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. S

    Hamilton's Principle and the Principle of Virtual Work

    I understand that accepting Hamilton's principle will yield identical results as accepting Newton's laws. However, simply accepting that the integral of the difference between kinetic and potential energies is an extrema seems not intuitively obvious. The textbook that I used for my classical...
  2. S

    Heisenberg Uncertainty Principle Derivation

    I know I have seen this before a few time done a few different ways/starting points and I am always blown away when I see it worked out. Don't know why but I love seeing this derivation. Does anyone want to post the derivation for all to see, it being done a few different ways would be great.
  3. E

    Heisenberg's Uncertainty Principle

    My source is the high school chemistry textbook: General Chemistry, 2nd Edition, by Donald A. McQuarrie and Peter A. Rock, published 1987 (This is not for a high school homework assignment.) According to Heisenberg's Uncertainty Principle, the product of the uncertainty in the momentum...
  4. A

    Pauli exclusion principle at distance

    Hi. New member to this Physics forum and not a physicist, although have an interest in physics from a layman's position. I saw a series of threads on a Twitter discussion posted about a year ago concerning Brian Cox and some other physicists concerning a statement made by Cox that the...
  5. L

    Archimedes principle - special case

    Homework Statement We have a chinese lantern (balloon) made of paper, cylinder shaped with the following sizes: base diameter - 45 cm, height - 70 cm. Mass of the balloon is 57 g (21 g from this is mass of "fuel" - the fuel is wax paper!). Fuel is then ignited at the centre of the base, which...
  6. Q

    How Does Halving the Volume Affect Gas Equilibrium in a Closed System?

    Homework Statement There is a constant volume in a cylinder filled with three gases - ammonia, nitrogen, and hydrogen. The volume is 1.0 L. The equilibrium constant of the reaction of nitrogen and hydrogen to form ammonia is 1.0. Find the number of moles of each gas at equilibrium, and what...
  7. B

    Heisenberg's uncertainty principle

    Homework Statement So here's the question An electron is confined within a region of width 1 X 10^-10 meters. Estimate the uncertainty in the x component of the electrons momentum Homework Equations ΔPΔx\geq(1/2)(h/2pi) The Attempt at a Solution The problem appears pretty...
  8. A

    What is the role of chemical potential in conduction?

    I am studying some simple models of how conduction arises on a microscopic level. A central idea is the following: Look at two leads attached by a conducting channel. Initially the system as a whole is in equilibrium which especially means that the chemical potential μ is the same everywhere...
  9. Y

    Fourier Transform, and the uncertainty principle

    Hello, Recently I've learned about Fourier Transform, and the uncertainty principle that is arose from it. According to Fourier Transform, if there is only one pulse in a signal, then it is composed from a lot more frequencies, compared to the number of frequencies that are building a...
  10. N

    Newton's Second Law / Archimedes Principle

    Homework Statement A block of wood floats in fresh water with 0.722 of its volume V submerged and in oil with 0.895 V submerged. Find the density of (a) the wood and (b) the oil. Homework Equations mass = density x volume f = ma conservation of mass F_{weight} - F_{buoyancy} =...
  11. B

    What is the Variational Principle for Estimating Energy of First Excited State?

    Homework Statement A particle of mass m is in a potential of V(x) = Kx4 and the wave function is given as ψ(x)= e^-(ax2) use the variational principle to estimate the ground state energy. Part B: The true ground state energy wave function for this potential is a symmetric function of x...
  12. M

    Variational Principle and Vectorial Identities

    Hello there, I am struggling in proving the following. The principle of Minimum energy for an elastic body (no body forces, no applied tractions) says that the equilibrium state minimizes $$\int_{\Omega} \nabla^{(s)} u D (\nabla^{(s)}u)$$ among all vectorial functions u satisfying the...
  13. Sudharaka

    MHB Reducing Quadratic Form to Principle Axes

    Hi everyone, :) Here's a question with the summary of my method of how to solve it. I would really appreciate if you could go through it and let me know if there are any mistakes with my approach. Also are there any easier methods? Problem: Find an orthogonal transformation that reduces the...
  14. N

    Planck's Constant/Uncertainty Principle

    I just got to the chapter of Quantum Physics in this book for amateurs. A couple of questions: If Planck's constant was smaller than it actually is, then how the uncertainty principle be affected? What if Planck's constant was zero? ALSO: When you flip a coin, the outcome is uncertain. Does...
  15. B

    What Are the Differences Between Law, Principle, and Theory in Science?

    Hi, I tried to google it, but I got much more confused with different and contradictory definitions. Can somebody please help me understand the difference between these terms? Is there overlapping between them? Some books say "Law of conservation of energy", but others say "Principle of...
  16. K

    Origin and Nature of Gauge Principle

    Gauge Principle is successful in strong electroweak force modeling in the the form U(1), SU(2), SU(3) and in GR but it fails in SU(5) or attempted gauge symmetry between leptons and quarks, it fails in Supersymmetry, fails in Supergravity, and even fails in String Theory. Don't we even know...
  17. edguy99

    Is Heisenbergs uncertainty principle valid?

    An article at physicsworld.com suggests that arbitrarily small measurements can be made. Ozawa: "My theory suggests if you use your measuring apparatus as suggested by the maker, you can make better measurement than Heisenberg's relation" Regarding his opposition: "They now prove that if...
  18. Ravi Mohan

    Pauli's exclusion principle

    How do we prove Pauli's exclusion principle? My professor makes a Slater determinant and then merrily shows how it disappears when two columns or rows are same. That is not Pauli's principle, is it? It is based on an assumption that certain particles are described by certain states. So my...
  19. M

    What Is the Speed of a Block Launched by a Compressed Spring?

    Homework Statement So we have a test this Friday over several topics in modern mechanics. Our professor gave us last year's test to use as a study guide, but hasn't posted the solutions yet. I'm not sure I'm doing this one right so I was hoping someone could check my work! I feel like my...
  20. F

    Feynman Lectures and Uncertainty Principle

    I read the Quantum Physics section of the online version of Feynman lectures http://feynmanlectures.caltech.edu/I_02.html#Ch2-S3 and I don't understand how he can deduce electron momentum from the Uncertainty Principle. I agree that the momentum is uncertain but how can he deduce that it is very...
  21. C

    Relationship between Principle of Least Action and Continuity Equation

    Is there a profound relationship between Principle of Least Action and Continuity Equation? Can we derive one from another?
  22. B

    Uncertainty principle asymmetric with respect to time?

    We take for granted the fact that we can infer more about the past than the future. Considering the only difference between past and future is entropy, I wonder if the reason it is possible to have records of the past and not the future is entropy related. At the quantum level is the...
  23. arivero

    Sommerfeld uncertainty principle

    Ok, I http://en.wikipedia.org/wiki/Academic_genealogy_of_theoretical_physicists#Arnold_Sommerfeld that Heisenberg had studied under Arnold Sommerfeld, but I have just now learned that Sommerfeld proposed a sort of uncertainty principle in the first Solvay meeting. That was 1911. It seems...
  24. I

    Deriving fermat's principle of least time (lagrangian stuff)

    original question: Consider light passing from one medium with index of refraction n1 into another medium with index of refraction n2. User Fermat's principle to minimize time, and derive snell's law (n1sintheta1=n2sintheta2). here is part of the solution key http://i.imgur.com/S1Pg9jC.png...
  25. I

    Bernoulli's principle in stagnant situations

    please note in the process proving bernoulli we use the CONTINUITY equation.Consider a situation wherein a pitot tube is used to measure pressure/velocity in a steady flow(picture attached for reference).Here we use bernoulli for pressure measurement.we apply the bernoulli theorem for a...
  26. A

    Archimedes principle vs Atwood's principle

    If we have an upward moving sphere with 0.4 m3 volume and 100 kg mass , according to Archimedes buoyancy force = 4000 N so acceleration = 40 m/s^2 But according to Atwood's principle , buoyancy force ≈ 1600 N acceleration = 16 m/s^2 what makes such big difference . ( ignore drag force and...
  27. T

    Diffusion Equation and Maximum Principle

    "Consider a solution of the diffusion equation ##u_{t} = u_{xx}## in {0 ≤ x ≤ L, 0 ≤ t ≤ ∞}. a) Let M(T) = the maximum of u(t,x) in the closed rectangle {0 ≤ x ≤ L, 0 ≤ t ≤ T}. Does M(T) increase or decrease as a function of T? b) Let m(T) = the minimum of u(t,x) in the closed rectangle {0 ≤ x ≤...
  28. B

    What Uncertainty Principle actually is?

    What Uncertainty Principle actually is? I searched on the internet and got an amazing answer that the energy could form out of thin air. Who could explain more specifically to me, who is in year 11.
  29. H

    Use uncertainty principle to obtain the result of Bohr's Model

    Problem Find the minimum energy of the hydrogen atom by using uncertainty principle a. Take the uncertainty of the position Δr of the electron to be approximately equal to r b. Approximate the momentum p of the electron as Δp c. Treat the atom as a 1-D system My step 1. Δr Δp ≥...
  30. S

    Need help understanding Superposition Principle

    need help understanding "Superposition Principle"..! hello everyone.. if we have a function y=f(x) then in-order to prove linearity we try to justify according to superposition principle as : let x1 and x2 be two inputs then f(x1+x2)=f(x1)+f(x2) please correct me if i am wrong upto here...
  31. A

    Archimedes principle and shapes of the body

    Archimedes principle of the force of upthrust = weight of water displaced by the body immersed , but is this law independent of the shape of this body. If we use the idea that the upthrust = P*A on the lower side - P*A on the upper side , if we use it on a rectangle or a square , the only change...
  32. S

    Maxwells equations from variational principle

    1. Hey, I have to find Maxwells equations using the variational principle and the electromagnetic action: S=-\intop d^{4}x\frac{1}{4}F_{\mu\nu}F^{\mu\nu} by using \frac{\delta s}{\delta A_{\mu(x)}}=0 therefore \partial_{\mu}F^{\mu\nu}=0 3. I have had a go at the...
  33. V

    What formula or principle governs this observed phenomena?

    Hello, Just discovered this forum as I'm so intensely curious about this question I sought out just such a place! I'm currently designing a circular patio using 6-in square blocks in concentric circles. While using a combination of Excel and Adobe Illustrator, I uncovered something...
  34. D

    Uncertainty principle for electron in nucleus

    Hello, I have this exercise and want to check if what i did is correct Homework Statement Nuclei, typically of size $10^{-14}$ m, frequently emit electrons with energies of 1-10 MeV. Use the uncertainty principle to show that electrons of energy 1 MeV could not be contained in the nucleus...
  35. N

    Calculators Using Ti-89 to solve for Three dimensional Principle Stresss

    I've got a TI-89 Titanium I've taking a class in Mechanical Design and my professor informs me that we need to be able to solve the equation (equation 3-15) shown in the attachment. I've been trying to find a way to program it into my calculator, but haven't had any real success. The...
  36. ajayguhan

    Examining Fermats Principle & Light's Path of Min Time

    Why does light follows a path of minimum time? I.e., is there any theory or explanation behind the fermats principle?
  37. J

    Schrodinger's Cat vs. The Principle of Explosion

    onsider the following link: http://en.wikipedia.org/wiki/Principle_of_explosion Now, consider Schrodinger's Cat. Does this not imply the following statements? - A) The cat is alive before the box is opened. - B) The cat is not alive before the box is opened. Aren't both of these statements...
  38. A

    Calculating Momentum with Constant Force

    Homework Statement At a certain instant a particle (mass=0.5 kg) is moving in the +x direction with velocity 14 m/s. During the next 0.15 seconds a constant force acts on the particle, with Fx= −8 N and Fy= Fz = 0 N. What is the magnitude of the momentum of the particle (in kg*m/s) at the end...
  39. P

    D'alembert's Principle: Doubts Explained

    /I'm having some doubt with D'alembert's Principle. The principle states that \sum_{i}(\vec {F}_i - \dot{\vec{p}}_i)\delta\vec{r}_i=0 but does that mean that each term of the summation must vanish too, or just the sum does? I know that mathematically there's no need that each term shall vanish...
  40. R

    A question about the equivalency principle

    If you had two synchronized clocks in two side by side elevators, and one sat on the surface of the Earth in 1 g for ten years, while the other one accelerated out into space, turned around, and then came back (all at one g, aside from escaping and reentering the Earth's gravity), would the...
  41. G

    Heisenberg uncertainty principle and wavelength of light

    I read somewhere that Heisenberg described his uncertainty principle by saying that you can't measure position more accurately than the wavelength of light (which makes sense), so Δx > λ. This is what I don't get. He then says that p=h/λ, so Δp > h/λ2 Δλ. He the multiplies and sets Δλ ≈ λ to...
  42. T

    Initial-Value Version of Hamilton's Principle?

    As far as I understand, Hamilton's principle (a.k.a. "least action" or "stationary action") requires that you know both the initial and final location of a particle. Then, based on the requirement that the action must be stationary along any "possible" path, it will tell you what path(s) the...
  43. J

    Equivalence Principle: Einstein's Dilemma

    Hey, you know the way einstein said that its impossible to tell if you`re in a box that is accelerating or in a gravitational field? But, don`t accelerating charges radiate? so theoretically, by looking at these charges you could in fact tell the difference ? I dunno, maybe I`m missing...
  44. R

    Galilean principle of relativity

    Galilean principle of relativity is for sure not absolutely true, isn't it? Say one has a pool full of water within a spaceship, if the spaceship is accelerated and then acceleration stops, one can look at the ripples in the pool, analyze their form and then tell, up to a certain degree of...
  45. D

    Generalized uncertainty principle

    Homework Statement A spin-half particle is in a known eigenstate of Sz. Show that the product <S^2_x> < S^2_y> is consistent with the Uncertainty principle Homework Equations The Attempt at a Solution I know that the generalized uncertainty principle gives ΔS_x ΔS_y ≥ |<[S_x...
  46. D

    Uncertainty Principle textbook equation

    I have been going through my Physics textbook to brush up on my Quantum Mechanics before starting my next QM course next academic year and the Heisenberg uncertainty principle for position and momentum is written as ΔxΔp ≥ h-bar when I thought it was ΔxΔp ≥ (h-bar)/2. Other sources say it is...
  47. E

    The Pauli Exclusion Principle and Repulsive Forces in Atoms

    Some time ago I asked about contact forces and was directed to read about the Pauli exclusion principle and the resulting L-J potential before a detailed explanation was given explaining why the electrons start interacting significantly, electromagnetically, even if the atom as a whole is...
  48. andrewkirk

    A Cosmological Principle: Trying to define large-scale homogeneity

    I have been trying to pin down a precise definition of large-scale homogeneity, in the context of saying, per the Cosmological Principle, that all constant-time hypersurfaces (CTHs) of a foliation are large-scale homogeneous. Here is my attempt: Let M represent any coordinate-independent...
  49. E

    Principle of least action used to predict particle movement?

    I am having a tough time understanding the principle of least action and I would really appreciate it if someone would clear it up for me. And from what I understand from it, it can be used to predict particle motion?
  50. T

    Holographic principle, mathematical universe & dimensional reduction

    If the holographic principle says that the the observable universe could be represented as a two dimensional surface, and we accept the idea that the universe is itself a mathematical object, and cantor showed that we could in some cases represent higher dimensional sets within lower dimensional...
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