What is Sign convention: Definition and 69 Discussions

In physics, a sign convention is a choice of the physical significance of signs (plus or minus) for a set of quantities, in a case where the choice of sign is arbitrary. "Arbitrary" here means that the same physical system can be correctly described using different choices for the signs, as long as one set of definitions is used consistently. The choices made may differ between authors. Disagreement about sign conventions is a frequent source of confusion, frustration, misunderstandings, and even outright errors in scientific work. In general, a sign convention is a special case of a choice of coordinate system for the case of one dimension.
Sometimes, the term "sign convention" is used more broadly to include factors of i and 2π, rather than just choices of sign.

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  1. M

    Sign conventions for Kirchhoff's loop rule

    Battery and Capacitor For a battery (or capacitor), the potential increases by ##\mathcal{E}## (or ##\displaystyle\frac{q}{C}##) as we move from -ve to +ve terminal (or plate) regardless of the direction of the assumed current. Resistor and Inductor Suppose we are traversing the loop in the...
  2. Johan M

    Simplified rocket lateral dynamics model sign convention

    Hi everyone :smile: . I had came across a simplified simplified rocket lateral dynamics model :https://github.com/build-week/hover-jet/blob/feature/start-design-scripts/design-scripts/jet_vane_speed.ipynb . It has vanes at the exit which generate lift force and can control the rocket...
  3. A

    Sign convention of internal forces in vertical bars for bending moment

    Hello: I was looking for a widespread convention (akin to Hibbeler's, Beer's, etc) that deals with the sign convention of a vertical bar for bending moments. For example, without knowing in advance, how do I draw the bending moment at a cut passing through point E in the figure attached? Beam...
  4. E

    Thermodynamic sign convention for heat (i.e. in heat engines)

    Just to clarify, I'm aware of the two equivalent expressions of the first law ##\Delta U = Q + W## and ##\Delta U = Q - W## when applied to a certain system, though my question is primarily about ##Q## - for which, as far as I am aware, the convention is almost universally that ##Q > 0## if heat...
  5. bagasme

    B Different Sign Conventions used with Kirchhoff's Law?

    Hello, As far as I know, the standard sign convention for Kirchhoff's law are: For resistor: the potential difference (voltage) is negative if the chosen loop direction is the same as chosen current direction through the resistor, positive otherwise. For battery: the voltage is positive if the...
  6. J

    Simple clarification regarding the sign convention for work

    Precursor : Sign conventions regarding Work by the system/on the system may even vary among Physics textbooks and among Chemistry textbooks and as a rule of thumb it is better to clearly mark out the convention used by the text being referred to specifically and remain consistent with it...
  7. WeiShan Ng

    [Fluid Mechanics] Negative sign on the viscous work term

    Homework Statement I am revising on the derivation of the differential equation of energy (White's Fluid Mechanics 7th ed) and I'm having trouble understanding the sign convention used in the viscous work term. The textbook first define an elemental control volume and list out the inlet...
  8. WeiShan Ng

    [Calc] Sign Convention in damping spring system

    Homework Statement A carriage is mounted on a spring, as shown in the diagram. The bottom of the spring is fixed to the ground. The carriage (loaded with its passenger) has a mass of 150kg. The carriage can only move vertically. The natural length of the spring is 10m and its spring...
  9. M

    How does the sign convention work in ray optics?

    Homework Statement I needed help to undestand some concepts in Ray Optics for my assignment. The topics which U am concerned are: Part 1: Mirror Equation, Refraction at singly curved surface, lens maker's equation, combination of thin lenses in contact. Part 2: Magnification Power of simple...
  10. N

    Does the Direction of Current Affect Ohm's Law Sign Convention?

    Here the voltage is negative so the current is negative. does it matter if I traverse the circuit through the other direction?
  11. S

    What is the Cartesian sign convention for microscope and telescope distances?

    Homework Statement Cartesian sign convetion for microscope,compound microscope and telescope. Homework Equations distances in the direction of incident ray are negative and thsoe along it are positive. all distances are measured from pole of lens or mirror. The Attempt at a Solution 1.FOR...
  12. F

    Sign convention of fixed end moment at cantilever beam

    Homework Statement I don't understand why the fixed end moment at AE is 10KNm ( clockwise) . Homework EquationsThe Attempt at a Solution Forger about the sign convention here , i have the resultant moment = 10KNm , which ic clockwise , why in the 2nd picture ( SFD and BMD part) , the moment...
  13. F

    Moments -- sign convention in beam

    Homework Statement In this question , we can see that in teh second picture , the 12.86kNm is in clockwise direction at the right of span BC . Homework EquationsThe Attempt at a Solution In meachnic of materials , i learned that when the beam curve upwards ( U shape) , then it has positive...
  14. F

    Sign convention of shear stress confusion

    Homework Statement In my book , i was told that when the shear stress turn a plane of element clockwise , then the shear stress is positive , In this question , we can notice the shear stress of 40 will turn the plane clockwise , why the author plot the stress at A = (80,-40) ? Shouldn't it be...
  15. F

    Sign convention problem in momentum calulations

    Homework Statement Particle A has a mass of 1kg and velocity 2x10^8m/s to the right and collides with a stationary particle B that has a mass of 4kg. after the collision, particle A moves to the left with a velocity(v) and particle B moves to the right with a velocity of 1x10^7 m/s. calcuate...
  16. C

    Sign convention of shear stress

    Homework Statement In the in the first image , it's shown that the shear stresses has positive sign, whereas in the second image e (circled blue part) has different sign convention,which is positive and negative, which is correct? Homework EquationsThe Attempt at a Solution IMO, the second...
  17. C

    Sign convention in Mohr's circle

    Homework Statement I doubt that is the following notes (in the first picture), correct or not? Because i couldn't find any proof in the other book... Homework EquationsThe Attempt at a Solution Can I say that consider σ positive to right and τ positive downward, orientation (θ) clockwise as...
  18. C

    Sign convention of shear force and moment on beam

    Homework Statement I don't understand why the sign of moment of beam at left (clockwise) and moment at the right of beam (anticlockwise) are considered as positive... Why we can't assume the clockwise moment as positive and anticlockwise moment as negative? Homework EquationsThe Attempt at a...
  19. C

    Sign convention of shear force and bending moment

    Homework Statement http://nptel.ac.in/courses/112107146/22[/B] in the notes, the author define upward shear force as positive (LHS) , he take downward shear force as positive as positive (RHS)? Homework EquationsThe Attempt at a Solution why the author didnt fix downward shear force as...
  20. UchihaClan13

    I Why is b>1 and x,y positive in logarithm definition?

    A simple doubt came to my mind while browsing through logarithmic functions and natural logarithms we define $$\log_b(xy) = \log_b(x) + \log_b(y)$$ Here why is the condition imposed that b>1 and b is not equal to zero and that x and y are positive numbers? Is it something to do with the...
  21. sharmax

    A Raychaudhuri Shear Equation in Different Sign Conventions

    Hi, I derived Raychaudhuri Equations in both (- + + +) and (+ - - -) sign conventions from metric. In Robert Wald and Sean Carroll books, (- + + +) sign convention and I derived correctly in that Sign Convention as given in the books. In the other convention, I am having one sign difference in...
  22. UchihaClan13

    Sign Convention for Work (In Thermodynamics)

    Okay so after reading various books on sign conventions for work I am told by my teachers that in chemistry,work done on the system is positive while work done by the system is negative while in physics it's the exact opposite I only use the former convention And modify the first law of...
  23. droidofthevoid

    Identifying sign conventions for torque and tension

    My question arises from trying to get the correct sign convention for a pulley that has two boxes attached by a massless rope. one box is to the left of the pulley and the other box is hanging down below the pulley. If I take the counter clockwise direction to be negative, and set up my...
  24. Calpalned

    What is the relationship between dℓ and dR in the sign convention for voltage?

    In the derivation for equation 24-2 in the picture below., the line integral was chosen along a path anti-parallel to the field lines. As a result, cos(180) = -1, which made the integral for the voltage positive. However, in examples 24-2 and 24-3, the line integral is also taken as...
  25. Calpalned

    Does Moving in the Same Direction Affect Work Done by an Electric Field?

    If the work done by an electric field is N, then the work done by me will be negative N? Perhaps this applies if I move a charge against the field lines, but what if I move in the same direction?
  26. L

    What is the sign convention for work in dE = q - w

    Why is it not just dE = q + w because I assume if you use general sign convention and take work to be negative when it is done by the system and positive when it is done on the system, you should get the right answer. Silly question but I'd appreciate some clarity on this formula please?
  27. AdityaDev

    Thermodynamics: sign convention

    In physics book, if work is done by the system, it is positive in my chemistry book, if work is done by the system, it is taken as negetive. when I asked my chemistry teacher, he said the sign convention is different in physics and chemistry. but why are they different?
  28. V

    Passive Sign Convention Misconception?

    The Passive Sign Convention from Wiki: "In electrical engineering, the passive sign convention (PSC) is a sign convention or arbitrary standard rule adopted universally by the electrical engineering community for defining the sign of electric power in an electric circuit. The convention defines...
  29. O

    Understanding Passive Sign Convention

    Problem statement I'm trying to understand how passive sign convention works, and how to read the voltages/currents direction in a circuit. For different set of voltages and currents I'm trying to calculate power. I'm using the picture below as a reference. a) i=10A v=125V b) i=5a v=-240V c)...
  30. X

    Understanding Passive Sign Convention: Power Calculation Example (P=VI)

    Homework Statement I am having trouble understanding PSC. Please help on this question. Teach me like I am 5. My tutor couldn't break it to me. (SEE ATTACHED) and find whether the power is being given or taken Homework Equations p=vi p=-vi The Attempt at a Solution I can't...
  31. 7

    Regarding r and sign convention for Position vectors for moments

    Homework Statement The Attempt at a Solution I'm having trouble understanding how r1 = -1.5j and r2= r3 = 0. Can anyone make this a little clearer for me? I've spent quite a while trying to wrap my head around it but to no avail.
  32. morrobay

    Sign Convention For v and x In Delta t = gamma(delta t' + v delta x')

    With Δt = γ ( Δt' + v Δx' ) For calculating delta t for stationary twin with values of traveling twin. Is it conventional that velocity (v) outbound is positive and distance (x') is negative. Then at turnaround point sign of velocity changes to negative. And for inbound v is negative and...
  33. Z

    Mesh Analysis / Sign Convention - Conceptual Problem

    Homework Statement I am having a little trouble using mesh analysis. I'm trying to stick rigorously to the passive sign convention, as I'm told that this is sensible. Consider the circuit diagram attached, where I have added the mesh currents in orange. In red are the polarities for the...
  34. STEMucator

    Passive Sign Convention Homework: Understanding the Concept and Solving Problems

    Homework Statement This is a problem I made myself, I was curious to see if I'm understanding this convention properly. The link: http://gyazo.com/24c7f7f82f6453a3396d96d67c2a067d Homework Equations ##P=IV## The Attempt at a Solution Okay, from what I understand, the power...
  35. C

    Simple voltage drop / sign convention

    Homework Statement Find Vad and VebHomework Equations voltage drop, KVLThe Attempt at a Solution I actually did: Vad -24V - 4V + 6V = -22V but this is wrong.I know going the other way Vad: 6V + 8V + 12V = 26V, because it is only drop, and plus to minus on drops means they are...
  36. S

    Understanding the Sign Convention for Lens Formula Proofs

    I googled for lens formula's proof on the internet and found one here http://www.tutorvista.com/content/physics/physics-ii/light-refraction/convex-lens-formula.php The problem is that during the course of derivation, it is assumed that image distance is -u following the sign convention...
  37. L

    Does this obey passive sign convention? (Circuits)

    I thought passive sign convention is obeyed when you have a positive current flowing into the positive terminal of the element first. But how about this case? While it is drawn that way, the positive terminal is actually the NEGATIVE terminal because the voltage is negative, so you have to flip...
  38. C

    Understanding Sign Convention and Its Application in Vectors

    Homework Statement What is sign convention, and how is it applied to vectors? I was reading an economics paper when I came across this term. There's a ":" sign between the vector in question and an entry, which is all supposed to output a real number. What exactly is sign convention, and how...
  39. P

    Minkowski Metric Sign Convention

    Hello, I believe this is a really stupid question but I can't seem to figure it out. So given a Minkowski spacetime one can choose either the convention (-+++) or (+---). Supposedly it's the same. But given the example of the four momentum: Choosing (+---) in a momentarily comoving...
  40. S

    Sign Convention For Momentum Operators

    Greetings, How do we decide on which sign to take when using the momentum operator? The question may be very simple but I need a push in the right direction. Many thanks.
  41. H

    Confusion on sign convention for surface integrals

    Homework Statement Compute the surface integral for F = [3x^2, y^22, 0] and S being a portion of the plane r(u,v)=[u,v,2u+3v], 0≤u≤2, −1≤v≤1.The Attempt at a Solution I managed to get the correct answer, because with some luck I defined the normal in the correct direction. I am just confused...
  42. S

    Frictional force- sign convention

    Frictional force-- sign convention For a horizontal spring mass system is the frictional force considered negative (-μmg)? i guess it will be negative; the minus sign used to convey the force is trying to impede the motion of block then with that guessing work; i think friction force will be...
  43. C

    Question about sign convention for R1 and R2 when using the lens maker's formula

    Hi! I'm having some trouble with optics. When we have convex, planoconvex, concave, planoconcave, and meniscus lenses, I don't understand how to substitute with the sign for R1 and R2. How do I determine whether the radius of a side is negative or positive? A book that I'm using said to take...
  44. C

    Another sign convention issue (optics)

    I read through all of the previous sign convention posts on this forum (and googled a lot and checked a bunch of textbooks) before posting this question. I don't understand the sign convention properly for geometric optics. There are two conventions that I've understood so far, and I've got a...
  45. G

    Sign Convention for Spring and Pendulum Equations of Motion

    If a mass was hanging vertically from a spring under its own weight. In the static equilibrium position, using up as positive, the spring force is +kx and gravity is -mg. Add them up to get: kx - mg = 0. Using down as positive, the spring force is -kx and gravity is +mg. Add them: -kx + mg =...
  46. O

    Moments Problem Regarding Sign Convention?

    Homework Statement The handle of the hammer is subjected to the force of F = 20. Determine the moment of this force about the point A. http://s3.amazonaws.com/answer-board-image/3214a2db-1a55-4648-b582-5f36f0f34451.jpeg Homework Equations Principle of moments: Mo = FxY - FyX The...
  47. A

    Sign convention for work of a gas

    if a gas is expanding,what will be the sign of the work done by it?
  48. P

    Sign convention in geometric optics.

    Until today I learned in geometric optics that Object distance +ve for real object else -ve Image distance +ve for real image else -ve Radius of curvature +ve for if light comes to the surcace from the side lying center of curvature else -ve On the basis of this the lens formula...