What is Principle: Definition and 1000 Discussions

A principle is a proposition or value that is a guide for behavior or evaluation. In law, it is a rule that has to be or usually is to be followed. It can be desirably followed, or it can be an inevitable consequence of something, such as the laws observed in nature or the way that a system is constructed. The principles of such a system are understood by its users as the essential characteristics of the system, or reflecting system's designed purpose, and the effective operation or use of which would be impossible if any one of the principles was to be ignored. A system may be explicitly based on and implemented from a document of principles as was done in IBM's 360/370 Principles of Operation.
Examples of principles are, entropy in a number of fields, least action in physics, those in descriptive comprehensive and fundamental law: doctrines or assumptions forming normative rules of conduct, separation of church and state in statecraft, the central dogma of molecular biology, fairness in ethics, etc.
In common English, it is a substantive and collective term referring to rule governance, the absence of which, being "unprincipled", is considered a character defect. It may also be used to declare that a reality has diverged from some ideal or norm as when something is said to be true only "in principle" but not in fact.

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  1. D

    Huygens Principle: Light & Diffraction Explained

    Hello, I have the following question: Physics books say that in order to see diffraction you need a wave to encounter an object that is comparable in size to the wavelength. This is presumably the reason why light does not bend through an opened door but sound does. My question is, what...
  2. bcrowell

    Correspondence principle for a step potential

    Consider the Schrodinger equation with the step-function potential V(x)=\begin{cases} 0, & x<0 \\ U, & x>0 \end{cases}\qquad . A pulse with E>U comes in from the left with unit amplitude and undergoes partial reflection. The reflection has an amplitude (ignoring phase) that is given by the...
  3. R

    Bernoulli's Principle alternative view

    Hi guys, Could anyone explain Bernoulli's Principle to me so that it makes sense from an alternative point of view? http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html I can make sense of the maths but I'm trying to understand what actually happens so I can visualise it. At first...
  4. A

    Equal probability principle

    Hey Guys, I have got a question. English is not my first language. Is the "equal-probability-principle" the same as "the equal-a-priori-probability-postulate"? Do both names describe the same process? Thx Abby o:)
  5. Spinnor

    Does string theory, in principle, answer why matter comes in chunks ?

    Does string theory, in principle, answer why matter comes in "chunks"? Does string theory, in principle, answer why matter comes in "chunks" of the same amount (say the mass of an electron or muon), and why the energy in the electromagnetic field of a given frequency also comes in a "chunks"...
  6. J

    Solving differential equation from variational principle

    I have the following differential equation which I obtained from Euler-Lagrange variational principle \frac{\partial}{\partial x}\left(\frac{1}{\sqrt{y}}\frac{dy}{dx}\right)=0 I also have two boundary conditions: y\left(0\right)=y_{1} and y\left(D\right)=y_{2} where D, y_{1} and y_{2} are...
  7. I

    Pauli exclusion principle

    I was told that the pauli exclusion principle states that no two electrons in an atom can have the same quantum numbers is it ture need answers!
  8. M

    Bernoulli principle - negative pressure

    I will write a program about principle bernoulli, but i have a problem. My input data: Pressure(p1) = 1000 Cross-section(A1) = 1 Velocity(V1) = 1 Cross-section(A2) = 0.5 Velocity(v2)= (V1A1)/A2 to simplify: h1=h2 i'm counting the pressure p2 : p2 = p1 + 0.5*v1*v1 - 0.5*v2*v2...
  9. icystrike

    Inclusion-Exclusion Principle (Probability) - Bonferroni inequalities

    Homework Statement Hi PF! I am studying from a book - A first course in probability by Sheldon Ross, and I have came across this section whereby the are trying to prove the upper bound (equations 4.1 and 4.3) and lower bound (equation 4.2) of the inclusion-exclusion principle from basic...
  10. A

    Scope of Pauli's Exclusion Principle

    What is the scope of Pauli's exclusion principle? When we say two particles in a system cannot have the same quantum state, how do we choose the system?
  11. A

    Principle of Work and Energy

    Homework Statement If the cord is subjected to a constant force of F= 300 and the 16kg smooth collar starts from rest at A, determine the velocity of the collar when it reaches point B. Neglect the size of the pulley. Homework Equations (\frac{1}{2}mv12) + \sumU1-2 =...
  12. S

    Uncertainty about Heisenberg Uncertainty Principle

    I'm not certain as to the true meaning of the Heisenberg Uncertainty Principle, and wonder if anyone can provide insight. I understand that there is a reciprocal relation between the certainties as to the position and momentum of a particle such as an electron; the more precisely position is...
  13. G

    Pauli Exclusion Principle and Entanglement

    How do fermions, which have vast amounts of empty space, know not to occupy the same space as another fermion? Do physicists say that the two fermions become entangled and that is what enables them to be "aware" of the "existence" of the other fermion? Is entanglement used as an explanation...
  14. tomwilliam2

    Simultaneity of Uncertainty Principle

    All of the statements of Heisenberg's uncertainty principle that I've read seem to state that there is a fundamental limit on the precision to which you can measure the values of conjugate pairs (like position and momentum) at the same time. So is this simultaneity necessary? I ask because...
  15. L

    Uncertainty principle in terms of expectations values in Dirac notatio

    Homework Statement Show that (\Delta A)^{2} = \langle \psi |A^{2}| \psi \rangle - \langle \psi |A| \psi \rangle ^{2}\\ \phantom{(\Delta A)^{2} }=\langle \psi | (A - \langle A \rangle )^{2} | \psi \rangle , where \Delta A is the uncertainty of an operator A and \langle A \rangle is the...
  16. A

    Working Principle of Split Ring Resonators

    What is the physical working principle of SRR's ? How do they result in the negative permeability at a macro level? (either of explanation or decent links will do!) Thanks :biggrin:
  17. 7

    Gaussian function to derive Heisenberg's uncertainty principle

    At our QM intro our professor said that we derive uncertainty principle using the integral of plane waves ##\psi = \psi_0(k) e^{i(kx - \omega t)}## over wave numbers ##k##. We do it at ##t=0## hence ##\psi = \psi_0(k) e^{ikx}## \psi = \int\limits_{-\infty}^{+\infty} \psi_0\!(k) \cdot...
  18. D

    I Why is the Pauli Exclusion Principle not a force?

    This is a question I've had for some time now. Why is the exchange interaction not considered a force, like the other 4 fundamental forces? When reading solid-state physics texts, for example, I come across explanations of this kind: the atoms cannot get too close together because of the...
  19. Q

    Heisenberg's uncertainty principle invalidated?

    Heisenberg's uncertainty principle asserts a fundamental limit to the precision with which certain pairs of physical properties of a particle, such as position and momentum, can be known simultaneously. The more you know about one, the less you know about the other. Why does the following...
  20. N

    Superposition principle? How to algebraically add up two equations?

    Superposition principle? How to algebraically add up two equations?? How do you add up (λ (√3)) / (4pi(ε naught)R) (√3/2 i hat - 1/2 j hat) + (λ (√3)) / (4pi(ε naught)R) (√3/2 i hat + 1/2 j hat)? the 1/2 j hat cancels out right? but what do i do with everything else?
  21. C

    Testing Equivalence Principle: Prying Gravity From Inertia

    Hi, So I came across this article on the internet, and it was about the Equivalence Principle. So I were interested and read it all, and in the article it said that the EP might be only approximately true. What interested me more about the article was it said that any theory of quantum...
  22. A

    Who was the creator of principle of inertia

    Hi all, It is really a history question. I read that Galileu developed the concept of inertia, but today I knew it from Newton's 1st law. So my question is: Both of them arrived at the idea independently? Was Newton aware about Galileu's work and used the law because it is the basis for his...
  23. Q

    What is the concept behind the Uncertainty Principle?

    I'm trying to understand the uncertainty principle. ΔxΔp >= h/4∏ from my understanding of the concept, its not possible to know the value of the position and the momentum simultaneously. yet the problems that i see floating around on the internet seem to just plug in values for mass and...
  24. B

    Frame of reference question about equivalence principle

    I've been giving some thought to the new scientist article about the equivalence principle. What initially seems like a very simple issue is remarkably tantalising. (please note- I'm not a physicist so this may be quite stupid) This led me on a little to the question of gravity and direction...
  25. S

    Understanding the Principle of Equivalence: A Closer Look

    What’s your understanding of the principle of equivalence? In the literature, I find two meanings: (1) Gravitational mass is numerically equal to inertial “mass.” (This is a postulate.) (2) A mass at rest in a frame is equivalent to being in a “gravitational field” in an accelerated frame...
  26. A

    How does Le Chatelier's Principle work?

    It is said that if there is a change in pressure in an equilibrium state, the chemical reactions that diminish the number of molecules will be favorised. Therefore, there will be less molecules, so the pressure will decrease to reach equilibrium again. However, since the kinetic energy of the...
  27. S

    Photons violate Uncertainty Principle?

    A photon is considered as a quantum particle, right? However since we know the speed of a photon(speed of light) and hence can predict its position, isn't it violating the Heisenberg Uncertainty Principle? Where am I going wrong? Is it false to believe that a photon is a quantum particle?
  28. P

    What is the Franck-Condon principle?

    hi guys I want to know what is the Franck-Condon principle?... please in details thanks for all
  29. J

    What is the principle of moments?

    Homework Statement I'm not sure on the principles of moments Homework Equations The Attempt at a Solution Is it just that moments are a turning force?
  30. P

    Quantum entanglement and the uncertainty principle

    This question may have been posted before, but I didn't have much luck finding it. We can't know the momentum and position of a particle at the same time, but if one were to entangle two particles (I.e. photons), wouldn't it be possible to measure one quality on each and thus deduce both the...
  31. P

    Implications of Heisenberg uncertainty principle

    Hi does anyone know how bogulobov transformations are related to the Heisenberg uncertainty principle?
  32. P

    Buoyancy and Archimedes Principle, volume ratio/density question

    A geode is a hollow rock with a solid shell and an air-filled interior. Suppose a particular geode weighs twice as much in air as it does when completely submerged in water. If the density of the solid part of the geode is 3100 km/m^3 , what fraction of the geode's volume is hollow? The...
  33. M

    Principle of Equivalence: Understanding the Foundations of General Relativity

    I have just began my journey in General Theory of relativity through the book of Weinberg-Gravitation and cosmology principles and applications of the general theory of relativity I don't know if this topic is just a repeat of an older one, if it is please let me know the link to it... I...
  34. D

    Principle of conservation of momentum question

    [b]1. Somebody help explain how to do this for me, am not looking answer, an explanation so i can solve it would be great. A space vehicle traveling at a velocity of 7000km/h separates into two sections of mass 1100kg and 200kg. the two parts continue moving in the same direction with the...
  35. C

    Solution to Momentum Principle Problem

    View the attached picture that is the problem that I need help with.
  36. V

    Buoyancy (Archimede's Principle) Problem

    Question: Suppose a person weighing 530 Newtons is floating in a salt lake (concentration of 20% NaCl) with a specific gravity of 1.148. How much less of the person's body would be in the salt water as compared to if he were floating in ordinary water (w/ density 1.00g/cm^3)? Comment: One...
  37. E

    Wave Interference - the principle of superposition

    Homework Statement The problems are attached in the photo below (or at least I'm hoping they are, doing this from my Ipad makes this somewhat diffcult) Homework Equations Not really an equation, but the Principle of Superposition states that the amplitude of two interfering waves are added...
  38. anorlunda

    Principle of least action as a law?

    We say that the laws of nature (e.g. Newton's Laws, relativity, ...) must be confirmed and must be checked for invariance (e.g Lorentz, gauge, ...). Yet many of these laws may be directly derived from the principle of least action. Why do we not consider the principle of least action a law...
  39. ShayanJ

    Uncertainty principle,examining the «principle» part

    In some experiments like Heisenberg's microscope and Davisson–Germer experiment,we see that by using light to understand the actual path of electrons,the electrons are disturbed and so the result is changed(in the second case,the interference pattern is replaced by the distribution of classical...
  40. M

    Has the Uncertainty Principle been defeated?

    According to this article, scientists were able to bypass the Heisenberg Uncertainty Principle, creating relative certainty in measurement of both position and momentum. Any thoughts, objections, comments?
  41. I

    The uncertainity principle, a new take?

    http://www.livescience.com/18567-wac...principle.html are these once again bold claims? they are saying the uncertainty principle isn't just in measurement but in the very nature of the quantum. It seems to be bold claims and not reperated anywhere on any other sites and certainly doesn't...
  42. H

    What is this scientific principle called?

    Greetings, This is more of a question about the philosophy of science than physics. In fact, it's just a vocabulary question, so should have a one-word or one-phrase answer (similar to "Occam's razor"), if there is one. Is there a word or phrase for the following scientific principle (my...
  43. J

    Equivalence principle implies uniformly accelerated charge doesn't radiate?

    Does a uniformly accelerated charged particle radiate em waves? The equivalence principle says that a particle in uniform acceleration is equivalent to a particle at rest in a gravitational field. A particle at rest in a gravitational field is clearly not going to radiate em waves therefore by...
  44. S

    Equivalence Principle and Gravity

    I'm reading a book wherein the Earth's rotation is supposedly slowing down. The author claims that a ball thrown in the air would fall faster and harder... But if the rotation slows, wouldn't the equivalence principle say that the smaller acceleration could also be interpreted as a smaller force...
  45. P

    Using Wolfgang Pauli's Exclusion Principle for communication

    has anyone worked on using the spin of certain sister pairs of subatomic particles as digital communication that is that to have a device that spins one sister particle one way or another, that spins its sister particle in a receiver on the other side of the world or universe where a sensors...
  46. snoopies622

    The uncertainty principle and electric field strength

    As my knowledge of an electric field strength becomes more precise, what physical quantity becomes correspondingly less precisely known?
  47. N

    19-gon and Pigeonhole principle

    Hey! I have got some question for you. Decide if you can choose seven tops of the regular 19-gon and four of them are tops of trapezoid. (I think - Pigeonhole principle, but how?)
  48. ShayanJ

    A peculiarity in uncertainty principle

    Consider two hermitian operators A and B. Imagine a system is in state |\psi\rangle ,then we have: \langle \psi|[A,B]|\psi\rangle=\langle \psi|AB-BA|\psi\rangle=B^{\dagger}A^{\dagger}|\psi\rangle-BA|\psi\rangle=BA|\psi\rangle-BA|\psi\rangle=0 This just seems a little strange,for example...
  49. nomadreid

    Confused about the principle of least action

    To make the confusions both concise and explicit, I will put down some incorrect calculations, and ask for corrections Take the Lagrangian KE - PE = T - V Action = S=∫ L dt (with given limits) Principle of least action: δS= 0: S(t1)-S(t2) =0 if t1-t2 is small (using the (.) as function...
  50. nomadreid

    In holographic principle, the boundary is wrt which light cone?

    It is stated that in the holographic principle (e.g., in http://en.wikipedia.org/wiki/Holographic_principle) that the the description of a volume of space is encoded on a light-like boundary to the region. But with respect to which position in the volume? In a black hole, it is clear, because...
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