Conventions in Physics

 

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Numbering and formatting figures and tables

 

Figures and tables in physics and physics-related fields follow the same basic principles as in other disciples, which you can learn about at length at our page on using visuals. For your reference, however, we have included here a set of author guidelines from the American Physical Society as one example of how you might choose to configure visual aids in your paper.

 

Numbering

  • Tables should be numbered in Roman numerals (I, II, III, etc.)
  • Figures should be numbered in Arabic numerals (1, 2, 3, etc.)
  • Parts of figures should be labeled 1a, 1b, etc.

 

Formatting tables

  • Tables should be titled in all caps with a roman numeral, such as “TABLE I.”
  • Include a short caption that explains the table content and defines any abbreviations used throughout the table (but not any that have already been defined in the text).
  • Align table entries by one of the following: flush left, centered, by unit, by operator, or by digit/decimal. Use whichever style makes the entries most readable. When possible, combine these alignment styles, such as:

table1

 

Formatting figures

  • Figure titles should be abbreviated and in all-caps followed by a period and an Arabic numeral, such as “FIG. 1.”
  • Like tables, figures should include a concise caption after the title that explains the figure and any abbreviations, acronyms, or symbols used that are not previously defined in the text.

 

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Presenting mathematical terms and equations

 

The following are general guidelines for presenting mathematics in your paper. Although these summarize common conventions in physics papers, remember that different journals will have different rules, which may deviate from these guidelines.

 

Displaying and numbering equations

  • Display all equations out of the line of text if they are one of the following:
    • particularly important
    • require a number (for later reference)
    • are over 25 characters
    • are complicated (e.g. involve multiple fractions, matrices, etc.)
  • Number each equation you use sequentially, as in (1), (2), etc.
  • Place equation numbers at the extreme right of the equation, as in:
                                                                         x + y = 0                                                                      (1)
  • Do not refer back to such an equation by its number alone. Instead, write (for example): “as in equation (45)”

 

Arithmetic symbols

  • To avoid confusion with dot products and vector products, do not use explicit multiplication signs, such as dots or crosses, except for in scientific notation, grids, and vector operators.

 

Grouping equations with brackets

  •  Group first with ( ), then [ ], then { }. If more groups are needed, use the same sequence again, but in bold. For example:
    equation0
  • If groupings require larger-sized bracketing, start the larger grouping over with the beginning of the bracket sequence, i.e. ( ). For example:equation1

Fraction styles

  • Three different types of equation styles are acceptable in most cases:
    • Built-up, as in:
      equation2
    • Slashed, as in:
      equation3
    • Or written with a negative exponent, as in:
      equation4
  • When the equation involves 3 or more simple fractions, use the built-up format throughout the equation.
  • In subscripts, superscripts, and limits, use the slashed format.
  • In all cases, use whichever style makes your meaning most clear

 

 

Using symbols to express physical quantities

 

One great example of a scientific convention is the use of particular symbols to signify physical quantities. For example, we conventionally use the boldface Greek symbol Φ to express magnetic flux, but there is no definite reason we could not use a boldface A or ζ or even ♥ instead if we define it as such. But using a conventional symbol—in this case, Φ—helps alleviate confusion among different scientists and contexts.

Although most physicists will opt to use the conventional symbol, one may choose to adopt a different symbol in his or her paper. One reason for this is that there is some overlap in symbols, such as T often representing both temperature and period, creating the need to define a new symbol for period (such as W) in a paper where both period and temperature are used in equations.

Despite the usefulness of having this type of convention, every symbol you use in your paper should still be defined, however obvious it may seem. The following examples demonstrate some acceptable ways to define a symbol within text:

a. Data yield the following equations for the partial pressures of water (pH2O) and nitrogen (pN2).

b. Data yield the following equations for the partial pressures of water pH2O and nitrogen pN2.

c. Data yield the following equations … where pH2O is the partial pressure of water, g is gravity, and F is heat flux from the planetary surface.

Pay careful attention to the distinction between physical symbols and units. A symbol takes the place of a quantity; a unit denotes the magnitude of that quantity. You should not define the meaning of a unit abbreviation in your paper, as in “femtoseconds (fs).”

 

Common symbols used to express physical quantities
Quantity Common symbols Quantity Common symbols
acceleration a, a length, distance l, d, r, x, y, z, s
angle θ, φ magnetic induction B
angular acceleration α magnetic field H, B
angular frequency ω magnetic flux Φ
angular momentum L, L magnetization M
area A mass m
capacitance C moment of inertia I
charge q, Q momentum p, p
charge density, linear λ period T, t
charge density, surface σ power P
charge density, volume ρ pressure p
current I resistance R
density ρ specific heat c
electric displacement D temperature T
electric field E time t
electric polarization P torque T, τ, Γ
electric potential V, Φ velocity v, u, v
energy, work, heat W, K, E, U, Q volume V
force F, F wavelength λ
frequency ν, f
inductance L

 

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Using and formatting citations and references

 

Once you are familiar with why we use citations and references the way we do, the biggest hurdle can be knowing exactly how you’re expected to format this information. As always, it varies from journal to journal, each of which will clearly delineate how they want you to present this information. But for your convenience, below we summarize the most common ways citations and references are formatted in physics journals.

 

Citations in Text

Three styles are most common, as follows:

  1. Preferred: Using a superscript number, as in3
  2. Using bracketed numbers, as in [3]
  3. By the last name of the author(s) and the date of publication, as in (James 2001), without a comma before the date. For papers with two authors, join author names with an ampersand (Press & Rybicki 1992). Papers by three or more authors are cited by the first author followed by “et al.” and the date (Goodman et al. 2003).

 

Reference List

Two styles are most common, as follows:

  1. (If citation styles numbers 1 or 2, above, are used,) order references such as they appear in the text. Write the rest of the reference with the authors, journal name, volume number, page or article number, and year of publication. For example:

    [31] K. Itakura, O. Morimatsu, and H. Otomo, Phys. Rev. D 77, 014014 (2008)

  2. (If citation style number 3, above, is used,) order references alphabetically, starting with the last name of the first author, followed by the first author’s initial(s), and so on for each additional author. For papers with more than five authors, the last name and initials of the first three authors only should be listed, followed by a comma and “et al.” The rest of the reference should include the year of publication, journal name, volume number, and page number. For example:

    Martinez, E. L., Rebolo, R., & Zapatero Osorio, M. R. 1996, ApJ, 469, 706

 

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