanishes at densities1and0,corresponding to?=±1.Writing E for the coarse-grained counterpart of E,pointing along unit vector?y,the simplest form is therefore j E=E(1??2)[1+O(?)]?y,where
the O(?)corrections will turn out to be irrelevant for universal properties. Next,we consider possible anisotropies.Clearly,all?2operators should be split into components parallel(?2)and transverse(?2⊥)to E,accompanied
by di?erent coe?cients.For example,the anisotropic version of the Model B termτ?2?will read(τ ?2+τ⊥?2⊥)?,with two di?erent“di?usion”coe?cients for the parallel and transverse subspaces.Should we expect both of these to vanish as T approaches T c(E)?Or just one of them-but which one?Recalling that the lowering ofτ,in the equilibrium system,is a consequence of the
presence of interparticle interactions,we argue that E,stirring parallel jumps much more e?ectively than transverse ones,should counteract this e?ect in the parallel direction,having less of an impact in the transverse subspace. Thus,we anticipate that genericallyτ >τ⊥,so that criticality,in particular, is marked byτ⊥vanishing at positiveτ .This is borne out by the structure of typical ordered con?gurations,namely,single strips aligned with E,indicating that“antidi?usion”,i.e.,τ⊥<0,dominates in the transverse directions below T c.Finally,the drive also induces anisotropies in the noise terms so that the second moment of their distribution should be taken as<ηiηj>=2σiδijδ(x?
x′)δ(t?t′).Since the transverse subspace is still fully isotropic,we de?ne σ1=...=σd?1≡σ .Generically,however,we should expectσd≡σ⊥=σ . Summarizing,we write down the full Langevin equation:
?t?(x,t)=λ{(τ⊥??2⊥)?2⊥?+(τ ??2)?2??2α×?2?2⊥?+
+
u
δ?.In this case,P?is simply proportional to
exp(?H/σ),irrespective of the choice of N.We will refer to such a dynamics, in an operational sense[6],as“satisfying the?uctuation-dissipation theorem”(FDT)[13].Model B falls into this category,with N=??2.In contrast, for our driven system,F is just the expression in the{...}brackets of(1)
and?≡??η,so that N=?(σ
?2+σ⊥?2⊥).Thus,this F is clearly not
Hamiltonian!In this fashion,our continuum theory re?ects the fact that we are dealing with a generic non-equilibrium steady state.For completeness, we should point out that certain microscopically non-Hamiltonian dynamics may become Hamiltonian if viewed on su?ciently large length scales.The discussion of these subtleties[14–16],while intriguing,is beyond the scope of these lectures.
3Surprising Singular Behavior,near and far from Criticality
For the d=2Ising model in equilibrium,thermodynamic quantities are typ-ically analytic,except at a single point,i.e.,T c.In particular,being a model with only short-range interactions,correlation functions are short-ranged far above T c,decaying exponentially with distance and controlled by a?nite cor-relation length.Far below criticality,a half?lled system in a square geometry will display two strips of equal width,as a result of the co-existence of a particle rich(dense)phase with hole-rich one.Correlations of the?uctuations within each phase are also short-ranged.More interestingly,the interfaces between the phases do represent soft degrees of freedom,being the Goldstone mode of a broken translational invariance[17].One consequence is a divergent struc-ture factor(Fourier transform of the two-point correlation,of deviations from being straight).Although this behavior is singular,it is well understood and can be related to that of a simple random walk[18].Only at criticality does the system possess non-trivial singular properties,the nature of which became transparent only in the70’s[19]despite Onsager’s tour-de-force in1944[20]. When driven into non-equilibrium steady states,this picture changes dramat-ically.In particular,non-trivial singular behavior appears at all T.While the transition itself remains second order,its properties fall into a non-Ising class. Here,we will present some of these surprising features brie?y,referring the reader to[6]for more details.
3.1T far above T c
Focusing on the behavior far above criticality,let us study the two-point cor-relation G(r)(r=(x,y))and its transform,the structure factor,S(k).We?rst point out that the drive clearly induces an asymmetry between x and y,i.e., anisotropy beyond that due to the lattice.Thus,we should not be surprised if the familiar Ornstein Zernike form of S(∝(1+ξ2k2))were to become el-liptical rather than circular,e.g.,similar to Fig.1a.However,simulations[2] showed that there is a discontinuity singularity in S at the origin,as in Fig.1b. To be more precise,we may de?ne
R≡lim k
y→0
S(0,k y)
Fig.1.Ising model in system. Strange the con-text ofτ andτ⊥are positive.a stable min-imum at nonlinearities are the system in the Langevin
?t?(x,t)=λ(τ⊥?2⊥+τ ?2)???η.(3) With the help of a Fourier transform,we can easily?nd?(k,t)for any real-ization ofη,and then compute averages over the Gaussian distributedη,with its second moment given by(1).This forms the starting point for the discus-sion of the disordered phase,quanti?ed by,e.g.,equal-time structure factors or three-point functions[21].
Here,we focus on the former,S(k)≡,which can be easily computed:
S(k)=σ⊥k2x+σ k2y
so that R= σ τ⊥ / σ⊥τ .In order for this S to reduce to the equilibrium Ornstein-Zernike form,the FDT has to be invoked,which constrains R to
unity.For the driven case,R is no longer constrained,so that a discontinuity develops.One consequence of such a singularity in S is that G becomes long-ranged,decaying as r?2at large|r|.The amplitude,in addition to being∝(R?1),has a dipolar angular dependence,so that an appropriate angular average of G is again short-ranged[6].
To end this discussion,let us follow G.Grinstein’s argument[22]that such long-ranged power law decays should be expected.Starting with the dynamic two-point correlation G(r,t),we know that the conservation law leads to the
autocorrelation:G(0,t)→t?d/2.In addition,being a di?usive system,we
√
should expect r～
L.In our case,if we study an interface aligned along the y axis,then,to a good approximation,we may specify the con?guration by its position along x by the“height”function h(y).Known as capillary waves[24],these?uctuations have a venerable his-tory.Since the interface is a manifestation of broken translational invariance, h(y)are the soft Goldstone modes[17],so that the associated structure fac-
tor S h(q)≡ h(q)h(?q) diverges as q?2.Of course,this property is just the
√
Fourier version of the
plotting the widths vs.L p with various values of p ,we found that the curves did not straighten out with p as low as 0.05[25].As a phenomenon,roughness suppression is not novel,gravity being the most common example.However,the drive here is parallel to the interface,reminiscent of wind driving across a water surface,which has a destabilizing e?ect by contrast.Subsequently,a more detailed simulation study [26]of the interface with L y ≤600showed that S (q )～q ?2for large q ,crossing over to q ?0.67for small q (Fig.2).Since 1/L q
?0.67dq =O (1),the small q behavior is consistent with p =0.Though 0.67appears temptingly close to a simple rational:2/3,there is no viable theory so far (despite two valiant attempts [27]).Finally,let us note that the crossover from rough to smooth occurs at the scale of q ～E .Although no precise Monte Carlo analysis of this crossover has been performed,it is con-sistent with E having the units of 1/length.In this respect,such a length is comparable to the capillary length which controls the crossover in the gravi-tationally stabilized interface.Of course,in that case,the small q behavior is simply q 0!
q
1/S (q )
h Fig.2.Log-log plot of interface structure factor vs.wavevector.
3.3Critical Properties
Finally,we turn to the critical region,described by τ⊥ 0in (1).In contrast to the situation for T ?T c ,we now need fourth-order terms in ?⊥to stabilize the system against large-wavelength ?uctuations.Similarly,the nonlinear terms are required to ensure a stable ordered phase below T c .However,we still have τ >0,so that fourth-order parallel gradients may safely be neglected.Thus,near criticality,the leading terms on the right hand side of (1)are
(?2⊥)2?and ?2?,implying that parallel and transverse wave vectors,k ⊥and k ,scale naively as |k |～|k ⊥|2,i.e.,parallel gradients are less relevant than transverse ones.More systematically,a naive dimensional analysis [28]reveals
that the nonlinearity associated with E is the most relevant one,having an upper critical dimension d c=5,distinct from the usual value of4for the Ising model.Dropping irrelevant terms,(1)simpli?es to
?t?(x,t)=λ{(τ⊥??2⊥)?2⊥?+?2?+u
Here,μis just a scaling factor.Eqn.(6)can be viewed as a de?nition of the critical exponentsν,z,ηand?.The appearance of the latter is of course consistent with our earlier discussion.Di?erent universality classes are dis-tinguished by the characteristic values of these exponents,expressed,e.g., through their?-expansions.For our model,all exponents,except?,take their
mean-?eld values:ν=1
3.A separate analysis
yields the order parameter exponentβ=1
1+?.Since most
simulations have focused on two-dimensional systems,we set?=3,predicting ?=2andη′
=2/3.Monte Carlo data for the pair correlation along the
drive beautifully display the expected r?2/3
decay[32].Numerical evidence for
the strong anisotropy exponent is somewhat more indirect,extracted from an anisotropic?nite size scaling analysis[7].Convincing data collapse is obtained, using anisotropic systems of size L ×L⊥with?xed“aspect”ratio L /L1+?⊥, for the theoretically predicted values of the exponents.
It is intriguing that the signals of continuous phase transitions in or near equilibrium,namely,a diverging length scale,scale invariance and universal behavior,also mark such transitions in far-from-equilibrium scenarios.
4Some Recent Developments
Beyond the topics discussed in the previous section,we may arbitrarily name three other“levels”of non-equilibrium steady-state systems.The?rst are as-sociated with the KLS model itself,including fascinating results on higher correlations[21],failure of the Cahn-Hilliard approach to coarsening dynam-ics[33],models with shifted periodic and/or open boundary conditions[34], and systems with AC or random drives[35,16].Topics in the next“level”in-volve various generalizations of KLS,such as other interactions[36,37],multi-
layer systems,multi-species models,and systems with quenched impurities [38].Further“a?eld”is a wide range of driven systems,e.g.,surface growth, electrophoresis and sedimentation,granular and tra?c?ow,biological and ge-ological systems,etc.In these brief lecture notes,we present two topics at the “intermediate level”:a driven bi-layer system and a model with two species.
4.1Phase Transitions in a Bi-layer Lattice Gas
The physical motivation for considering multi-layered systems may be traced to intercalated compounds[39].The process of intercalation,where foreign atoms di?use into a layered host material,is well suited for modeling by a driven lattice gas of several layers.In both physical systems and Monte Carlo simulations of a model with realistic parameters[40],?nger formation has been observed.Both are transient phenomena,so that we might ask:are there any novel phenomena in the driven steady states?On the theoretical front,there are two motivations.To study critical behavior of the single layer KLS model, it is necessary to set the overall density at1/2,with the consequence that two interfaces develop below criticality.These interfaces seriously complicate the analysis,since their critical behavior is quite distinct from the bulk.One way to avoid these di?culties is to consider a bi-layer system[41],with no inter-layer interactions.Through particle exchange between the layers,the system may order into a dense and a hole-rich layer,neither of which has interfaces. Certainly,this is expected(and observed)in the equilibrium case,where the steady state is just two Ising models,decoupled except for the overall conser-vation law.The other is using multilayer models as“interpolations”from two-to three-dimensional systems[42].Motivations aside,what the simulations reveal is entirely unexpected.
In the simplest generalization,a bi-layer system is considered as a fully periodic L×L×2model.The?rst simulations were carried out on the case with zero inter-layer interactions[42].Therefore,within each layer,all aspects are identical to the KLS model.Particle exchanges across the layers,una?ected by the drive,are updated according to the local energetics alone,so that these jump rates are identical to a system in equilibrium.The overall particle density is?xed at1/2.In the absence of E,there would be a single second order transition at the Onsager temperature.Since the inter-layer coupling is zero,phase segregation at low temperatures will occur across the layers, i.e.,the ordered phase is characterized by homogeneous layers of di?erent densities(opposite magnetizations,in spin language).Having no interfaces, the free energy of the system is clearly lowest in such a state.At T=0,one layer will be full and the other empty,so that this state will be labeled by FE. Intriguingly,when E is turned on,two transitions appear[42]!As T is lowered,
the homogeneous,disordered(D)phase?rst gives way to a state with strips
in both layers,reminiscent of two entirely unrelated,yet aligned,single layer driven systems.We will refer to this state as the strip phase(S).As T is lowered
further,a?rst order transition takes the S phase into the FE state.Why the S state should interpose between the D and FE states was not understood.
This puzzle,together with the presence of interlayer couplings in intercalated
compounds,motivated our study of the interacting bi-layer driven system[43]. Within this wider context,the presence of two transitions is no longer a total
mystery.On the other hand,this study reveals several unexpected features,
leading to interesting new questions.
Our system consists of two L2Ising models with attractive interparticle in-teractions of strength unity.Arranged in a bi-layer structure,the inter-layer
interaction is speci?ed by J.Thus,the“internal”Hamiltonian is just H≡
?4 nn′?4J nn′′,where the?rst sum is over nearest neighbors within a given layer while n and n′′di?er only by the layer index.The external drive,
E,is imposed through the jump rates,as in Section II.For simulations,we kept the overall density at1/2and chose J∈[?10,10].Note that negative J’s are especially appropriate for intercalated materials[39,40].The eventual goal of such a study is to map out the phase diagram in the T-J-E space.So far,we have data for only three(positive)values of E.Not expecting further surprises,we believe that we have uncovered the main features of the phase diagram.
To begin,let us point out various features in the equilibrium case.Here,the J>0and the J<0systems can be mapped into each other by a gauge transformation.Thus,in the thermodynamic limit,the phase diagram in the T-J plane is exactly symmetric.However,for the lattice gas constrained by a conservation law,the low temperature states of these two systems are not the same,being S or FE,respectively.While the D-S or D-FE transitions are second order(shown as a line in Fig.3),the S-FE line will be?rst order in nature(T axis from0to1in Fig.3).Due to the presence of interfaces,the FE domain includes the J≡0line.For the same reason,it“intrudes”into the J>0half plane by O(1/L),for?nite systems.Of course,on the T axis lies a pair of decoupled Ising systems,so that T c(J=0)assumes the Onsager value,as L→∞.In general,T c(J)is not known exactly,though T c(J=±∞)is precisely2T c(J=0),since every con?guration of the two layers can be mapped into one in the usual Ising model.The lesson from equilibrium is now clear: the S state is as generic as the FE state in this wider context.As we will see below,the presence of two transitions does not represent a qualitative change from the equilibrium phase structure.
Turning to driven systems,we can no longer expect a symmetric phase dia-gram,since the drive breaks the Ising symmetry.The data for E=25are shown in Fig.3,in which the?rst/second order transitions are marked as
Fig.3.second order
shift the phase
remains
lowering of the critical temperature for large|J|.Given that T c(E)>T c(0)in the KLS model,it is quite unexpected that T c(|J|?1,E?1)is less than its equilib-rium counterpart.Perhaps more notable is the presence of a small region in the J<0half plane,in which an S-phase is stable.Since particle-rich strips lie on top of each other,such a phase could not exist if either energy or entropy were to dominate the steady state.Concerned that its presence might be a ?nite size e?ect,we performed simulations using L’s up to100,with J=?0.1 and T∈[1.00,1.20].In all cases,the S-phase prevailed,leading us to conjec-ture that this region exists even in the thermodynamic limit.On the other hand,for lower T,the FE-phase penetrates into the J>0half plane,as in equilibrium cases.Energetics seem to take the upper hand here.Though we believe that,as L→∞,this part of the phase boundary will collapse onto the J=0line,we have not checked the?nite size e?ects explicitly.Of course, these“intrusions”into the positive/negative J region by the FE/S-phase are responsible for the appearance of two transitions in the studies of the J≡0 model[42].
It would clearly be useful to develop some intuitive arguments which can“pre-dict”these qualitative modi?cation of the equilibrium phase structure.Since the usual energy-entropy considerations appear to fail,our attempt[43]is based on the competition between suppression of short-range correlations[44] and enhancement of long-range ones[32],as a result of driving.Let us focus on the two-point correlations in the disordered phase with T being lowered from ∞and see how they are a?ected by the drive.On the one hand,the nearest-
neighbor correlations are found to be suppressed by E,consistent with the picture that the drive acts as an extra noise in breaking bonds.Taken alone, this suppression would lead to the lowering of the critical temperature,as pointed out in Section II.On the other hand,as we showed above,the drive changes signi?cantly the large distance behavior of G,from an exponential to a power law.Further,the amplitude is positive(negative)for correlations parallel(transverse)to E.Both the positive longitudinal correlations and the negative transverse ones should help the process of ordering into strips parallel to the?eld.In other words,the enhanced long-range parts favor the S-phase. Taken alone,we expect this e?ect to increase T c(E).Evidently,for the single layer case,the latter e?ect“wins”.For a bi-layer system,we need to take into account cross-layer correlations,which are necessarily short-ranged. Focusing?rst on the J=0case,where short-range e?ects due to cross-layer interactions are absent,we are led to T c(0,E)/T c(0,0)>1.Indeed,this ratio is comparable to that in the single-layer case.Next,we consider systems with positive J.Without the drive,T c(J,0)is of course enhanced over T c(0,0). For non-vanishing drive,it is not possible to track the competition between the short-and long-range properties of the transverse correlations.Evidently, for small J,the long-range part still dominates,so that T c(J,E)/T c(J,0)> 1.However,for J?J0,the presence of E e?ectively lowers J,since the latter is associated with only short-range correlations.Since a lower e?ective J naturally gives rise to a lower T c,we would“predict”that T c(J,E)/T c(J,0) could decrease considerably as J increases.From Fig.3,we see that T c(J,E)< T c(J,0)for J≥5!The interplay of the competing e?ects is so subtle that either can dominate,in di?erent regions of the phase diagram.Finally,we turn to the J<0case.Here,the two e?ects tend to co-operate rather than compete,since the long-range parts favor the S-phase over the FE-phase.As a result,the domain of FE is smaller everywhere.In particular,note that the J<0branch of T c(J,E)is signi?cantly lower than the J>0branch. The small region of the S-phase can be similarly understood,at least at the qualitative level.In this picture,we may argue that the bicritical point and its trailing?rst order line should be“driven”to the J<0half-plane.
To end this subsection,we point out a few other interesting features.The order parameters of the S and FE phases are conserved and non-conserved, respectively.Based on symmetry arguments,we may expect that the critical properties of the two second order branches will fall into di?erent universality classes.In contrast,the gauge symmetry forces static equilibrium properties to be identical along these two branches[45].Since the drive breaks this sym-metry,we can only speculate that the D-S transitions belong to the same class as the single layer,KLS model[30]while the D-FE ones should remain in the equilibrium Ising class[14].Since these two classes are distinct,we can also expect a rich crossover structure near the bi-critical point.Assuming these conjectures are con?rmed,we can truly marvel at the novelties a driving?eld
can bring.
4.2Biased Di?usion of Two Species
If the particles in the KLS model are considered“charged”,it is natural to
explore the e?ects of having both positive and negative charges.De?ned on a fully periodic L x×L y square lattice,a con?guration of such a“two species”model can be characterized by two local occupation variables,n+r and n?r,
which equal1(0),if a positive or negative particle is present(absent)at site r≡(x,y).In spin language,this corresponds to a spin-1model,and novel behavior is to be expected,since we have altered the internal symmetries of the local order parameter.For simplicity,we will restrict ourselves to zero to-tal charge,Q≡ r(n+r?n?r)=0.Pursuing the analogy to electric charges, positive(negative)charges move preferentially along(against)the drive E which is directed along the?y axis.As a?rst step,we assume that there are no interparticle interactions apart from an excluded volume constraint. Thus,our model can be viewed as the high-temperature,large E limit of a more complicated interacting system.To model biased di?usion,a parti-cle with charge q=±1jumps onto a nearest-neighbor empty site accord-ing to min[1,e?qEδy],whereδy=0,±1is the change in the y-coordinate of the particle.To allow charge exchange between neighboring particles,two nearest-neighbor sites carrying opposite charge may exchange their content with probabilityγmin[1,e?qEδy].Now,δy is the change in the y-coordinate of the positive particle.The parameterγsets the ratio of the characteristic time scales controlling,respectively,charge exchange and di?usion.
Physical motivations for this model come from various directions:fast ionic
conductors with several species of mobile charges[1],electric breakdown of
water-in-oil microemulsions[46],and gel electrophoresis of charged polymers [47].It can also be interpreted as a simple model for some tra?c or granular ?ows[48,49].
Summarizing our simulation data,we?rst map out the phase diagram of the
model,in the space spanned by E,the total mass densityˉm≡ r(n+r+ n?r)/(L x L y),andγ[50].Focusing on smallγ,the di?usive dynamics is limited by the excluded volume constraint.For su?ciently small E andˉm,typical con?gurations are disordered,characterized by homogeneous charge and mass densities and fairly large currents.Nevertheless,this phase is highly nontrivial, supporting anomalous two-point correlations reminiscent of the KLS model:In generic directions,the familiar r?d decay prevails,with the remarkable excep-
tion of the?eld direction,where a novel r?(d+1)/2
dominates[51].With increas-
ing E orˉm,the tendency of the particles to impede one another becomes more pronounced,until a phase transition into a spatially inhomogeneous“ordered”
phase occurs.Beyond the transition line E c (ˉm,γ),typical con?gurations are charge-segregated,consisting of a strip of mostly positive charges “?oating”on a similar strip of negative charges,surrounded by a background of holes.To explore these transitions more quantitatively,we need to de?ne a suitable order parameter.Taking the Fourier transforms of the local hole and charge densities,?φ(k ⊥,k )≡1L x L y r [n +r ?n ?r
]exp {ik ⊥x +ik y },we select the components Φ≡|?φ(0,2πL y )|since these are most sensitive to ordering into a single transverse strip.Naturally,we choose their averages <Φ>and <Ψ>as our
order parameters.To distinguish ?rst from second order transitions,we also measure their ?uctuations and histograms.The resulting phase diagram is shown in Fig.4.Typically,hysteresis loops in <Φ>,<Ψ>and the average current are observed only for ˉm <ˉm o (γ),indicating that the transitions are ?rst order in this regime.On the other hand,?uctuations develop sharp peaks for larger mass densities,signalling continuous https://www.doczj.com/doc/c714629803.html,rger values of γfavor the disordered phase,so that the whole transition line shifts to higher E ,the region between the spinodals narrows,and ˉm o (γ)moves to higher mass densities.Thus,we observe a surface of ?rst order transitions,separated from a surface of continuous ones by a line of multicritical points:E c (ˉm o (γ),γ).
m
E
01
2
3
4
5
Fig.4.Phase diagram for the driven two-species model.The system size is 30×30.The ?lled circles mark the lines of continuous transitions,while the open circles denote the spinodal lines associated with ?rst order transitions.The inset shows a typical ordered con?guration at ˉm =0.40,E =3.00and γ=0.02with the open (?lled)circles representing positive (negative)charges.
While it is not surprising that the disordered phase is stable for large E pro-vided the mass density is su?ciently small,it may appear rather counterintu-itive that it should also dominate theˉm 1region.However,settingˉm≡1 eliminates every hole in the system so that the dynamics is carried entirely by the charge exchange mechanism.Relabelling positive charges as“particles”and negative ones as“holes”,the model becomes equivalent to a noninteract-ing(i.e.,J=0)KLS model.Also termed the asymmetric simple exclusion process(ASEP)in the literature,its steady-state solution is exactly known [52]to be homogeneous.Thus,for allγ>0,our two-species model remains disordered along the entireˉm≡1line.For su?ciently largeγ,the data in-dicate quite clearly that a?nite region of disorder persists,for any E,just below complete?lling.For smallerγ,however,it is less obvious whether such a region exists in the thermodynamic limit,since even a single hole can suf-?ce to induce spatial inhomogeneities in a?nite system:Acting as a catalyst for the charge segregation process,the hole creates a strip of predominantly positive charge,separated by a rather sharp interface from a similar,nega-tively charged domain,located“downstream”.The charge exchange mecha-nism tends to remix the charges but has little e?ect for smallγ.Eventually, the hole ends up trapped in the interfacial region!Finally,we turn to larger values ofγ,speci?cally,γ 0.62:here,the charge exchange mechanism dom-inates over the excluded volume constraint and suppresses the ordered phase completely.
So far,our discussion has mostly drawn upon Monte Carlo results.It is grat-ifying,however,that the same qualitative picture emerges from a mean-?eld theory,based upon a set of equations of motion for the local densities.We brie?y summarize the analytic route.Since the numbers of both positive and negative charges are conserved,we begin with a continuity equation for the coarse-grained densitiesρ±(r,t):?tρ±+?j±=0.For simplicity,we fo-cus on the caseγ=0,i.e.,particle-hole exchanges,?rst.As for the KLS model,j±can be written as the sum of a di?usive piece and an Ohmic term, j±=λ±{??δH
δρ±is taken
at?xedρ?since we are focusing on particle-hole exchanges here.To model the charge exchange mechanism,we simply add a similar term to j±,namely, j±=λ{??δH
δρ±|?±E?y}whereλ′=ρ+ρ?vanishes with bothρ+andρ?,and the functional derivative is taken at?xed hole density. Expressing the resulting equations in the more convenient variables?and charge densityψ≡ρ+?ρ?,we obtain
?t?=?{??+E?ψ?y}
?tψ=?{γ?ψ+(1?γ)[??ψ?ψ??]?E?(1??)?y(7)?γ
1+γ
du .The potential V(u),given
explicitly in[50],exhibits a minimum for a range of J’s,so that inhomoge-neous,periodic solutions of(7)exist.We note in passing that these may even be found explicitly,providedγ=0[53].Otherwise,numerical integration is of course always possible,yielding rather impressive agreement with simula-tion pro?les.Interestingly,our equations of motion predict that E enters only through the scaling variable E L y.If plotted accordingly,our simulation data con?rm this scaling by collapsing rather convincingly,at least within their error bars.
However,the mere existence of two types of steady-state solutions is not suf-?cient to provide evidence for a phase transition.Therefore,we seek insta-bilities of these solutions,as the control parameters E,ˉm andγare var-ied.Performing a linear stability analysis,we?nd that a homogeneous so-lution with massˉm becomes unstable if the drive E exceeds a threshold value E H(ˉm,γ)=2π1?ˉm+γˉm
2?γ<ˉm<1,so that,in particular,the homogeneous phase is always
stable forγ 1.Clearly,we cannot identify this mean-?eld stability boundary with the true transition line:we have not considered the locus of instabilities associated with inhomogeneous solutions,and?uctuations have been neglected throughout.However,it is quite remarkable how well it mirrors the qualitative shape of the phase diagram.
Finally,let us consider the order of the transitions.In principle,two routes
can be pursued here.One of these,namely,the computation of the stability
boundary of the inhomogeneous phase,E I(ˉm,γ),is rather subtle,involving three Goldstone modes[54].Once E I(ˉm,γ)is known,values(ˉm,γ)for which
E I and E H coincide can be identi?ed as loci of continuous transitions;other-wise,E H and E I mark the“spinodals”,near a?rst order transition,where the
homogeneous/inhomogeneous phases become linearly unstable.Alternatively,
the adiabatic elimination of the fast modes results in an e?ective equation of motion for the slow ones which can then be analyzed.Since the technical details of this approach can be found in[50],we need only focus on the result which combines the expected with the surprising.Letting M(q⊥,t)denote the complex amplitude of the unique slow mode,associated with the band of wave vectors(q⊥,2π/L y),its equation of motion takes the Ginzburg-Landau form:?t M=?{(τ+q2⊥)M+gM|M|2+O(M5)}.Here,τis the soft eigenvalue, which vanishes on the stability boundary and g=g(E,ˉm,γ,L y)is a rather complicated function.As in standard Landau theory,the sign of g determines the order of the transition:if positive,the transition is continuous while neg-ative g signals a?rst order one.Sinceτ?0,we can set E=E H and discuss g on the stability boundary itself:being positive forˉm 1,it has a unique zero at a criticalˉm o(γ),below which it becomes negative.Sinceˉm o(γ)increases withγ,we recover the qualitative behavior of the multicritical line observed in the simulations.The surprising aspect of this equation of motion,however, resides in the fact that M is O(2)-symmetric and q⊥spans just a single spatial dimension.Given these symmetries,the Mermin-Wagner theorem[55]should forbid the existence of long-range order!One might hope that a careful analy-sis of?nite-size e?ects in this system would contribute to the disentanglement of this puzzle.
We conclude with two comments.First,we noted earlier that the caseˉm≡1 is exactly soluble.There are,in fact,two other surfaces in the phase dia-gram,corresponding toγ=1andγ=2,for which certain distributions are exactly known.Settingγ=1implies that the rates for particle-hole and particle-particle exchanges become equal,so that any given particle,e.g.,a positive charge,cannot distinguish between negative charges and holes.Thus, it experiences biased di?usion,equivalent to the non-interacting KLS model. Accordingly,the marginal steady-state distribution of the occupation numbers of one species is uniform,i.e.,P[{n±r}]= {n?r}P[{n+r,n?r}]∝1,and observ-ables pertaining to a single species are trivial.For example,the two-point correlation functions for equal charges,
Second,it is natural to wonder about the consequences of having non-zero
电动机分类及介绍 电动机是一种旋转式电动机器，它将电能改动为机械能，它首要包含一个用以发作磁场的电磁铁绕组或散布的定子绕组和一个旋转电枢或转子。在定子绕组旋转磁场的效果下，其在电枢鼠笼式铝框中有电流转过并受磁场的效果而使其翻滚。这些机器中有些类型可作电动机用，也可作发电机用。它是将电能改动为机械能的一种机器。通常电动机的作功有些作旋转运动，这种电动机称为转子电动机;也有作直线运动的，称为直线电动机。以下为电动机的各种分类。 几种多见电动机介绍直流电动机将直流电能改换为机械能的电动机。因其超卓的调速功用而在电力拖动中得到广泛运用。直流电动机按励磁办法分为永磁、他励和自励3类,其间自励又分为并励、串励和复励3种。沟通电动机将沟通电的电能改动为机械能的一种机器。沟通电动机首要由一个用以发作磁场的电磁铁绕组或散布的定子绕组和一个旋转电枢或转子构成。电动机运用通电线圈在磁场中受力翻滚的景象而制成的。沟通电动机由定子和转子构成，并且定子和转子是选用同一电源，所以定子和转子中电流的方向改动老是同步的。沟通电动机即是运用这个原理而作业的。三相电动机三相电机是指当电机的三相定子绕组(各相差120度电视点)，通入三相
沟通电后，将发作一个旋转磁场，该旋转磁场切开转子绕组，然后在转子绕组中发作感应电流(转子绕组是闭合通路)，载流的转子导体在定子旋转磁场效果下将发作电磁力，然后在电机转轴上构成电磁转矩，驱动电动机旋转，并且电机旋转方向与旋转磁场方向相同。三相异步电动机的三相定子绕组每相绕组都有两个引出线头，总共六个引出线头，别离以U1、U2;V1、V2;W1、W2标明。这六个引出线头引进电机接线盒的接线柱上。单相电动机单相电机通常是指用单相沟通电源(AC220V)供电的小功率单相异步电动机。单相异步电动机通常在定子上有两相绕组，转子是通常鼠笼型的。两相绕组在定子上的散布以及供电状况的纷歧样，能够发作纷歧样的起动特性和作业特性。下图是带正回转开关的接线图，通常这种电机的起动绕组与作业绕组的电阻值是相同的，即是说电机的起动绕组与作业绕组是线径与线圈数完全一同的。通常洗衣机用得到这种电机。这种正回转操控办法简略，不必杂乱的改换开关。步进电动机步进电动机又称脉冲电机，是数字操控体系中的一种首要的施行元件，它是将电脉冲信号改换成转角或转速的施行电动机，其角位移量与输入电脉冲数成正比;其转速与电脉冲的频率成正比。在负载才调方案内，这些联络将不受电源电压、负载、环境、温度等要素的影响，还可在很宽的方案内完毕调速，活络主张、制动和回转。伺服电动
电机及电机学概念 （electric machine and electric machine theory concept） 电机定义：是指依据电磁感应定律实现电能的转换或传递的一种电磁装置。 电动机也称电机（俗称马达），在电路中用字母"M"（旧标准用"D"）表示。它的主要作用是产生驱动转矩，作为用电器或各种机械的动力源。 电动机的种类 1．按工作电源分类根据电动机工作电源的不同，可分为直流电动机和交流电动机。其中交流电动机还分为单相电动机和三相电动机。 2．按结构及工作原理分类电动机按结构及工作原理可分为直流电动机，异步电动机和同步电动机。 同步电动机还可分为永磁同步电动机、磁阻同步电动机和磁滞同布电动机。 异步电动机可分为感应电动机和交流换向器电动机。感应电动机又分为三相异步电动机、单相异步电动机和罩极异步电动机等。交流换向器电动机又分为单相串励电动机、交直流两用电动机和推斥电动机。 直流电动机按结构及工作原理可分为无刷直流电动机和有刷直流电动机。有刷直流电动机可分为永磁直流电动机和电磁直流电动机。电磁直流电动机又分为串励直流电动机、并励直流电动机、他励直流电动机和复励直流电动机。永磁直流电动机又分为稀土永磁直流电动机、铁氧体永磁直流电动机和铝镍钴永磁直流电动机。 3．按起动与运行方式分类电动机按起动与运行方式可分为电容起动式单相异步电动机、电容运转式单相异步电动机、电容起动运转式单相异步电动机和分相式单相异步电动机。 4．按用途分类电动机按用途可分为驱动用电动机和控制用电动机。 驱动用电动机又分为电动工具（包括钻孔、抛光、磨光、开槽、切割、扩孔等工具）用电动机、家电（包括洗衣机、电风扇、电冰箱、空调器、录音机、录像机、影碟机、吸尘器、照相机、电吹风、电动剃须刀等）用电动机及其它通用小型机械设备（包括各种小型机床、小型机械、医疗器械、电子仪器等）用电动机。 控制用电动机又分为步进电动机和伺服电动机等。 5．按转子的结构分类电动机按转子的结构可分为笼型感应电动机（旧标准称为鼠笼型异步电动机）和绕线转子感应电动机（旧标准称为绕线型异步电动机）。 6．按运转速度分类电动机按运转速度可分为高速电动机、低速电动机、恒速电动机、调速电动机。
舌尖上的中国分集简介 第一集自然的馈赠 本集导入作为一个美食家，食物的美妙味感固然值得玩味，但是食物是从哪里来的？毫无疑问，我们从大自然中获得所有的食物，在我们走进厨房，走向餐桌之前，先让我们回归自然，看看她给我们的最初的馈赠。本集将选取生活在中国境内截然不同的地理环境（如海洋，草原，山林，盆地，湖泊）中的具有代表性的个人、家庭和群落为故事主角，以及由于自然环境的巨大差异（如干旱，潮湿，酷热，严寒）所带来的截然不同的饮食习惯和生活方式为故事背景，展现大自然是以怎样不同的方式赋予中国人食物，我们又是如何与自然和谐相处，从而了解在世代相传的传统生活方式中，通过各种不同的途径获取食物的故事。 美食介绍 烤松茸油焖春笋雪菜冬笋豆腐汤 腊味飘香腌笃鲜排骨莲藕汤椒盐藕夹酸辣藕丁煎焖鱼头泡饼煎焗马鲛鱼酸菜鱼松鼠桂鱼侉炖鱼本集部分旁白中国拥有世界上最富戏剧性的自然景观，高原， 第一集: 自然的馈赠 山林，湖泊，海岸线。这种地理跨度有助于物种的形成和保存，任何一个国家都没有这样多潜在的食物原材料。为了得到这份自然的馈赠，人们采集，捡拾，挖掘，捕捞。穿越四季，本集将展现美味背后人和自然的故事。香格里拉，松树和栎树自然杂交林中，卓玛寻找着一种精灵般的食物——松茸。松茸保鲜期只有短短的两天，商人们以最快的速度对松茸进行精致的加工，这样一只松茸24小时之后就会出现在东京的市场中。松茸产地的凌晨3点，单珍卓玛和妈妈坐着爸爸开的摩托车出发。穿过村庄，母女俩要步行走进30公里之外的原始森林。雨让各种野生菌疯长，但每一个藏民都有识别松茸的慧眼。松茸出土后，
卓玛立刻用地上的松针把菌坑掩盖好，只有这样，菌丝才可以不被破坏，为了延续自然的馈赠，藏民们小心翼翼地遵守着山林的规矩。为期两个月的松茸季节，卓玛和妈妈挣到了5000元，这个收入是对她们辛苦付出的回报。 舌尖上的中国DVD 老包是浙江人，他的毛竹林里，长出过遂昌最大的一个冬笋。冬笋藏在土层的下面，从竹林的表面上看，什么也没有，老包只需要看一下竹梢的叶子颜色，就能知道笋的准确位置，这完全有赖于他丰富的经验。笋的保鲜从来都是个很大的麻烦，笋只是一个芽，是整个植物机体活动最旺盛的部分。聪明的老包保护冬笋的方法很简单，扒开松松的泥土，把笋重新埋起来，保湿，这样的埋藏方式就地利用自然，可以保鲜两周以上。在中国的四大菜系里，都能见到冬笋。厨师偏爱它，也是因为笋的材质单纯，极易吸收配搭食物的滋味。老包正用冬笋制作一道家常笋汤，腌笃鲜主角本来应该是春笋，但是老包却使用价格高出20倍的遂昌冬笋。因为在老包眼里，这些不过是自家毛竹林里的一个小菜而已。在云南大理北部山区，醒目的红色砂岩中间，散布着不少天然的盐井，这些盐成就了云南山里人特殊的美味。老黄和他的儿子在树江小溪边搭建一个炉灶，土灶每年冬天的工作就是熬盐。云龙县的冬季市场，老黄和儿子赶到集市上挑选制作火腿的猪肉，火腿的腌制在老屋的院子里开始。诺邓火腿的腌制过程很简单，老黄把多余的皮肉去除，加工成一个圆润的火腿，洒上白酒除菌，再把自制的诺盐均匀的抹上，不施锥针，只用揉、压，以免破坏纤维。即使用现代的标准来判断，诺邓井盐仍然是食盐中的极品，虽然在这个古老的产盐地，盐业生产已经停止，但我们仍然相信诺邓盐是自然赐给山里人的一个珍贵礼物。圣武和茂荣是兄弟俩，每年9月，他们都会来到湖北的嘉鱼县，来采挖一种自然的美味。这种植物生长在湖水下面的深深的淤泥之中，茂荣挖到的植物的根茎叫做莲藕，是一种湖泊中高产的蔬菜——藕。作为职业挖藕人，每年茂荣和圣武要只身出门7个月，采藕的季节，他们就从老家安徽赶到有藕的地方。较高的人工报酬使得圣武和茂荣愿意从事这个艰苦的工作。挖藕的人喜欢天气寒冷，这不是因为天冷好挖藕，而是天气冷买藕吃藕汤的人就多一些，藕的价格就会涨。整整一湖的莲藕还要采摘5个月的时间，在嘉鱼县的珍湖上，300个职业挖藕人，每天从日出延续到日落，在中国遍布淡水湖的大省，这样场面年年上演。今天当我们有权远离自然，享受美食的时候，最应该感谢的是这些通过劳动和智慧成就餐桌美味的人们。 第二集主食的故事 本集导入主食是餐桌上的主要食物，是人们所需能量的主要来源。从远古时代赖以充饥的自然谷物到如今人们餐桌上丰盛的、让人垂涎欲滴的美食，一个异彩纷呈、变化多端的主食世界呈现在你面前。本集着重描绘不同地域、不同民族、不同风貌的有关主食的故事，展现人们对主食的样貌、口感的追求，处理和加工主食的智慧，以及中国人对主食的深
电机 电机泛指能使机械能转化为电能、电能转化为机械能的一切机器。特指发电机、电能机、电动机。是指依据电磁感应定律实现电能的转换或传递的一种电磁装置。电动机也称（俗称马达），在电路中用字母“M”（旧标准用“D”）表示。它的主要作用是产生驱动转矩，作为用电器或各种机械的动力源。 基本介绍 电机是指依据电磁感应定律实现电能的转换或传递的一种电磁装置。电机也称电机（俗称马达），在电路中用字母“M”（旧标准用“D”）表示。它的主要作用是产生驱动转矩，作为用电器或各种机械的动力源。 主要分类 分类概述 1、按工作电源种类划分：可分为直流电机和交流电机。 1）直流电动机按结构及工作原理可划分：无刷直流电动机和有刷直流电动机。 有刷直流电动机可划分：永磁直流电动机和电磁直流电动机。 电磁直流电动机划分：串励直流电动机、并励直流电动机、他励直流电动机和复励直流电动机。 永磁直流电动机划分：稀土永磁直流电动机、铁氧体永磁直流电动机和铝镍钴永磁直流电动机。 2）其中交流电机还可划分：单相电机和三相电机。 2、按结构和工作原理可划分：可分为直流电动机、异步电动机、同步电动机。 1）同步电机可划分：永磁同步电动机、磁阻同步电动机和磁滞同步电动机。 2）异步电机可划分：感应电动机和交流换向器电动机。 感应电动机可划分：三相异步电动机、单相异步电动机和罩极异步电动机等。 交流换向器电动机可划分：单相串励电动机、交直流两用电动机和推斥电动机。 3、按起动与运行方式可划分：电容起动式单相异步电动机、电容运转式单相异步电动机、电容起动运转式单相异步电动机和分相式单相异步电动机。 4、按用途可划分：驱动用电动机和控制用电动机。 1）驱动用电动机可划分：电动工具（包括钻孔、抛光、磨光、开槽、切割、扩孔等工具）用电动机、家电（包括洗衣机、电风扇、电冰箱、空调器、录音机、录像机、影碟机、吸尘器、照相机、电吹风、电动剃须刀等）用电动机及其它通用小型机械设备（包括各种小型机床、小型机械、医疗器械、电子仪器等）用电动机。 2）控制用电动机又划分：步进电动机和伺服电动机等。
《舌尖上的中国》解说词 第一集《自然的馈赠》 中国拥有世界上最富戏剧性的自然景观，高原，山林，湖泊，海岸线。这种地理跨度有助于物种的形成和保存，任何一个国家都没有这样多潜在的食物原材料。为了得到这份自然的馈赠，人们采集，捡拾，挖掘，捕捞。穿越四季，本集将展现美味背后人和自然的故事。 香格里拉，松树和栎树自然杂交林中，卓玛寻找着一种精灵般的食物——松茸。松茸保鲜期只有短短的两天，商人们以最快的速度对松茸的进行精致的加工，这样一只松茸24小时之后就会出现在东京的市场中。 松茸产地的凌晨3点，单珍卓玛和妈妈坐着爸爸开的摩托车出发。穿过村庄，母女俩要步行走进30公里之外的原始森林。雨让各种野生菌疯长，但每一个藏民都有识别松茸的慧眼。松茸出土后，卓玛立刻用地上的松针把菌坑掩盖好，只有这样，菌丝才可以不被破坏，为了延续自然的馈赠，藏民们小心翼翼地遵守着山林的规矩。 为期两个月的松茸季节，卓玛和妈妈挣到了5000元，这个收入是对她们辛苦付出的回报。 老包是浙江人，他的毛竹林里，长出过遂昌最大的一个冬笋。冬笋藏在土层的下面，从竹林的表面上看，什么也没有，老包只需要看一下竹梢的叶子颜色，就能知道笋的准确位置，这完全有赖于他丰富的经验。 笋的保鲜从来都是个很大的麻烦，笋只是一个芽，是整个植物机体活动最旺盛的部分。聪明的老包保护冬笋的方法很简单，扒开松松的泥土，把笋重新埋起来，保湿，这样的埋藏方式就地利用自然，可以保鲜两周以上。 在中国的四大菜系里，都能见到冬笋。厨师偏爱它，也是因为笋的材质单纯，极易吸收配搭食物的滋味。老包正用冬笋制作一道家常笋汤，腌笃鲜主角本来应该是春笋，但是老包却使用价格高出20倍的遂昌冬笋。因为在老包眼里，这些不过是自家毛竹林里的一个小菜而已。 在云南大理北部山区，醒目的红色砂岩中间，散布着不少天然的盐井，这些盐成就了云南山里人特殊的美味。老黄和他的儿子树江小溪边搭建一个炉灶，土灶每年冬天的工作就是熬盐。 云龙县的冬季市场，老黄和儿子赶到集市上挑选制作火腿的猪肉，火腿的腌制在老屋的院子里开始。诺邓火腿的腌制过程很简单，老黄把多余的皮肉去除，加工成一个圆润的火腿，洒上白酒除菌，再把自制的诺盐均匀的抹上，不施锥针，只用揉、压，以免破坏纤维。 即使用现代的标准来判断，诺邓井盐仍然是食盐中的极品，虽然在这个古老的产盐地，盐业生产已经停止，但我们仍然相信诺邓盐是自然赐给山里人的一个珍贵礼物。 圣武和茂荣是兄弟俩，每年9月，他们都会来到湖北的嘉鱼县，来采挖一种自然的美味。这种植物生长在湖水下面的深深的淤泥之中，茂荣挖到的植物的根茎叫做莲藕，是一种湖泊中高产的蔬菜——藕。
《舌尖上的中国》电视节目策划书 一、背景浅析 改革开放以来，国人物质生活更加富足，在吃方面也变得更加讲究。食客、美食家、吃货等的出现，则从另一方面体现国人口味的变化。衣食住行为人之根本，古人读万卷书、行万里路已是满足，今人在此之外加上品万种美食，才称得上是真正的享受生活。而中国幅员辽阔，有八大菜系，淮扬菜、粤菜、鲁菜、湘菜，每种菜系都有其特色美食，而且烹调方法各有不同，可以说，国人够有口福的了。在《舌尖上的中国》这部纪录片中，导演和摄制组带着观众探访祖国各地美食，了解各式各样与美食有关的人和事，总共七集、每集50分钟的容量，节奏紧凑、制作精良，尝遍美食的同时又能遍览祖国风物，纪录片煞是好。而随着中国经济的飞速发展，人们的物质生活水平也得到了极大地提高，已经不再局限于吃饱穿暖这样基本的要求。民以食为天，在物质文明丰富至如斯的今天，人们开始重视对于生活品质更高层次的追求。对于吃，食不厌精、烩不厌细，不只吃味道，还要吃环境、吃服务、吃文化，吃健康。 二、企划动机 《舌尖上的中国》这部以美食为主题的纪录片，前些日子风头力压各种电视连续剧，成为连日来的微博“刷屏利器”并高居话题榜榜首，甚至让许多早已抛弃了电视的80后“吃货”们，纷纷锁定夜间的央视坐等这部“吃货指南”。人们为美食流口水，为美食的故事流眼泪，为美食的文化而感动自豪，自豪于中国饮食文化的博大精深，甚至有人高调地赞美“这才是最好的爱国主义教育片”。看要应对新的发展形势，就必须有新的态度与措施。饮食行业发展到今日，大街小巷、五花八门，各种价位、各种地域乃至跨国界的美食比比皆是。如何从
千篇一律的店面经营中脱颖而出？不仅仅是商家绞尽脑汁，着力打开销售额与知名度。美食老饕们也寻寻觅觅，力图享受到有特色、有内涵的精彩美食。《舌尖上的中国》这档美食资讯节目，是继《舌尖上的中国》这个记录片之后的后续美食资讯类节目，更实现其为商家和消费者两者搭建一个交流的平台，优秀的商家可以尽展己之所长，以此为基础展示自身特色美食功夫。而美食爱好者们则可以通过节目的推介，节省大量的时间、精力，接触到平时踏破铁鞋无觅处，只能通过口口相传难辨优劣，隐藏在街头巷尾的各种美食。大可以凭此慕名而来、亦可尽兴而去。 一、节目名称——《舌尖上的中国》 二、节目类别——继纪录片《舌尖上的中国》之后，观众参与度高的一档各地美食资讯节目 三、节目主旨——让爱美食的人走进节目让看节目的人走近美食 四、节目目标 借助最近《舌尖上的中国》的热点，重点打造一档精品的美食资讯节目，不仅仅做美食，还囊括了相关的综合资讯。力图达到“美食主动靠过来，观众自然看进去”的效果，打造口碑，铸就精品。 五、节目定位 这是一档集继纪录片《舌尖上的中国》之后的观众参与度高的一档各地美食资讯节目。将美食推介节目与菜肴烹饪节目于一身，力图更好的开发各地特色美食资源的服务资讯类美食节目，兼顾一定的娱乐性质。采用立体全面的推介方式，为现场和电视机前的观众提供优质精选的美食信息，商家现场展示、与美食爱好者们现场沟通交流。
《舌尖上的中国》解说词1-7集 第一季：1.自然的馈赠 作为一个美食家，食物的美妙味感固然值得玩味，但是食物是从哪里来的？毫无疑问，我们从大自然中获得所有的食物，在我们走进厨房，走向餐桌之前，先让我们回归自然，看看她给我们的最初的馈赠。 本集将选取生活在中国境内截然不同的地理环境(如海洋,草原,山林,盆地,湖泊)中的具有代表性的个人、家庭和群落为故事主角，以及由于自然环境的巨大差异(如干旱，潮湿，酷热，严寒)所带来的截然不同的饮食习惯和生活方式为故事背景，展现大自然是以怎样不同的方式赋予中国人食物，我们又是如何与自然和谐相处，从而了解在世代相传的传统生活方式中，通过各种不同的途径获取食物的故事。 第一季：2.主食的故事 主食是餐桌上的主要食物，是人们所需能量的主要来源。从远古时代赖以充饥的自然谷物到如今人们餐桌上丰盛的、让人垂涎欲滴的美食，一个异彩纷呈、变化多端的主食世界呈现在你面前。本集着重描绘不同地域、不同民族、不同风貌的有关主食的故事，展现人们对主食的样貌、口感的追求，处理和加工主食的智慧，以及中国人对主食的深厚情感。 第一季：3.转化的灵感 腐乳、豆豉、黄酒、泡菜，都有一个共同点，它们都具有一种芳香浓郁的特殊风味。这种味道是人与微生物携手贡献的成果。而这种手法被称作“发酵”。中国人的老祖宗，用一些坛坛罐罐，加上敏锐的直觉，打造了一个食物的新境界。要达到让食物转化成美食的境界，这其中要逾越障碍，要营造条件，要把握机缘，要经历挫败，从而由“吃”激发出最大的智慧。 第一季：4.时间的味道 腌制食品，风干晾晒的干货，以及酱泡、冷冻等是中国历史最为久远的食物保存方式。时至今日，中国人依然对此类食品喜爱有加。 本集涉及的美食主要有腊肉，火腿，烧腊，咸鱼(腌鱼)，腌菜，泡菜，渍菜，以及盐渍，糖渍，油浸，晾晒，风干，冷冻等不同食物保存方法，展现以此为基础和原材料的各种中国美食。贮藏食物从早先的保存食物 方便携带发展到人们对食物滋味的不断追求，保鲜的技术中蕴涵了中国人的智慧，呈现着中国人的生活，同时“腌制发酵保鲜”也蕴含有中国人的情感与文化意象，如对故乡的思念，内心长时间蕴含的某种情感等等。第一季：5.厨房的秘密 与西方“菜生而鲜，食分而餐”的饮食传统文化相比，中国的菜肴更讲究色、香、味、形、器。而在这一系列意境的追逐中，中国的厨师个个都像魔术大师，都能把“水火交攻”的把戏玩到如火纯青的地步，这是 8000年来的修炼。我们也在这漫长的过程中经历了煮、蒸、炒三次重要 的飞跃，他们共同的本质无非是水火关系的调控，而至今世界上懂得蒸菜和炒菜的民族也仅此一家。本集将主要透过与具有精湛美食技艺的人有关的故事，一展中国人在厨房中的绝技。 第一季：6.五味的调和
电动摩托车用小型电机的种类、特性报告电动机是电动摩托车的一个重要部件，目前在电动摩托车驱动领域，主要有直流电动机驱动系统、交流永磁电动机驱动系统和开关磁阻电动机驱动系统应用范围比交广泛。 1 直流电动机驱动系统 1.1 直流电机的工作原理 下图为直流电动机的物理模型，N、S为定子磁极，abcd是固定在可旋转导磁圆柱体上的线圈，线圈连同导磁圆柱体称为电机的转子或电枢。线圈的首末端a、d连接到两个相互绝缘并可随电枢一同旋转的换向片上。转子线圈与外电路的连接是通过放置在换向片上固定不动的电刷进行的。换向片与电刷滑动接触。把电刷A、B接到直流电源上，电刷A接正极，电刷B接负极。此时电枢线圈中将有电流流过，根据左手定则，电枢逆时针旋转。 当电枢旋转几何中性线时，F=0，电枢由于惯性继续运转。取下图所示位置，原N极性下导体ab转到S极下，受力方向从左向右，原S 极下导体cd转到N极下，
受力方向从右向左，该电磁力形成逆时针方向的电磁转矩，线圈在该电磁力形成的电磁转矩作用下继续逆时针方向旋转，直流电动机在直流电源作用下转动起来。 1.2 直流电机的数学方程 1.2.1 电枢中感应电动势 电枢中通入直流电流后，产生电磁转矩，使电机在磁场中转动。而通电线圈在磁场中转动，又会在线圈中产生感应电动势E 。 感应电动势为：n K E E φ= 式中，K E ——与电机结构相关的参数； Φ——电机每极磁通量，Wb ； n ——电机转速，r/min 。 1.2.2 电枢电压方程 如图所示，a a R I E U += M U R a + - E
式中，U ——外接电源电压，V ； I a ——电枢电流，A ； R a ——电枢电阻，Ω。 1.2.3 电磁转矩方程 式中，T ——电磁转矩，Nm ； K T ——与电机结构相关的常数。 1.3 直流电机的机械特性 直流电机按照励磁方式可分为两种，永磁式和电励磁式。电磁式是由磁性材料提供磁场；电励磁式是由磁极上绕线圈，在线圈中通入直流电，来产生电磁场。根据励磁线圈和转子绕组的连接关系，励磁式的直流电机可以分为：串励电机、他励电机、并励电机、复励电机。 1.3.1 串励直流电机的机械特性 串励直流电机数学模型： 式中，R f ——串励绕组电阻，Ω。 K Φ——励磁系数。 由此可推出： 这种励磁方式的特点是：当电压U 一定时，随负载转矩的增大，n 下降很快， n T T 0
《舌尖上的中国》7集（全）解说词 《舌尖上的中国》是纪录频道推出的第一部高端美食类系列纪录片，从2011年3月开始大规模拍摄，是国内第一次使用高清设备拍摄的大型美食类纪录片。共在国内拍摄60个地点方，涵盖了包括港澳台在内的中国各个地域，它全方位展示博大精深的中华美食文化。向观众，尤其是海外观众展示中国的日常饮食流变，千差万别的饮食习惯和独特的味觉审美，以及上升到生存智慧层面的东方生活价值观，让观众从饮食文化的侧面认识和理解传统和变化着的中国。 而台词优美而清新，听着解说看着画面，让人心醉··· 第一集《自然的馈赠》 中国拥有世界上最富戏剧性的自然景观，高原，山林，湖泊，海岸线。这种地理跨度有助于物种的形成和保存，任何一个国家都没有这样多潜在的食物原材料。为了得到这份自然的馈赠，人们采集，捡拾，挖掘，捕捞。穿越四季，本集将展现美味背后人和自然的故事。 香格里拉，松树和栎树自然杂交林中，卓玛寻找着一种精灵般的食物——松茸。松茸保鲜期只有短短的两天，商人们以最快的速度对松茸的进行精致的加工，这样一只松茸24小时之后就会出现在东京的市场中。 松茸产地的凌晨3点，单珍卓玛和妈妈坐着爸爸开的摩托车出发。穿过村庄，母女俩要步行走进30公里之外的原始森林。雨让各种野生菌疯长，但每一个藏民都有识别松茸的慧眼。松茸出土后，卓玛立刻用地上的松针把菌坑掩盖好，只有这样，菌丝才可以不被破坏，为了延续自然的馈赠，藏民们小心翼翼地遵守着山林的规矩。 为期两个月的松茸季节，卓玛和妈妈挣到了5000元，这个收入是对她们辛苦付出的回报。老包是浙江人，他的毛竹林里，长出过遂昌最大的一个冬笋。冬笋藏在土层的下面，从竹林的表面上看，什么也没有，老包只需要看一下竹梢的叶子颜色，就能知道笋的准确位置，这完全有赖于他丰富的经验。 笋的保鲜从来都是个很大的麻烦，笋只是一个芽，是整个植物机体活动最旺盛的部分。聪明的老包保护冬笋的方法很简单，扒开松松的泥土，把笋重新埋起来，保湿，这样的埋藏方式就地利用自然，可以保鲜两周以上。 在中国的四大菜系里，都能见到冬笋。厨师偏爱它，也是因为笋的材质单纯，极易吸收配搭食物的滋味。老包正用冬笋制作一道家常笋汤，腌笃鲜主角本来应该是春笋，但是老包却使用价格高出20倍的遂昌冬笋。因为在老包眼里，这些不过是自家毛竹林里的一个小菜而已。在云南大理北部山区，醒目的红色砂岩中间，散布着不少天然的盐井，这些盐成就了云南山里人特殊的美味。老黄和他的儿子树江小溪边搭建一个炉灶，土灶每年冬天的工作就是熬盐。云龙县的冬季市场，老黄和儿子赶到集市上挑选制作火腿的猪肉，火腿的腌制在老屋的院子里开始。诺邓火腿的腌制过程很简单，老黄把多余的皮肉去除，加工成一个圆润的火腿，洒上白酒除菌，再把自制的诺盐均匀的抹上，不施锥针，只用揉、压，以免破坏纤维。 即使用现代的标准来判断，诺邓井盐仍然是食盐中的极品，虽然在这个古老的产盐地，盐业生产已经停止，但我们仍然相信诺邓盐是自然赐给山里人的一个珍贵礼物。 圣武和茂荣是兄弟俩，每年9月，他们都会来到湖北的嘉鱼县，来采挖一种自然的美味。这
第一集《自然的馈赠》 中国拥有世界上最富戏剧性的自然景观，高原，山林，湖泊，海岸线。这种地理跨度有助于物种的形成和保存，任何一个国家都没有这样多潜在的食物原材料。为了得到这份自然的馈赠，人们采集，捡拾，挖掘，捕捞。穿越四季，本集将展现美味背后人和自然的故事。 香格里拉，松树和栎树自然杂交林中，卓玛寻找着一种精灵般的食物——松茸。松茸保鲜期只有短短的两天，商人们以最快的速度对松茸的进行精致的加工，这样一只松茸24小时之后就会出现在东京的市场中。 松茸产地的凌晨3点，单珍卓玛和妈妈坐着爸爸开的摩托车出发。穿过村庄，母女俩要步行走进30公里之外的原始森林。雨让各种野生菌疯长，但每一个藏民都有识别松茸的慧眼。松茸出土后，卓玛立刻用地上的松针把菌坑掩盖好，只有这样，菌丝才可以不被破坏，为了延续自然的馈赠，藏民们小心翼翼地遵守着山林的规矩。 为期两个月的松茸季节，卓玛和妈妈挣到了5000元，这个收入是对她们辛苦付出的回报。 老包是浙江人，他的毛竹林里，长出过遂昌最大的一个冬笋。冬笋藏在土层的下面，从竹林的表面上看，什么也没有，老包只需要看一下竹梢的叶子颜色，就能知道笋的准确位置，这完全有赖于他丰富的经验。 笋的保鲜从来都是个很大的麻烦，笋只是一个芽，是整个植物机体活动最旺盛的部分。聪明的老包保护冬笋的方法很简单，扒开松松的泥土，把笋重新埋起来，保湿，这样的埋藏方式就地利用自然，可以保鲜两周以上。 在中国的四大菜系里，都能见到冬笋。厨师偏爱它，也是因为笋的材质单纯，极易吸收配搭食物的滋味。老包正用冬笋制作一道家常笋汤，腌笃鲜主角本来应该是春笋，但是老包却使用价格高出20倍的遂昌冬笋。因为在老包眼里，这些不过是自家毛竹林里的一个小菜而已。 在云南大理北部山区，醒目的红色砂岩中间，散布着不少天然的盐井，这些盐成就了云南山里人特殊的美味。老黄和他的儿子树江小溪边搭建一个炉灶，土灶每年冬天的工作就是熬盐。 云龙县的冬季市场，老黄和儿子赶到集市上挑选制作火腿的猪肉，火腿的腌制在老屋的院子里开始。诺邓火腿的腌制过程很简单，老黄把多余的皮肉去除，加工成一个圆润的火腿，洒上白酒除菌，再把自制的诺盐均匀的抹上，不施锥针，只用揉、压，以免破坏纤维。 即使用现代的标准来判断，诺邓井盐仍然是食盐中的极品，虽然在这个古老的产盐地，盐业生产已经停止，但我们仍然相信诺邓盐是自然赐给山里人的一个珍贵礼物。 圣武和茂荣是兄弟俩，每年9月，他们都会来到湖北的嘉鱼县，来采挖一种自然的美味。这种植物生长在湖水下面的深深的淤泥之中，茂荣挖到的植物的根茎叫做莲藕，是一种湖泊中高产的蔬菜——藕。 作为职业挖藕人，每年茂荣和圣武要只身出门7个月，采藕的季节，他们就从老家安徽赶到有藕的地方。较高的人工报酬使得圣武和茂荣愿意从事这个艰苦的工作。挖藕的人喜欢天气寒冷，这不是因为天冷好挖藕，而是天气冷买藕吃藕汤的人就多一些，藕的价格就会涨。
《舌尖上的中国》中提到的所有美食统计 第一集自然的馈赠 1，香格里拉松茸 2，江浙地区冬笋油焖冬笋 3，广西柳州酸笋黄豆酸笋小黄鱼 4，云南大理诺邓山区诺邓盐血肠火腿莴笋炒火腿火腿炒饭 5，湖北嘉鱼藕莲藕炖排骨 6，吉林查干湖湖水大鱼鱼头泡饼（北京） 7，海南香煎马鲛鱼酸菜鱼汤水煮红螺 第二集主食的故事 1，山西襄汾县花馍花卷油卷 2，陕西绥德黄馍馍（糜子面） 3，新疆库车馕饼 4，中原地区馒头（馍馍，古时又叫炊饼，现在想通武大郎为啥喊“炊饼”了） 长期发展形成南稻北麦，2000年前五谷排行顺序：稻，黍shu，稗bai，麦，菽shu；现在以稻，麦，玉米为主。5，贵州黎平米粉汤粉 6，广州沙河河粉干炒牛河 7，陕西西安凉皮汉中米皮（这是本人自己加上的，因为说到米粉，河粉，如果不说凉皮就说不过去了，导 演组失职啊） 8，陕西肉夹馍，牛羊肉泡馍，粉蒸肉夹馍（也叫荷叶饼夹馍，这个也是按自己加的） 9，兰州拉面 10，广州竹升面云吞捞面 11，中原地区手擀面 12，陕西岐山臊子面 13，嘉兴粽肉子蛋黄棕（关于粽子南方有肉粽，蛋黄粽等，北方一般都是蜜枣棕，北方人吃不惯肉粽，咸棕）
15，北方饺子焖面（陕西河南） 第三集转化的灵感 本集介绍三大部分1豆腐，2，酒，3醋（第六集时会讲一下醋所以这集就说了一下），4酱油，5酱油，6大酱 豆腐篇 1，云南红河建木县碳烤豆腐球石屏县老豆腐 2，中原地区石膏豆腐（各种豆制品，相信大家都是很喜欢吃豆腐的吧 ，尤其是嫩的。） 3，内蒙古锡林郭勒旗奶茶奶豆腐奶制品 4，云南白族豆腐皮 5，北京蒙古餐厅烤羊背 6，浙江天台山僧人的素食中豆制品很重要 7，安徽毛豆腐 酒篇 8，绍兴黄酒 9，安徽休林糯米酒 10 ，安昌镇腊肠 11，东北大豆酱酸菜 第四集时间的味道 这一集是介绍腌制品，脱水，酱菜 1，黑龙江绥化市朝鲜族泡菜， 2，广粤地区腊肠各种腊制品煲仔饭荔芋腊鸭煲南安腊鸭 3，湖南靖州县腌鱼腊鸭 4，徽州臭鳜鱼 5，安徽黔县腊八豆腐刀板鱼 黄山火腿咸肉 6，浙江金华火腿蜜汁上方 7，上海的三阳南货店经营各种腌制品腊肉等： 杭州酱鸭 上海腌笃鲜 温州黄鱼鲞xiang 宁波笋干 绍兴梅干菜梅干菜烧肉 8，上海醉虾醉蟹 9，福建霞浦紫菜 10，台湾云朴县乌鱼子 11，香港大奥海盐产地咸鱼虾膏虾酱 12，中原地区各种酱菜（腌萝卜，鬼子姜，辣椒，黄瓜各种蔬菜） 第五集厨房的秘密 这一集最后一句“厨房的秘密就是没有秘密”很是狗血啊。
电机与拖动基础讨论课课题：各种电机应用现状调研 班级：13级工程机械1班 组员：李昊天张晨阳崔超钰 陈杰董亚飞潘帅 时间：2016年4月5日 指导老师：马云飞
目录 前言 (3) 1、电机产业现状 (4) 2、电机行业格局 (4) 3、电机行业前景预测 (4) 4、电机的分类及应用概述 (5) 5、信号电机 (6) 5.1、位置信号电机 (6) 5.2、感应同步器 (6) 5.3、自整角机 (7) 5.4、速度信号电机 (7) 6、功率电动机 (8) 6.1、直流电动机 (8) 6.2、交流电机 (8) 6.3、同步电动机 (9) 6.4、异步电动机 (10) 7、控制电机 (10) 7.1伺服电机 (10) 7.2、步进电动机 (11) 7.3、力矩电动机 (12) 7.4、开关磁阻电机 (13) 7.5、无刷直流电动机 (13) 参考文献 (15)
前言 从广义上讲，电机是电能的变换装置，包括旋转电机和静止电机。旋转电机是根据电磁感应原理实现电能与机械能之间相互转换的一种能量转换装置；静止电机是根据电磁感应定律和磁势平衡原理实现电压变化的一种电磁装置，也称其为变压器。本文我们主要讨论旋转电机，旋转电机的种类很多，在现代工业领域中应用极其广泛，可以说，有电能应用的场合都会有旋转电机的身影。与内燃机和蒸汽机相比，旋转电机的运行效率要高的多；并且电能比其它能源传输更方便、费用更廉价，此外电能还具有清洁无污、容易控制等特点，所以在实际生活中和工程实践中，旋转电机的应用日益广泛。不同的电机有不同的应用场合，随着电机制造技术的不断发展和对电机工作原理研究的不断深入，目前还出现了许多新型的电机，例如，美国EAD公司研制的无槽无刷直流电动机，日本SERVO公司研制的小功率混合式步进电机，我国自行研制适用于工业机床和电动自行车上的大力矩低转速电机等。本文下面主要讨论部分电机种类及应用情况。
舌尖上的中国中英文介绍 《舌尖上的中国》第二季于2013年1月10日在京正式启动，该片于2014年4月18日至6月6日，每周五在中央电视台综合频道（CCTV-1）21点档、中央电视台纪录频道（CCTV-9）22点档同步开播，同时在爱奇艺，乐视网等网络平台播出。 A bite of China2 was started on January 10th,2013 in Beijing. This documentary will be on from April 18th,2014 to June 6th,2014.Each Friday, audience can enjoy this documentary in CCTV1 at 21.pm, or CCTV9,22pm.In the meanwhile, it shows in IQIYI or Letv and other network platform. 《舌尖上的中国》讲述了从远古时代赖以充饥的自然谷物到如今人们餐桌上丰盛的、让人垂涎欲滴的美食，一个异彩纷呈、变化多端的主食世界呈现在你面前。这部纪录片将着重描绘不同地域、不同民族、不同风貌的有关主食的故事，展现人们对主食的样貌、口感的追求，处理和加工主食的智慧，以及中国人对主食的深厚情感。据导演透露《舌尖上的中国2》将加重川菜的部分。 A bite of China describes food’s transition from ancient to nowadays. In ancient, humans just could solve the hunger , they could not eat delicious food. But now, there are various of food on people’s dinner-table. A colorful and changeable major food world emerges your eyes. This documentary pays most attention on describing the stories about major food in different areas, different races and features. It shows the intelligence about Chinese. They went after food’s features, tastes, solution and process. Of course, it also reflects Chinese feelings. From the director, A bite of China will introduce the food of Sichuan most detail. 《舌尖上的中国2》共分为《脚步》、《心传》、《时节》、《家常》、《秘境》、《相逢》、《三餐》，第八集则为拍摄花絮。每集50分钟 A bite of China covers 8 segments, including 《Steps》，《Heart inheriting》，《Seasons》,《The daily life of a family》，《Nulls》，《Meeting》，《Three meals》. The eighth part is tidbits. Each section lasts 50 minutes. 第一集《脚步》 The first segment:《Steps》. 【汗水中的苦辣酸甜】 Four tastes in sweat, bitter, hot, acid and sweet. “路菜”是先人保存食物的智慧，进而被演化成标志性的中国美食。味觉记忆的强大，往往让人们对故乡食物的迷恋十分牢固，甚至被赋予“乡愁”这样的文学语汇。舌尖第二季分集《脚步》，将跟随那些奔波在路上的人们，品尝辛劳与汗水中的苦辣酸甜。 “Lucai” is wisdom of Chinese ancestor which comes from food’s keeping. It has became a symbol of Chinese delicious food. The strong strength of gustation
第一章概述 第一节电机的定义 电机可泛指所有实施电能生产、传输、使用和电能特性变换的机械或装置。然而，由于生产、传输、使用电能和电能特性变换的方式很多，原理各异，如机械摩擦、电磁感应、光电效应、磁光效应、热电效应、压电效应、化学效应等等，内容广泛，不可能由一门课程包括。电机学的主要研究范畴仅限于那些依据电磁感应定律和电磁力定律实现机电能量转换和信号传递与转换的装置。 一、电机的能量交换原理 根据电机学定义，电机是一种将电能转换成机械能或将机械能转换成电能的设备，其中将机械能变为电能的称发电机；将电能变为机械能的称为电动机。电机实现能量转换都基于两个基本定律： 1、法拉第-楞次定律（感应电动势定律） 导体在磁场中运动切割磁力线，在导体两端必然会产生感应电动势，感应电动势e的大小与导体运动速度v、导体的长度L以及磁场的磁感应强度B成正比，即：e=BLV 感应电动势的方向符合右手定律。 2、毕奥-萨戈尔定律（电磁力定律） 载流导体在磁场中必然会受到电磁力的作用。电磁力的大小与导体所载电流I、磁场的磁感应强度B以及导体的长度L成正比，即：F=BIL 电磁力的方向符合左手定律。 所有电机都是根据上述两个定律基本原理设计的。电机类型很多，结构类型很多，结构形式也各不相同，但其能量转换和转矩产生原理是相同的，由于励磁方式不同，各类电机磁场性能和变化不同，才有电机的不同类型和不同特性。 电机内部实现能量转换包括电系统、耦合系统和机械系统三个环节，如图1-1。电动机通常是在其接线端子上输入电能，在轴上输出机械功率。外电路电能通过耦合系统在在机械系统中产生机械能，在机械系统轴头以机械能输出。发电机则是在其轴上输入机械功率，其端子上输出电功率。驱动机械的机械能通过耦合系统，在电系统中产生电能输出。在上述能量转换过程中，三个系统都会产生部分能量损失，最终都以热量形式向周围环境散逸。
哈尔滨工业大学寒假社会调查报告 题目：电机种类及其使用情况调查 指导教师：江善林 院系：电气学院电气工程系 班级：11306108 姓名：况麒麒 学号：1130610823 日期：2013年03月01日
电机类型及其使用情况调查 1 调查背景 从1820年丹麦物理教学家奥斯特发现了电流的磁效应以来，各种发现不断地涌现出来。1821年，英国科学家法拉第总结了载流导体在磁场中受力并发生机械运动的现象他的模型可以认为是现代直流电动机的雏形。1824年，阿拉果发现了旋转磁场，为交流感应电动机的发明奠定了基础。1831年，法拉第发现了电磁感应定律，并发明了单极直流电动机。1834年俄国物理学家雅可比设计并制成了第一台实用的直流电动机。1864年，英国物理学家麦克斯韦创立了完整的经典电磁学理论体系为电机电磁场分析奠定了基础。1888年，特斯拉发明了感应电动机。随着时代的进步，各种各样功能不同的电机被不断地发明出来。 电机的发展历史大致可分为四个阶段，1. 直流电机阶段，2. 交流电机阶段，3. 控制电机阶段，4. 特种电机阶段。 本次调查主要是想通过调查能让我们加深对电机的了解，为以后的学习工作打下一定的基础。 2 调查内容 2.1 时间安排：本次调查主要安排在寒假假期。 2.2 调查对象：本次调查主要调查电机的类型和电机的使用情况。 2.3 调查内容：电机类型有很多，分类方法也各种各样。下面主要介绍按工作电源和按结构和工作原理两种分类方法中的电机。 1. 按工作电源分，包括直流电动机和交流电动机，其中交流电动机又分为单相电动机和三相电动机；直流电动机分为无刷直流电动机和有刷直流电动机，有刷直流电动机又分为永磁直流电动机和电磁直流电动机。永磁直流电动机包括稀土永磁直流电机，铁氧体永磁直流电机，铝镍钴永磁直流电机三类; 电磁直流电机包括串励直流电机，并励直流电机，和复励直流电机三类。 直流电动机 无刷直流 电动机 永磁直流电动机 稀土永磁直流电动机 铁氧体永磁直流电动机 电动机 交流电动机 有刷直流 电动机 电磁直流电动机 铝镍钴永磁直流电动机 串励直流电动机 并励直流电动机 他励直流电动机 复励直流电动机 单相电动机 三相电动机
步进电机的种类、结构及工作原理 步进式伺服驱动系统是典型的开环控制系统。在此系统中，执行元件是步进电机。它受驱动控制线路的控制，将代表进给脉冲的电平信号直接变换为具有一定方向、大小和速度的机械转角位移，并通过齿轮和丝杠带动工作台移动。由于该系统没有反馈检测环节，它的精度较差，速度也受到步进电机性能的限制。但它的结构和控制简单、容易调整，故在速度和精度要求不太高的场合具有一定的使用价值。 1.步进电机的种类 步进电机的分类方式很多，常见的分类方式有按产生力矩的原理、按输出力矩的大小以及按定子和转子的数量进行分类等。根据不同的分类方式，可将步进电机分为多种类型，如表5-1所示。 表5-1 步进电机的分类 分类方式具体类型 按力矩产生的原理（1）反应式：转子无绕组，由被激磁的定子绕组产生反应力矩实现 步进运行 （2）激磁式：定、转子均有激磁绕组（或转子用永久磁钢），由电 磁力矩实现步进运行 按输出力矩大小 （1）伺服式：输出力矩在百分之几之几至十分之几（N·m）只能驱动较小的负载，要与液压扭矩放大器配用，才能驱动机床工作台等较大 的负载 （2）功率式：输出力矩在5-50 N·m以上，可以直接驱动机床工作 台等较大的负载 按定子数（1）单定子式（2）双定子式（3）三定子式（4）多定子式按各相绕组分布 （1）径向分布式：电机各相按圆周依次排列 （2）轴向分布式：电机各相按轴向依次排列 2.步进电机的结构
目前，我国使用的步进电机多为反应式步进电机。在反应式步进电机中，有轴向分相和径向分相两种，如表5--1所述。 图5--2是一典型的单定子、径向分相、反应式伺服步进电机的结构原理图。它与普通电机一样，分为定子和转子两部分，其中定子又分为定子铁心和定子绕组。定子铁心由电工钢片叠压而成，其形状如图中所示。定子绕组是绕置在定子铁心6个均匀分布的齿上的线圈，在直径方向上相对的两个齿上的线圈串联在一起，构成一相控制绕组。图5--2所示的步进电机可构成三相控制绕组，故也称三相步进电机。若任一相绕组通电，便形成一组定子磁极，其方向即图中所示的NS极。在定子的每个磁极上，即定子铁心上的每个齿上又开了5个小齿，齿槽等宽，齿间夹角为9°，转子上没有绕组，只有均匀分布的40个小齿，齿槽也是等宽的，齿间夹角也是9°，与磁极上的小齿一致。此外，三相定子磁极上的小齿在空间位置上依次错开1/3齿距，如图5--3所示。当A相磁极上的小齿与转子上的小齿对齐时，B相磁极上的齿刚好超前(或滞后)转子齿1/3齿距角，C相磁极齿超前(或滞后)转子齿2/3齿距角。 图5-2 单定子径向分相反应式伺服步进电机结构原理图