Huygens–Fresnel principle: Difference between revisions

From formulasearchengine
Jump to navigation Jump to search
en>EmausBot
m r2.7.2+) (Robot: Modifying ko:하위헌스 원리
 
Line 1: Line 1:
Are you constantly having problems with your PC? Are we constantly trying to find methods to strengthen PC performance? Then this is the article you have been interested in. Here we will discuss a few of the most asked concerns when it comes to having we PC serve you well; how can I create my computer quicker for free? How to make my computer run quicker?<br><br>You are able to reformat your computer to make it run faster. This usually reset your computer to when we first used it. Always remember to back up all files plus programs before doing this since this may remove a files from the database. Remember before you do this we need all motorists plus installation files and this ought to be a last resort should you are trying to find slow computer tricks.<br><br>It doesn't matter whether you're not especially obvious about what rundll32.exe is. However remember that it plays an important role in maintaining the stability of our computers and the integrity of the system. When several software or hardware couldn't reply usually to a program operation, comes the rundll32 exe error, that might be caused by corrupted files or missing information inside registry. Usually, error content will shows up at booting or the beginning of running a program.<br><br>The computer was truly quickly when we first bought it. Because a registry was very clean and without errors. After time, the computer begins to run slow and freezes up today plus then. Because there are errors accumulating in it and several information is rewritten or even completely deleted by the wrong uninstall of programs, improper operations, malware or alternative factors. That is the reason why the computer performance decreases gradually and become fairly unstable.<br><br>There are a lot of [http://bestregistrycleanerfix.com/registry-reviver registry reviver] s. Which one is the best is not effortless to be determined. But when you wish To stand out one among the multitude you should consider several items. These are features, scanning speed time, total mistakes detected, total mistakes repaired, tech help, Boot time performance and price. According to these products Top Registry Cleaner for 2010 is RegCure.<br><br>If you think which there are issues with all the d3d9.dll file, then you need to substitute it with a modern working file. This can be completed by performing a series of steps plus you can start by obtaining "d3d9.zip" within the server. Then you have to unzip the "d3d9.dll" file on the hard drive of the computer. Proceed by locating "C:\Windows\System32" and then finding the existing "d3d9.dll" on a PC. Once found, rename the file "d3d9.dll to d3d9BACKUP.dll" and then copy-paste this modern file to "C:\Windows\System32". After that, press "Start" followed by "Run" or look "Run" on Windows Vista & 7. As soon as a box shows up, kind "cmd". A black screen may then appear plus we have to sort "regsvr32d3d9.dll" and then click "Enter". This task will enable you to substitute the old file with the fresh copy.<br><br>As the hub center of the computer, all of the significant settings are stored the registry. Registry is structured because keys and every key relates to a system. The system reads the keys and utilizes the info to launch plus run programs. However, the big issue is that there are too numerous unwanted settings, useless info occuping the useful room. It makes the program run slowly and huge amounts of settings become unreadable.<br><br>Registry cleaners have been tailored to fix all of the broken files inside your program, allowing the computer to read any file it wants, when it wants. They function by scanning by the registry and checking each registry file. If the cleaner sees that it is corrupt, then it may substitute it automatically.
In [[quantum mechanics]], the '''Hamiltonian''' is the [[Operator (physics)|operator]] corresponding to the total [[energy]] of the system. It is usually denoted by ''H'', also ''Ȟ'' or ''Ĥ''. Its [[Spectrum of an operator|spectrum]] is the set of possible outcomes when one measures the total energy of a system.  Because of its close relation to the time-evolution of a system, it is of fundamental importance in most formulations of quantum theory.
 
==Introduction==
 
{{main|Operator (physics)#Operators in quantum mechanics}}
 
The Hamiltonian is the sum of the kinetic energies of all the particles, plus the potential energy of the particles associated with the system. For different situations or number of particles, the Hamiltonian is different since it includes the sum of kinetic energies of the particles, and the potential energy function corresponding to the situation.
 
==The Schrödinger Hamiltonian==
 
===One particle===
 
By analogy with [[classical mechanics]], the Hamiltonian is commonly expressed as the sum of [[Hermitian operators|operators]] corresponding to the [[kinetic energy|kinetic]] and [[potential energy|potential]] energies of a system in the form
 
:<math> \hat{H} = \hat{T} + \hat{V} </math>
 
where
:<math> \hat{V} = V = V(\bold{r},t) </math>
is the [[potential energy]] operator and
:<math>\hat{T} = \frac{\bold{\hat{p}}\cdot\bold{\hat{p}}}{2m} = \frac{\hat{p}^2}{2m} = -\frac{\hbar^2}{2m}\nabla^2</math>
is the [[kinetic energy]] operator in which ''m'' is the [[mass]] of the particle, the dot denotes the [[dot product]] of vectors, and
:<math> \hat{p} = -i\hbar\nabla </math>
is the [[momentum operator]] wherein ∇ is the [[del|gradient]] [[operator (mathematics)|operator]]. The [[dot product]] of ∇ with itself is the [[Laplacian]] ∇<sup>2</sup>. In three dimensions using [[Cartesian coordinates]] the Laplace operator is
:<math>\nabla^2 = \frac{\partial^2}{{\partial x}^2} + \frac{\partial^2}{{\partial y}^2} + \frac{\partial^2}{{\partial z}^2}</math>
 
Although this is not the technical definition of the [[Hamiltonian mechanics|Hamiltonian in classical mechanics]], it is the form it most commonly takes. Combining these together yields the familiar form used in the [[Schrödinger equation]]:
 
:<math>\begin{align} \hat{H} & = \hat{T} + \hat{V} \\
& = \frac{\bold{\hat{p}}\cdot\bold{\hat{p}}}{2m}+ V(\mathbf{r},t) \\
& = -\frac{\hbar^2}{2m}\nabla^2+ V(\mathbf{r},t)
\end{align} </math>
 
which allows one to apply the Hamiltonian to systems described by a [[wave function]] ''Ψ''('''r''', ''t''). This is the approach commonly taken in introductory treatments of quantum mechanics, using the formalism of Schrödinger's wave mechanics.
 
===Many particles===
 
The formalism can be extended to ''N'' particles:
 
:<math> \hat{H} = \sum_{n=1}^N \hat{T}_n + V </math>
 
where
:<math> V = V(\bold{r}_1,\bold{r}_2\cdots \bold{r}_N,t) </math>
is the potential energy function, now a function of the spatial configuration of the system and time (a particular set of spatial positions at some instant of time defines a configuration) and;
:<math> \hat{T}_n = \frac{\bold{p}_n\cdot\bold{p}_n}{2m_n} </math>
is the kinetic energy operator of particle ''n'', and ∇<sub>n</sub> is the gradient for particle ''n'', ∇<sub>n</sub><sup>2</sup> is the Laplacian for particle using the coordinates:
:<math>\nabla_n^2 = \frac{\partial^2}{\partial x_n^2} + \frac{\partial^2}{\partial y_n^2} + \frac{\partial^2}{\partial z_n^2}</math>
 
Combining these together yields the Schrödinger Hamiltonian for the ''N''-particle case:
 
:<math>\begin{align} \hat{H} & = \sum_{n=1}^N \hat{T}_n + V \\
& = \sum_{n=1}^N \frac{\bold{\hat{p}}_n\cdot\bold{\hat{p}}_n}{2m_n}+ V(\mathbf{r}_1,\mathbf{r}_2\cdots\mathbf{r}_N,t) \\
& = -\frac{\hbar^2}{2}\sum_{n=1}^N \frac{1}{m_n}\nabla_n^2 + V(\mathbf{r}_1,\mathbf{r}_2\cdots\mathbf{r}_N,t)
\end{align} </math>
 
However, complications can arise in the [[many-body problem]]. Since the potential energy depends on the spatial arrangement of the particles, the kinetic energy will also depend on the spatial configuration to conserve energy. The motion due to any one particle will vary due to the motion of all the other particles in the system. For this reason cross terms for kinetic energy may appear in the Hamiltonian; a mix of the gradients for two particles:
 
:<math>-\frac{\hbar^2}{2M}\nabla_i\cdot\nabla_j </math>
 
where ''M'' denotes the mass of the collection of particles resulting in this extra kinetic energy. Terms of this form are known as ''mass polarization terms'', and appear in the Hamiltonian of many electron atoms (see below). 
 
For ''N'' interacting particles, i.e. particles which interact mutually and constitute a many-body situation, the potential energy function ''V'' is ''not'' simply a sum of the separate potentials (and certainly not a product, as this is dimensionally incorrect). The potential energy function can only be written as above: a function of all the spatial positions of each particle.
 
For non-interacting particles, i.e. particles which do not interact mutually and move independently, the potential of the system is the sum of the separate potential energy for each particle,<ref name="QuantumPhysics">Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles (2nd Edition), R. Resnick, R. Eisberg, John Wiley & Sons, 1985, ISBN 978-0-471-87373-0</ref> that is
 
:<math> V = \sum_{i=1}^N V(\bold{r}_i,t) = V(\bold{r}_1,t) + V(\bold{r}_2,t) + \cdots + V(\bold{r}_N,t) </math>
 
The general form of the Hamiltonian in this case is:
 
:<math>\begin{align} \hat{H} & = -\frac{\hbar^2}{2}\sum_{i=1}^{N}\frac{1}{m_i}\nabla_i^2 + \sum_{i=1}^N V_i\\
& = \sum_{i=1}^{N}\left(-\frac{\hbar^2}{2m_i}\nabla_i^2 + V_i \right) \\
& = \sum_{i=1}^{N}\hat{H}_i \\
\end{align}</math>
 
where the sum is taken over all particles and their corresponding potentials; the result is that the Hamiltonian of the system is the sum of the separate Hamiltonians for each particle. This is an idealized situation - in practice the particles are usually always influenced by some potential, and there are many-body interactions. One illustrative example of a two-body interaction where this form would not apply is for electrostatic potentials due to charged particles, because they certainly do interact with each other by the coulomb interaction (electrostatic force), shown below.
 
==Schrödinger equation==
{{Main|Schrödinger equation}}
The Hamiltonian generates the [[time]] evolution of quantum states. If <math> \left| \psi (t) \right\rangle</math> is the state of the system at time ''t'', then
 
:<math> H \left| \psi (t) \right\rangle = i \hbar {\partial\over\partial t} \left| \psi (t) \right\rangle.</math>
 
This equation is the [[Schrödinger equation]]. It takes the same form as the [[Hamilton–Jacobi equation]], which is one of the reasons ''H'' is also called the Hamiltonian. Given the state at some initial time (''t'' = 0), we can solve it to obtain the state at any subsequent time. In particular, if ''H'' is independent of time, then
 
:<math> \left| \psi (t) \right\rangle = e^{-iHt/\hbar} \left| \psi (0) \right\rangle.</math>
 
The [[Matrix exponential|exponential]] operator on the right hand side of the Schrödinger equation is usually defined by the corresponding [[Exponential function#Formal definition|power series]] in ''H''. One might notice that taking polynomials or power series of [[unbounded operator]]s that are not defined everywhere may not make mathematical sense. Rigorously, to take functions of unbounded operators, a [[functional calculus]] is required. In the case of the exponential function, the [[continuous functional calculus|continuous]], or just the [[holomorphic functional calculus]] suffices. We note again, however, that for common calculations the physicists' formulation is quite sufficient.
 
By the *-[[homomorphism]] property of the functional calculus, the operator
 
:<math> U = e^{-iHt/\hbar} </math>
 
is a [[unitary operator]]. It is the ''[[time evolution]] operator'', or ''[[propagator]]'', of a closed quantum system. If the Hamiltonian is time-independent, {U(t)} form a [[Stone's theorem on one-parameter unitary groups|one parameter unitary group]] (more than a [[C0 semigroup|semigroup]]); this gives rise to the physical principle of [[detailed balance]].
 
==Dirac formalism==
 
However, in the [[bra-ket notation|more general formalism]] of [[Paul Dirac|Dirac]], the Hamiltonian is typically implemented as an operator on a [[Hilbert space]] in the following way:
 
The eigenkets ([[eigenvectors]]) of ''H'', denoted <math>\left| a \right\rang</math>, provide an [[orthonormal basis]] for the Hilbert space. The spectrum of allowed energy levels of the system is given by the set of eigenvalues, denoted {''E''<sub>a</sub>}, solving the equation:
 
:<math> H \left| a \right\rangle = E_a \left| a \right\rangle.</math>
 
Since ''H'' is a [[Hermitian operator]], the energy is always a [[real number]].
 
From a mathematically rigorous point of view, care must be taken with the above assumptions. Operators on infinite-dimensional Hilbert spaces need not have eigenvalues (the set of eigenvalues does not necessarily coincide with the [[spectrum of an operator]]). However, all routine quantum mechanical calculations can be done using the physical formulation.{{clarify|date=December 2011}}
 
==Expressions for the Hamiltonian==
 
Following are expressions for the Hamiltonian in a number of situations.<ref>Quanta: A handbook of concepts, P.W. Atkins, Oxford University Press, 1974, ISBN 0-19-855493-1</ref> Typical ways to classify the expressions are the number of particles, number of dimensions, and the nature of the potential energy function - importantly space and time dependence. Masses are denoted by ''m'', and charges by ''q''.
 
===General forms for one particle===
 
===Free particle===
 
The particle is not bound by any potential energy, so the potential is zero and this Hamiltonian is the simplest. For one dimension:
 
:<math>\hat{H} = -\frac{\hbar^2}{2m}\frac{\partial^2}{\partial x^2} </math>
 
and in three dimensions:
 
:<math>\hat{H} = -\frac{\hbar^2}{2m}\nabla^2 </math>
 
===Constant-potential well===
 
For a particle in a region of constant potential ''V'' = ''V''<sub>0</sub> (no dependence on space or time), in one dimension, the Hamiltonian is:
 
:<math>\hat{H} = -\frac{\hbar^2}{2m}\frac{\partial^2}{\partial x^2} + V_0 </math>
 
in three dimensions
 
:<math>\hat{H} = -\frac{\hbar^2}{2m}\nabla^2 + V_0 </math>
 
This applies to the elementary "[[particle in a box]]" problem, and [[step potential]]s.
 
===Simple harmonic oscillator===
 
For a [[simple harmonic oscillator]] in one dimension, the potential varies with position (but not time), according to:
 
:<math>V = \frac{k}{2}x^2 = \frac{m\omega^2}{2}x^2  </math>
 
where the [[angular frequency]], effective [[spring constant]] ''k'', and mass ''m'' of the oscillator satisfy:
 
:<math>\omega^2 = \frac{k}{m}</math>
 
so the Hamiltonian is:
 
:<math>\hat{H} = -\frac{\hbar^2}{2m}\frac{\partial^2}{\partial x^2} + \frac{m\omega^2}{2}x^2 </math>
 
For three dimensions, this becomes
 
:<math>\hat{H} = -\frac{\hbar^2}{2m}\nabla^2 + \frac{m\omega^2}{2} r^2 </math>
 
where the three dimensional position vector '''r''' using cartesian coordinates is (''x'', ''y'', ''z''), its magnitude is
 
:<math>r^2 = \bold{r}\cdot\bold{r} = |\bold{r}|^2 = x^2+y^2+z^2 </math>
 
Writing the Hamiltonian out in full shows it is simply the sum of the one-dimensional Hamiltonians in each direction:
 
:<math>\begin{align} \hat{H} & = -\frac{\hbar^2}{2m}\left( \frac{\partial^2}{\partial x^2} + \frac{\partial^2}{\partial y^2} + \frac{\partial^2}{\partial z^2} \right) + \frac{m\omega^2}{2} (x^2+y^2+z^2) \\
& = \left(-\frac{\hbar^2}{2m}\frac{\partial^2}{\partial x^2} + \frac{m\omega^2}{2}x^2\right) + \left(-\frac{\hbar^2}{2m}\frac{\partial^2}{\partial y^2} + \frac{m\omega^2}{2}y^2 \right ) + \left(- \frac{\hbar^2}{2m}\frac{\partial^2}{\partial z^2} +\frac{m\omega^2}{2}z^2 \right) \\
\end{align}</math>
 
===Rigid rotor===
 
For a [[rigid rotor]] – i.e. system of particles which can rotate freely about any axes, not bound in any potential (such as free molecules with negligible vibrational [[Degrees of freedom (physics and chemistry)|degrees of freedom]], say due to [[double bond|double]] or [[triple bond|triple]] [[chemical bond]]s), Hamiltonian is:
 
:<math> \hat{H} = -\frac{\hbar^2}{2I_{xx}}\hat{J}_x^2 -\frac{\hbar^2}{2I_{yy}}\hat{J}_y^2 -\frac{\hbar^2}{2I_{zz}}\hat{J}_z^2 </math>
 
where ''I<sub>xx</sub>'', ''I<sub>yy</sub>'', and ''I<sub>zz</sub>'' are the [[moment of inertia]] components (technically the diagonal elements of the [[moment of inertia#Moment of inertia tensor|moment of inertia tensor]]), and <math> \hat{J}_x \,\!</math>, <math> \hat{J}_y \,\!</math> and <math> \hat{J}_z \,\!</math> are the total [[angular momentum]] operators (components), about the ''x'', ''y'', and ''z'' axes respectively.
 
===Electrostatic or coulomb potential===
 
The [[Coulomb potential energy]] for two point charges ''q''<sub>1</sub> and ''q''<sub>2</sub> (i.e. [[charged particle]]s, since particles have no spatial extent), in three dimensions, is (in [[SI units]] - rather than [[Gaussian units]] which are frequently used in [[electromagnetism]]):
 
:<math>V = \frac{q_1q_2}{4\pi\epsilon_0 |\bold{r}|}</math>
 
However, this is only the potential for one point charge due to another. If there are many charged particles, each charge has a potential energy due to every other point charge (except itself). For ''N'' charges, the potential energy of charge ''q<sub>j</sub>'' due to all other charges is (see also [[Electric potential energy#Electrostatic potential energy stored in a configuration of discrete point charges|Electrostatic potential energy stored in a configuration of discrete point charges]]):<ref>Electromagnetism (2nd edition), I.S. Grant, W.R. Phillips, Manchester Physics Series, 2008 ISBN 0-471-92712-0</ref>
 
:<math>V_j = \frac{1}{2}\sum_{i\neq j} q_i \phi(\mathbf{r}_i)=\frac{1}{8\pi\varepsilon_0}\sum_{i\neq j} \frac{q_iq_j}{|\mathbf{r}_i-\mathbf{r}_j|}</math>
 
where ''φ''('''r'''<sub>i</sub>) is the electrostatic potential of charge ''q<sub>j</sub>'' at '''r'''<sub>i</sub>. The total potential of the system is then the sum over ''j'':
 
:<math>V = \frac{1}{8\pi\varepsilon_0}\sum_{j=1}^N\sum_{i\neq j} \frac{q_iq_j}{|\mathbf{r}_i-\mathbf{r}_j|}</math>
 
so the Hamiltonian is:
 
:<math>\begin{align}\hat{H} & = -\frac{\hbar^2}{2}\sum_{j=1}^N\frac{1}{m_j}\nabla_j^2 + \frac{1}{8\pi\varepsilon_0}\sum_{j=1}^N\sum_{i\neq j} \frac{q_iq_j}{|\mathbf{r}_i-\mathbf{r}_j|} \\
& = \sum_{j=1}^N \left ( -\frac{\hbar^2}{2m_j}\nabla_j^2 + \frac{1}{8\pi\varepsilon_0}\sum_{i\neq j} \frac{q_iq_j}{|\mathbf{r}_i-\mathbf{r}_j|}\right) \\
\end{align}</math>
 
===Electric dipole in an electric field===
 
For an [[electric dipole moment]] '''d''' constituting charges of magnitude ''q'', in a uniform, [[electrostatic field]] (time-independent) '''E''', positioned in one place, the potential is:
 
:<math>V = -\bold{\hat{d}}\cdot\bold{E} </math>
 
the dipole moment itself is the operator
 
:<math>V = -\bold{\hat{d}}\cdot\bold{E} </math>
 
Since the particle is stationary, there is no translational kinetic energy of the dipole, so the Hamiltonian of the dipole is just the potential energy:
 
:<math>\hat{H} = -\bold{\hat{d}}\cdot\bold{E} = -q\bold{E}\cdot\bold{\hat{r}}</math>
 
===Magnetic dipole in a magnetic field===
 
For a magnetic dipole moment '''μ''' in a uniform, magnetostatic field (time-independent) '''B''', positioned in one place, the potential is:
 
:<math>V = -\boldsymbol{\mu}\cdot\bold{B} </math>
 
Since the particle is stationary, there is no translational kinetic energy of the dipole, so the Hamiltonian of the dipole is just the potential energy:
 
:<math>\hat{H} = -\boldsymbol{\mu}\cdot\bold{B} </math>
 
For a [[Spin-½]] particle, the corresponding spin magnetic moment is:<ref>Physics of Atoms and Molecules, B.H. Bransden, C.J.Joachain, Longman, 1983, ISBN 0-582-44401-2</ref>
 
:<math>\boldsymbol{\mu}_S = \frac{g_s e}{2m} \bold{S} </math>
 
where ''g<sub>s</sub>'' is the spin [[gyromagnetic ratio]] (aka "spin [[g-factor (physics)|g-factor]]"), ''e'' is the electron charge, '''S''' is the [[spin (physics)#Pauli matrices and spin operators|spin operator]] vector, whose components are the [[Pauli matrices]], hence
 
:<math>\hat{H} = \frac{g_s e}{2m} \bold{S} \cdot\bold{B} </math>
 
===Charged particle in an electromagnetic field===
 
For a charged particle ''q'' in an electromagnetic field, described by the [[scalar potential]] ''φ'' and [[vector potential]] '''A''', there are two parts to the Hamiltonian to substitute for.<ref name="QuantumPhysics" /> The momentum operator must be replaced by the [[kinetic momentum]] operator, which includes a contribution from the '''A''' field:
 
:<math>\bold{\hat{\Pi}} = \bold{\hat{P}} - q\bold{A} </math>
 
where <math>\bold{\hat{P}}</math> is the canonical momentum operator given as the usual momentum operator:
 
:<math>\bold{\hat{P}} = -i\hbar\nabla </math>
 
so the corresponding kinetic energy operator is:
 
:<math>\hat{T} = \frac{\bold{\hat{\Pi}}\cdot\bold{\hat{\Pi}}}{2m} = \frac{1}{2m} \left ( \bold{\hat{P}} - q\bold{A} \right)^2 </math>
 
and the potential energy, which is due to the ''φ'' field:
 
:<math>V = q\phi </math>
 
Casting all of these into the Hamiltonian gives:
 
:<math>\hat{H} = \frac{1}{2m} \left ( -i\hbar\nabla - q\bold{A} \right)^2 + q\phi </math>
 
== Energy eigenket degeneracy, symmetry, and conservation laws ==
 
In many systems, two or more energy eigenstates have the same energy. A simple example of this is a free particle, whose energy eigenstates have wavefunctions that are propagating plane waves. The energy of each of these plane waves is inversely proportional to the square of its [[wavelength]]. A wave propagating in the ''x'' direction is a different state from one propagating in the ''y'' direction, but if they have the same wavelength, then their energies will be the same. When this happens, the states are said to be ''degenerate''.
 
It turns out that degeneracy occurs whenever a nontrivial [[Unitary matrix|unitary operator]] ''U'' [[commutation relation|commutes]] with the Hamiltonian. To see this, suppose that <math>|a\rang</math> is an energy eigenket. Then <math>U|a\rang</math> is an energy eigenket with the same eigenvalue, since
 
:<math>UH |a\rangle = U E_a|a\rangle = E_a (U|a\rangle) = H \; (U|a\rangle). </math>
 
Since ''U'' is nontrivial, at least one pair of <math>|a\rang</math> and <math>U|a\rang</math> must represent distinct states. Therefore, ''H'' has at least one pair of degenerate energy eigenkets. In the case of the free particle, the unitary operator which produces the symmetry is the [[Rotation operator (quantum mechanics)|rotation operator]], which rotates the wavefunctions by some angle while otherwise preserving their shape.
 
The existence of a symmetry operator implies the existence of a [[conservation law|conserved]] observable. Let ''G'' be the Hermitian generator of ''U'':
 
:<math> U = I - i \epsilon G + O(\epsilon^2) \,</math>
 
It is straightforward to show that if ''U'' commutes with ''H'', then so does ''G'':
 
:<math> [H, G] = 0 \,</math>
 
Therefore,
 
:<math>
\frac{\part}{\part t} \langle\psi(t)|G|\psi(t)\rangle
= \frac{1}{i\hbar} \langle\psi(t)|[G,H]|\psi(t)\rangle
= 0.
</math>
 
In obtaining this result, we have used the Schrödinger equation, as well as its [[bra-ket notation|dual]],
 
:<math> \langle\psi (t)|H = - i \hbar {\partial\over\partial t} \langle\psi(t)|.</math>
 
Thus, the [[expected value]] of the observable ''G'' is conserved for any state of the system. In the case of the free particle, the conserved quantity is the [[angular momentum]].
 
== Hamilton's equations ==
[[William Rowan Hamilton|Hamilton]]'s equations in classical [[Hamiltonian mechanics]] have a direct analogy in quantum mechanics. Suppose we have a set of basis states <math>\left\{\left| n \right\rangle\right\}</math>, which need not necessarily be eigenstates of the energy. For simplicity, we assume that they are discrete, and that they are orthonormal, i.e.,
 
:<math> \langle n' | n \rangle = \delta_{nn'}. </math>
 
Note that these basis states are assumed to be independent of time. We will assume that the Hamiltonian is also independent of time.
 
The instantaneous state of the system at time ''t'', <math>\left| \psi\left(t\right) \right\rangle</math>, can be expanded in terms of these basis states:
 
:<math> |\psi (t)\rangle = \sum_{n} a_n(t) |n\rangle </math>
 
where
 
:<math> a_n(t) = \langle n | \psi(t) \rangle. </math>
 
The coefficients ''a<sub>n</sub>(t)'' are [[complex number|complex]] variables. We can treat them as coordinates which specify the state of the system, like the position and momentum coordinates which specify a classical system. Like classical coordinates, they are generally not constant in time, and their time dependence gives rise to the time dependence of the system as a whole.
 
The expectation value of the Hamiltonian of this state, which is also the mean energy, is
 
:<math> \langle H(t) \rangle \ \stackrel{\mathrm{def}}{=}\  \langle\psi(t)|H|\psi(t)\rangle
= \sum_{nn'} a_{n'}^* a_n \langle n'|H|n \rangle </math>
 
where the last step was obtained by expanding <math>\left| \psi\left(t\right) \right\rangle</math> in terms of the basis states.
 
Each of the ''a<sub>n</sub>(t)'''s actually corresponds to ''two'' independent degrees of freedom, since the variable has a real part and an imaginary part. We now perform the following trick: instead of using the real and imaginary parts as the independent variables, we use ''a<sub>n</sub>(t)'' and its [[complex conjugate]] ''a<sub>n</sub>*(t)''. With this choice of independent variables, we can calculate the [[partial derivative]]
 
:<math>\frac{\partial \langle H \rangle}{\partial a_{n'}^{*}}
= \sum_{n} a_n \langle n'|H|n \rangle
= \langle n'|H|\psi\rangle
</math>
 
By applying [[Schrödinger's equation]] and using the orthonormality of the basis states, this further reduces to
 
:<math>\frac{\partial \langle H \rangle}{\partial a_{n'}^{*}}
= i \hbar \frac{\partial a_{n'}}{\partial t} </math>
 
Similarly, one can show that
 
:<math> \frac{\partial \langle H \rangle}{\partial a_n}
= - i \hbar \frac{\partial a_{n}^{*}}{\partial t} </math>
 
If we define "conjugate momentum" variables ''π<sub>n</sub>'' by
 
:<math> \pi_{n}(t) = i \hbar a_n^*(t) </math>
 
then the above equations become
 
:<math> \frac{\partial \langle H \rangle}{\partial \pi_n} = \frac{\partial a_n}{\partial t},\quad \frac{\partial \langle H \rangle}{\partial a_n} = - \frac{\partial \pi_n}{\partial t} </math>
 
which is precisely the form of Hamilton's equations, with the <math>a_n</math>s as the generalized coordinates, the <math>\pi_n</math>s as the conjugate momenta, and <math>\langle H\rangle</math> taking the place of the classical Hamiltonian.
 
==See also==
*[[Hamiltonian mechanics]]
*[[Operator (physics)]]
*[[Bra-ket notation]]
*[[Quantum state]]
*[[Linear algebra]]
*[[Conservation of energy]]
*[[Potential theory]]
*[[Many-body problem]]
*[[Electrostatics]]
*[[Electric field]]
*[[Magnetic field]]
 
==References==
{{reflist}}
 
{{physics operator}}
 
[[Category:Hamiltonian mechanics]]
[[Category:Operator theory]]
[[Category:Quantum mechanics]]
[[Category:Quantum chemistry]]
[[Category:Theoretical chemistry]]
[[Category:Computational chemistry]]

Revision as of 17:34, 18 January 2014

In quantum mechanics, the Hamiltonian is the operator corresponding to the total energy of the system. It is usually denoted by H, also Ȟ or Ĥ. Its spectrum is the set of possible outcomes when one measures the total energy of a system. Because of its close relation to the time-evolution of a system, it is of fundamental importance in most formulations of quantum theory.

Introduction

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church.

The Hamiltonian is the sum of the kinetic energies of all the particles, plus the potential energy of the particles associated with the system. For different situations or number of particles, the Hamiltonian is different since it includes the sum of kinetic energies of the particles, and the potential energy function corresponding to the situation.

The Schrödinger Hamiltonian

One particle

By analogy with classical mechanics, the Hamiltonian is commonly expressed as the sum of operators corresponding to the kinetic and potential energies of a system in the form

where

is the potential energy operator and

is the kinetic energy operator in which m is the mass of the particle, the dot denotes the dot product of vectors, and

is the momentum operator wherein ∇ is the gradient operator. The dot product of ∇ with itself is the Laplacian2. In three dimensions using Cartesian coordinates the Laplace operator is

Although this is not the technical definition of the Hamiltonian in classical mechanics, it is the form it most commonly takes. Combining these together yields the familiar form used in the Schrödinger equation:

which allows one to apply the Hamiltonian to systems described by a wave function Ψ(r, t). This is the approach commonly taken in introductory treatments of quantum mechanics, using the formalism of Schrödinger's wave mechanics.

Many particles

The formalism can be extended to N particles:

where

is the potential energy function, now a function of the spatial configuration of the system and time (a particular set of spatial positions at some instant of time defines a configuration) and;

is the kinetic energy operator of particle n, and ∇n is the gradient for particle n, ∇n2 is the Laplacian for particle using the coordinates:

Combining these together yields the Schrödinger Hamiltonian for the N-particle case:

However, complications can arise in the many-body problem. Since the potential energy depends on the spatial arrangement of the particles, the kinetic energy will also depend on the spatial configuration to conserve energy. The motion due to any one particle will vary due to the motion of all the other particles in the system. For this reason cross terms for kinetic energy may appear in the Hamiltonian; a mix of the gradients for two particles:

where M denotes the mass of the collection of particles resulting in this extra kinetic energy. Terms of this form are known as mass polarization terms, and appear in the Hamiltonian of many electron atoms (see below).

For N interacting particles, i.e. particles which interact mutually and constitute a many-body situation, the potential energy function V is not simply a sum of the separate potentials (and certainly not a product, as this is dimensionally incorrect). The potential energy function can only be written as above: a function of all the spatial positions of each particle.

For non-interacting particles, i.e. particles which do not interact mutually and move independently, the potential of the system is the sum of the separate potential energy for each particle,[1] that is

The general form of the Hamiltonian in this case is:

where the sum is taken over all particles and their corresponding potentials; the result is that the Hamiltonian of the system is the sum of the separate Hamiltonians for each particle. This is an idealized situation - in practice the particles are usually always influenced by some potential, and there are many-body interactions. One illustrative example of a two-body interaction where this form would not apply is for electrostatic potentials due to charged particles, because they certainly do interact with each other by the coulomb interaction (electrostatic force), shown below.

Schrödinger equation

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. The Hamiltonian generates the time evolution of quantum states. If is the state of the system at time t, then

This equation is the Schrödinger equation. It takes the same form as the Hamilton–Jacobi equation, which is one of the reasons H is also called the Hamiltonian. Given the state at some initial time (t = 0), we can solve it to obtain the state at any subsequent time. In particular, if H is independent of time, then

The exponential operator on the right hand side of the Schrödinger equation is usually defined by the corresponding power series in H. One might notice that taking polynomials or power series of unbounded operators that are not defined everywhere may not make mathematical sense. Rigorously, to take functions of unbounded operators, a functional calculus is required. In the case of the exponential function, the continuous, or just the holomorphic functional calculus suffices. We note again, however, that for common calculations the physicists' formulation is quite sufficient.

By the *-homomorphism property of the functional calculus, the operator

is a unitary operator. It is the time evolution operator, or propagator, of a closed quantum system. If the Hamiltonian is time-independent, {U(t)} form a one parameter unitary group (more than a semigroup); this gives rise to the physical principle of detailed balance.

Dirac formalism

However, in the more general formalism of Dirac, the Hamiltonian is typically implemented as an operator on a Hilbert space in the following way:

The eigenkets (eigenvectors) of H, denoted , provide an orthonormal basis for the Hilbert space. The spectrum of allowed energy levels of the system is given by the set of eigenvalues, denoted {Ea}, solving the equation:

Since H is a Hermitian operator, the energy is always a real number.

From a mathematically rigorous point of view, care must be taken with the above assumptions. Operators on infinite-dimensional Hilbert spaces need not have eigenvalues (the set of eigenvalues does not necessarily coincide with the spectrum of an operator). However, all routine quantum mechanical calculations can be done using the physical formulation.Template:Clarify

Expressions for the Hamiltonian

Following are expressions for the Hamiltonian in a number of situations.[2] Typical ways to classify the expressions are the number of particles, number of dimensions, and the nature of the potential energy function - importantly space and time dependence. Masses are denoted by m, and charges by q.

General forms for one particle

Free particle

The particle is not bound by any potential energy, so the potential is zero and this Hamiltonian is the simplest. For one dimension:

and in three dimensions:

Constant-potential well

For a particle in a region of constant potential V = V0 (no dependence on space or time), in one dimension, the Hamiltonian is:

in three dimensions

This applies to the elementary "particle in a box" problem, and step potentials.

Simple harmonic oscillator

For a simple harmonic oscillator in one dimension, the potential varies with position (but not time), according to:

where the angular frequency, effective spring constant k, and mass m of the oscillator satisfy:

so the Hamiltonian is:

For three dimensions, this becomes

where the three dimensional position vector r using cartesian coordinates is (x, y, z), its magnitude is

Writing the Hamiltonian out in full shows it is simply the sum of the one-dimensional Hamiltonians in each direction:

Rigid rotor

For a rigid rotor – i.e. system of particles which can rotate freely about any axes, not bound in any potential (such as free molecules with negligible vibrational degrees of freedom, say due to double or triple chemical bonds), Hamiltonian is:

where Ixx, Iyy, and Izz are the moment of inertia components (technically the diagonal elements of the moment of inertia tensor), and , and are the total angular momentum operators (components), about the x, y, and z axes respectively.

Electrostatic or coulomb potential

The Coulomb potential energy for two point charges q1 and q2 (i.e. charged particles, since particles have no spatial extent), in three dimensions, is (in SI units - rather than Gaussian units which are frequently used in electromagnetism):

However, this is only the potential for one point charge due to another. If there are many charged particles, each charge has a potential energy due to every other point charge (except itself). For N charges, the potential energy of charge qj due to all other charges is (see also Electrostatic potential energy stored in a configuration of discrete point charges):[3]

where φ(ri) is the electrostatic potential of charge qj at ri. The total potential of the system is then the sum over j:

so the Hamiltonian is:

Electric dipole in an electric field

For an electric dipole moment d constituting charges of magnitude q, in a uniform, electrostatic field (time-independent) E, positioned in one place, the potential is:

the dipole moment itself is the operator

Since the particle is stationary, there is no translational kinetic energy of the dipole, so the Hamiltonian of the dipole is just the potential energy:

Magnetic dipole in a magnetic field

For a magnetic dipole moment μ in a uniform, magnetostatic field (time-independent) B, positioned in one place, the potential is:

Since the particle is stationary, there is no translational kinetic energy of the dipole, so the Hamiltonian of the dipole is just the potential energy:

For a Spin-½ particle, the corresponding spin magnetic moment is:[4]

where gs is the spin gyromagnetic ratio (aka "spin g-factor"), e is the electron charge, S is the spin operator vector, whose components are the Pauli matrices, hence

Charged particle in an electromagnetic field

For a charged particle q in an electromagnetic field, described by the scalar potential φ and vector potential A, there are two parts to the Hamiltonian to substitute for.[1] The momentum operator must be replaced by the kinetic momentum operator, which includes a contribution from the A field:

where is the canonical momentum operator given as the usual momentum operator:

so the corresponding kinetic energy operator is:

and the potential energy, which is due to the φ field:

Casting all of these into the Hamiltonian gives:

Energy eigenket degeneracy, symmetry, and conservation laws

In many systems, two or more energy eigenstates have the same energy. A simple example of this is a free particle, whose energy eigenstates have wavefunctions that are propagating plane waves. The energy of each of these plane waves is inversely proportional to the square of its wavelength. A wave propagating in the x direction is a different state from one propagating in the y direction, but if they have the same wavelength, then their energies will be the same. When this happens, the states are said to be degenerate.

It turns out that degeneracy occurs whenever a nontrivial unitary operator U commutes with the Hamiltonian. To see this, suppose that is an energy eigenket. Then is an energy eigenket with the same eigenvalue, since

Since U is nontrivial, at least one pair of and must represent distinct states. Therefore, H has at least one pair of degenerate energy eigenkets. In the case of the free particle, the unitary operator which produces the symmetry is the rotation operator, which rotates the wavefunctions by some angle while otherwise preserving their shape.

The existence of a symmetry operator implies the existence of a conserved observable. Let G be the Hermitian generator of U:

It is straightforward to show that if U commutes with H, then so does G:

Therefore,

In obtaining this result, we have used the Schrödinger equation, as well as its dual,

Thus, the expected value of the observable G is conserved for any state of the system. In the case of the free particle, the conserved quantity is the angular momentum.

Hamilton's equations

Hamilton's equations in classical Hamiltonian mechanics have a direct analogy in quantum mechanics. Suppose we have a set of basis states , which need not necessarily be eigenstates of the energy. For simplicity, we assume that they are discrete, and that they are orthonormal, i.e.,

Note that these basis states are assumed to be independent of time. We will assume that the Hamiltonian is also independent of time.

The instantaneous state of the system at time t, , can be expanded in terms of these basis states:

where

The coefficients an(t) are complex variables. We can treat them as coordinates which specify the state of the system, like the position and momentum coordinates which specify a classical system. Like classical coordinates, they are generally not constant in time, and their time dependence gives rise to the time dependence of the system as a whole.

The expectation value of the Hamiltonian of this state, which is also the mean energy, is

where the last step was obtained by expanding in terms of the basis states.

Each of the an(t)'s actually corresponds to two independent degrees of freedom, since the variable has a real part and an imaginary part. We now perform the following trick: instead of using the real and imaginary parts as the independent variables, we use an(t) and its complex conjugate an*(t). With this choice of independent variables, we can calculate the partial derivative

By applying Schrödinger's equation and using the orthonormality of the basis states, this further reduces to

Similarly, one can show that

If we define "conjugate momentum" variables πn by

then the above equations become

which is precisely the form of Hamilton's equations, with the s as the generalized coordinates, the s as the conjugate momenta, and taking the place of the classical Hamiltonian.

See also

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

What is directory submission? How does it help in the web site promotion? Several think directory submission is comparable to the information entry jobs and they assume it to be very simple and effortless. Sites like yahoo, Google etc have large net directories equivalent to the telephone directory we use. These directories are shop homes of relevant data about company issues and net sites all more than the planet. Hence for advertising a web website its details are submitted to the directory in exchange for a little fee.

Once more this directory submission can either be accomplished manually or implemented by way of automated services. When someone submits his internet site to a directory human editors have to go through them to verify top quality and other guidelines. With millions of internet internet sites coming in daily it is not effortless or it is quite stressful to go through them and verify for quality. So automated directory submissions was invented to examine for the submission recommendations. The advantage of this automated submission is that it is easy and less time consuming. It automatically checks and submits without delay.

But what really matters is regardless of whether the web directories can accept automated directory submission as they are very strict about the recommendations. Moreover if the category to which the submission is made is not selected effectively then it loses its purpose. You can not anticipate an automotive service to analyze which category your submission is finest suited dont you? Therefore even though the time factor favors automated directory submission it nonetheless has a lengthy way to go to outbid manual solutions.

Any item or service hitting the public market place needs to be advertised. Visit linkliciousdiscussionsldd - StreetFire Member in US to read why to see about it. There are several methods and means by means of which this advertisement can reach the general public. The print media, roadside banners, brochures, graphic boards and so on are some of the familiar advertising and marketing tactics followed. Be taught more on this related URL by visiting linklicious discount. Designed a professional web site to promote the business is the new and trendy marketing and advertising tool undertaken today to expose a business. For another standpoint, please consider checking out: is linklicious safe. Crucial characteristics like the Search engine marketing, SEM, directory submission, link developing and so on support to improve the search outcome when somebody searches in the search engine.

The net internet site payout along with the catchy content material and tags should be competent to bring in buyer visitors. And to enable this numerous factors are concerned crucial like the Search engine optimisation, SEM, directory submission, link creating and so on. any organization concern which requires off as a fresher should give away a press release and do the On-line Directories and Search Engine Submissions. Press Release is nothing at all but a public relation announcement about the new organization prospects. The details provided should be one hundred% original and no false or exaggerated content ought to be focused on. This is the principal door which opens up to the public who will trust what they see or read from this press release. This can also be completed during a course of a company when they roll out a new product or have introduced or opened a new retailer..

When you have any concerns about in which and how to utilize health answers, it is possible to e-mail us from the web page.

  1. 1.0 1.1 Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles (2nd Edition), R. Resnick, R. Eisberg, John Wiley & Sons, 1985, ISBN 978-0-471-87373-0
  2. Quanta: A handbook of concepts, P.W. Atkins, Oxford University Press, 1974, ISBN 0-19-855493-1
  3. Electromagnetism (2nd edition), I.S. Grant, W.R. Phillips, Manchester Physics Series, 2008 ISBN 0-471-92712-0
  4. Physics of Atoms and Molecules, B.H. Bransden, C.J.Joachain, Longman, 1983, ISBN 0-582-44401-2