Principle Definition and 1000 Threads

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. K

    Optics: Huygens' principle and bending of light

    I know the technical details of why light bends towards "the normal" when meeting a more refractive medium must be complicated. But I was thinking about it in a more lay fashion. I was thinking if the bending can be explained using Christiaan Huygens' principle in that a light front is made of...
  2. Another

    I About the Heisenberg uncertainty principle

    heisenberg uncertainty principle ## Δx Δp ≥ ħ## where ##Δx = \sqrt{<\hat{x}^2>-<\hat{x}>^2}## ##Δp = \sqrt{<\hat{p}^2>-<\hat{p}>^2}## I don't know. Why ##Δx## equal to ## \sqrt{<\hat{x}^2>-<\hat{x}>^2} ## and ## Δp ## equal to ## \sqrt{<\hat{p}^2>-<\hat{p}>^2} ## What can I find out about...
  3. D

    Archimedes' principle -- Which box will sink first as we add coins....

    There are five empty box contains with the same weight. The upper side of each boxes are removed. These boxes are floating in a pool of water. Then to each box we put coins slowly one by one. Assuming the box does not tilt. Determine which of the following size of the box (p x l x t) can be...
  4. Jazzyrohan

    I Einstein's Equivalence Principle: Freely Falling Local Frames

    Einstein's equivalence principle states that: The sets of inertial frames in the real world that correspond to (portions of) the ideal set of inertial frames discussed in special relativity consist of freely falling local frames. In other words,can we say that since all the local frames are in...
  5. S

    I Question about the uncertainty principle

    The uncertainty principle says that you can't know position and velocity of particle at the same time. So particular we can not say that the particle is at rest at some point because then we would know it is not moving and we would know exactly where it is. So my question is if we send the...
  6. D

    B Heisenberg's uncertainty principle

    Instead of just taking one measurement of the particle, you take a 2nd measurement in addition thereby gathering more information about the particle then the uncertainty principle allows?It would be possible to extend out to an arbitrary number of follow-on measurements thereby measuring...
  7. Ssnow

    I On Heisenberg's uncertainty principle

    Hi everybody, my question is a curiosity on the (generalized) Heisenberg principle: ## \sigma_{x}\sigma_{p} \geq \frac{\hbar}{2},## where ##x,p## are the usual quantum operators and ##\hbar## the Planck constant divided by ##2\pi##. If I understood correctly, Gaussian states that are solution...
  8. T

    Bernoulli's Principle and Fans [split]

    That's confusing. If the velocity at the exit is increased, that simply means kinetic energy is increased. But as the pressure is decreased and suppose the process is adiabatic (in case of compressible fluid), that means the temperature too is decreased. That simply means the enthalpy of the...
  9. S

    What is the relation between Bernoulli's principle & Pascal?

    Bernoulli's principle states that under dynamic conditions total energy inside the fluid container remains constant. and if area is decreases then pressure decrease . and Pascal states that pressure = force/area . here if area decreases then pressure increase . I'm getting confusing...
  10. Zubair Ahmad

    I Fermat's Principle: Is Light Travel Time Min?

    Is time taken by light always minimum between two points. If yes then what is wrong with Fermat's original statement?
  11. C

    I Does the Principle of Least Action Have a Physical Meaning?

    I have found that some people say “yes, definitely”, and other days “no, definitely not”. Those who say “no” seem to regard PLA as merely a neat way of packaging the equations. Those who say “yes” seem to regard PLA as somehow fundamental. (There have actually been two recent books on this...
  12. Krushnaraj Pandya

    What is the Principle of a Capacitor?

    Homework Statement I was reading through the principle of a capacitor and I am unable to understand one of the statements. My textbook says the following- Consider an insulated metal plate A. Let it be connected to a voltage source and charged to a maximum. Now an uncharged plate B is brought...
  13. Math Amateur

    MHB ZFC and the Pairing Principle .... Searcoid Theorem 1.1.5 ....

    I am reading Micheal Searcoid's book: Elements of Abstract Nalysis ( Springer Undergraduate Mathematics Series) ... I am currently focussed on Searcoid's treatment of ZFC in Chapter 1: Sets ... I am trying to attain a full understanding of Searcoid's proof of the Pairing Principle ... The...
  14. F

    I Heisenberg - Uncertainty principle - lifetime of a particle

    I have seen that the more a particle has a high energy, i.e ##E##, the more its lifetime is short, respecting so the uncertainty principle. But by the definition of this uncertainty principle : ##E\,\Delta t \geq \dfrac{\hbar}{2}##, I can write : ##\Delta t \geq \dfrac{\hbar}{2E}##, then...
  15. Sandeep T S

    I Principle of Equivalence: Are You in a Gravitational Field?

    Consider 2 lifts ,one on ground and other on acceleration, principle of equivalance says you can't find you are on a gravitational field, or accelerating. g decrease when r increase so I can find which lift on gravitational field ?
  16. H

    B Maxwell's EM Theory vs Principle of Relativity

    What was the problem between Maxwell's EM theory and the principle of relativity? Why went the theory against the principle? I understand that the EM theory says that Light was a wave and ether is it's medium. On the other hand the principle of relativity says that there is no state of...
  17. facenian

    I Equivalence Principle: Controversy & Misinterpretations

    Is there a controversy over the EP? What I mean is: is it considered to be false beyond any doubts or on the contrary it is absolutely true and doubts about its validity are only misinterpretations? I know that the question it's not so simple because there are many EPs, strong, weak, etc. but I...
  18. C

    I Pauli principle and coupling term in Weizsäcker formula

    Consider the pairing term in Weizsäcker formula. Here https://en.wikipedia.org/wiki/Semi-empirical_mass_formula#Pairing_term it is claimed that: I don't understand how Pauli exclusion principle should be the cause of this. This term comes from spin-spin interaction (or "coupling"), but I do not...
  19. J

    I Does the Uncertainty Principle imply a linear cosmology?

    If the uncertainty in the age of the Universe is ##\Delta t## then the Uncertainty Principle implies that it has an uncertainty in its energy ##\Delta E## given by $$\Delta E \ \Delta t \sim h.\tag{1}$$ If this energy fluctuation excites the zero-point electromagnetic field of the vacuum then a...
  20. Paul Colby

    B Closed Time-Like Curves: Principles & Observations

    Hi, If I understand correctly, closed time like curves (CTC) are world lines that close upon themselves. What would an observer measure to demonstrate a CTC?
  21. D

    What is the operation principle of the LDR?

    Homework Statement Construct suitable circuit and record your finding to illustarte the operational principle of the sensors: sound, light Results: Distance from light source (cm) - light intensity (lx) 0 - 1000 10 - 400 20 - 260 30 - 200 40 - 190 50- 180 experiment involved moving a...
  22. J

    A Classical uncertainty principle

    What assumptions underlie the classical uncertainty principle? The principle doesn't seem to apply when I want to know the precise pitch of a tone from a bowed violin string, since I can measure the duration (as precisely as I want) of the beats produced when I interfere it with a sine wave of...
  23. dRic2

    I How Does Measurement Affect Uncertainty in Quantum Mechanics?

    I'm having trouble trying to remember this: suppose I have an operator ##A## and an eigenfunction ##\psi = c_1 \psi_1 + c_2 \psi_2## for ##A## so that $$ A\psi = a\psi$$ The expected value is ##\langle A \rangle = \langle \psi|A|\psi \rangle = a## After a measurement ##a## can be ##a_1##...
  24. T

    Prove Hausdorff's Maximality Principle by the W.O.P.

    Homework Statement Show Hausdorff's Maximality Principle is true by the Well-Ordering Principle. 2. Relevant propositions/axioms The Attempt at a Solution Case 1: ##\forall x,y\in X## neither ##x\prec y## or ##y\prec x## is true. Hence any singleton subset of ##X## is a maximal linear order...
  25. E

    I The energy-time uncertainty principle

    I'm watching a freshman-level lecture trying to take students through the energy-time uncertainty principle. (They've covered the position-momentum uncertainty principle). In the lecture, the professor starts by saying that we have a particle with some momentum, but that we can't know the...
  26. Mathmellow

    MHB Understanding induction proof of pigeonhole principle

    I am struggling to understand the induction proof of the pigeonhole principle in my textbook. The theorem and the proof, from Biggs Discrete Mathematics, is pasted below, and I will explain further (see bold text) what I am having trouble with. Theorem. Let m be a natural number. Then the...
  27. S

    I Pauli exclusion principle explained

    Hello! I am a bit confused about the Pauli exclusion principle. Let's say I have 3 electrons. Due to energy considerations the first 2 go to the ground state, and they can be only 2 electrons there, because the position wavefunction has only one option ##\psi_{100}## (and again due to energy...
  28. Z

    Derivation of the energy principle from Gregory Classical Mechanics textbook

    I'm working through Gregory's Classical Mechanics and came across his derivation of energy conservation for a system of N particles that is unconstrained. We get to assume all the external forces are conservative, so we can write them as the gradient of a potential energy. There's a step he...
  29. C

    Uncertainty Principle and the lifetime of an excited state

    Homework Statement [/B] Estimate the lifetime of the excited state of an atom whose natural emission line width is 3.00 × 10−4 eV. Homework Equations $$ \Delta E \Delta t = \frac{1}{2} \times \frac{h}{2\pi} $$ $$ \Delta E \Delta t = \frac{h}{2\pi} $$The Attempt at a Solution I think all I...
  30. e2m2a

    I Cosmological Redshift and Heisenberg Uncertainty Principle

    Has anyone conjectured that the redshift associated with Hubble's constant can be explained by Heisenberg's uncertainty principle? Another words, the further in space away from us a photon is emitted from a galaxy, the longer in time it takes for the photon to reach us. Since the photon has a...
  31. BookWei

    I Is the Action Always a Minimum in the Principle of Least Action?

    Hello, When we applying the principle of least action, we require ##\delta S=0##, which corresponding to the action S being an extremum. I am just wondering why do we say that the action is a minimum instead of a maximum for a physical path? Can I use the path integral to explain this problem...
  32. C

    I Local meaning of the equivalence principle

    [Moderator's note: Spun off from another thread due to topic change.] Can I say that "being at rest in a uniform gravitational field is locally equivalent to accelerating in flat spacetime, free falling in a uniform gravitational field is locally equivalent to nonacceleration in flat spacetime ?"
  33. Z

    Cyclotron Frequency: Is it Truly Independent of Energy and Mass?

    Do cyclotrons make use of the principle that the cyclotron frequency is independent of energy?
  34. S

    A What does this article mean? (The holographic principle and M-cosmology)

    I was reading an article about holographic principle and M-cosmology and how could they be used to interpret QM mechanics. But there's a part where it talks about holographic principle in a strange way to me. What does this article mean with this?: "Since for the participant observer the...
  35. maajdl

    Heat exchanger and the second principle

    Hello, I am developping a Python (/Pyomo) package for simulation and optimization of some industrial processes. I want to create global (simplified) models for heat exhangers (and more) and avoid differential equations. (to decrease the number of variables of the problem) Most often the...
  36. Clara Chung

    Deriving the Lens Maker formula of a plano-convex lens using Fermat's Principle

    Homework Statement Homework EquationsThe Attempt at a Solution I only need help on part c. I tried to add up t1 and t2 and differentiate it. However what variables should I differentiate with respect to? If I differentiate with respect to f I got f=root(2) * h, if i differentiate with respect...
  37. L

    MHB 3 newspapers- a question about Inclusion–exclusion principle

    There is a city with 100,000 people, which has 3 newspapers: A, B and C. 10% read A, 30% read B, 5% read C. 8% read A and B, 2% read A and C, 4% read B and C and only 1% read all of them. a) How much people read only one newspaper? b) How much people read at least two newspapers? c) If A and C...
  38. P

    I Time dilation vs the Equivalence principle

    Some time ago there was a similar thread https://www.physicsforums.com/threads/clock-hypothesis-gravity-time-dilation-and-equivalence-principle.929838/ but what I want to discuss is similar but not the same and I would like to specify my question in such way, that it hopefully won't go sideways...
  39. M

    B Archimedes principle and the density of a rock

    Hi, so I have a problem that I am working on and I think that I understand Archimedes principle but the density that I am calculating is just absurd. The problem is as follows: "A geologist finds that a moon rock whose mass is 9.28 kg has an apparent mass of 6.18 kg when submerged in water...
  40. synMehdi

    A Pontryagin minimum principle with control constraints

    Hi, I am trying to solve a control problem where I have to minimize the fuel consumption of a vehicle: $$J=\int_{0}^{T} L(x(t), u(t),t) + g(x(T),T)dt$$ ##L(u(t),v(t))=\sum\limits_{i,j=0}^{2} K_{i,j} u(t)^i v(t)^j ## is convex (quadratic) and the term ##g(x(T),T)## is to have a constraint in the...
  41. T

    B Understanding the Limits of Measurement in Quantum Mechanics

    Is it somehow possible to know both noncommutating properities of eg. a partcle, with using two indicators? Eg. two teams, with one is studing the momentum, and other one the position - then it's somehow impossible to exchange informations?
  42. J

    What's the superposition principle for group action?

    A very simple question: if given a vector ##v(t_0)## and two group functions ##G(t)## and ##G'(t)##, here ##t## is the parameter of time, the two group functions act on ##v(t_0)## simultaneously, then we can get a vector field ##v(t)##, then how to get ##v(t)##?
  43. S

    A Information encoding in the Holographic principle

    Can whatever type of information be encoded in a boundary in holographic principle? in a question some years ago regarding holography (https://physics.stackexchange.com/questions/75436/are-stokes-theorem-and-gausss-theorem-examples-of-the-holographic-principle) It is said that AdS/CFT is the...
  44. Physics345

    Increasing Yield of a System with Le Chatelier's Principle

    Homework Statement I am given 2NH3(g) + 2.5O2(g) ⇌ 2NO(g) + 3H2O(g)+heat. By using Le Chatelier's principle, describe and explain four ways in which an industrial chemist could increase the yield of nitrous oxide. Homework EquationsThe Attempt at a Solution For 2NH3(g) + 2.5O2(g) ⇌ 2NO(g)...
  45. T

    I Does Hyugen's principle apply in three dimensions?

    Does Hyugens principle apply in three dimensions ? If a surface wave (for simplicity an ocean wave) is propagating along the x-axis we know that this wave ray is a point source for wavelets on the y-axis but what about the z axis? If this diagram was 3d would we see a spherical wave front...
  46. E

    How do we use Bernoulli's Principle in this situation?

    Homework Statement A large water tank, open at the top, has a small hole in the bottom. When the water level is ## 30## ##m## above the bottom of the tank, the speed of the water leaking from the hole: A. is ##2.5## ##m/s## B. is ##24## ##m/s## C. is ##44## ##m/s## D. cannot be calculated...
  47. E

    When is Pascal's Principle applicable?

    Homework Statement Which of the following statements about Pascal’s principle is true? A. It is valid only for incompressible fluids B. It explains why light objects float C. It explains why the pressure is greater at the bottom of a lake than at the surface D. It is valid only for objects that...
  48. melo

    Understanding Bernoulli's Principle

    My understanding of Bernoulli's Principle is something like this: Pressure is inversely proportional to velocity. Fluid flowing through smaller cross-sectional area has increase velocity & decrease in pressure. Also P = F/A... so would force also decrease for fluid going through small area...
  49. .Scott

    I Event Horizons - Holographic Principle

    Another thread (now closed), got me thinking about the holographic principle. So I looked it up and found a couple of surprises. First it seems to be attached in some peculiar way to string theory. Second, it seems to be applied more restrictively than it I thought. It's especially that...
  50. L

    I Can anyone help me understand the Equivalence Principle

    Equivalence principles explains why lite bend towards massive objects, Einstein uses a moving lift to illustrate this, the light will seem to be bending if the lift is moving, but for a stationary lift, it will not because the position it strikes is stationary. So I think it is not correct...
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