# Explaining symbols in a scientific paper

Dear all,

Here are some questions about explaining symbols in a journal paper.

a) Do you explain every symbol in the equations? For instance, when you write down Maxwell's equations in a journal paper, do you, after the equations say

"where $$\vec{E}$$ denotes the electric field, $$\vec{H}$$ denotes the magnetic field, $$t$$ is time, blah ... ..."

b) Once you have a symbol explained, do you use the symbol in line with the rest of the text or spell out the full name in the rest of the text? For instance, assume $$\vec{E}$$ has been defined, do you say "the time variation of $$\vec{E}$$ gives ... ..." or "the time variation of the electric field gives ... ..."?

These might sound a little silly, but the picture has never been quite clear.

elgen

Yes, explain all variables used in an equation. For the second question, it is more a matter of taste and style than a hard and fast rule in physics.

Here are some questions about explaining symbols in a journal paper.

Also, with questions like this you could simply have a look at some journal papers and see what the norm appears to be. Especially if you have questions about publishing in a particular journal - look at published articles to see how their results are implemented.

Pythagorean
Gold Member
a) in my opinion, they should. But they don't always. I was just looking at a paper (Tsumoto, Bifurcations in the Morris Lecar model) that didn't. They did, however, reference the original paper by Morris and Lecar that had a full debriefing.

b) I prefer the symbol, personally, especially if there's more than one electric field.

ZapperZ
Staff Emeritus
Dear all,

Here are some questions about explaining symbols in a journal paper.

a) Do you explain every symbol in the equations? For instance, when you write down Maxwell's equations in a journal paper, do you, after the equations say

"where $$\vec{E}$$ denotes the electric field, $$\vec{H}$$ denotes the magnetic field, $$t$$ is time, blah ... ..."

b) Once you have a symbol explained, do you use the symbol in line with the rest of the text or spell out the full name in the rest of the text? For instance, assume $$\vec{E}$$ has been defined, do you say "the time variation of $$\vec{E}$$ gives ... ..." or "the time variation of the electric field gives ... ..."?

These might sound a little silly, but the picture has never been quite clear.

elgen

1. When in doubt, explain/define all symbols used. This is especially true if (i) you are not running against a page limit for that journal and (ii) if you are publishing in a journal that has a less-specialized topic, such as PRL, Nature, Science, etc.

2. Once you have the symbols defined, then you may just use the symbols after that.

3. In specialized journals, where the reader of that journal tends to be someone who is already familiar to that field of study, one can get by with not defining the "standard" symbols. Typically, one defines the unusual ones, and then state something like "...while the other symbols have their usual meanings". Typically, symbols such as e, h, c, etc.. have standard definitions. Other specialized areas may have more standard definitions that one can get away without defining.

4. The best thing to do is browse a large number of papers from that publication to see if there is a set of common symbols, and that there are papers that don't define all of them.

5. If something needs to be defined, you'll hear it back either from the editor, or the referee/s.

Zz.

a) Generally yes, because E could easily mean something else, and it's often the situation that you are dealing with multiple electric fields. Usually in astrophysics you read something like "Let E be the electric field of the black hole, and B be it's magnetic field. From Maxwell's equations we have...."

b) Usually you want to use the symbol in the text for the same reason as a). It's a bit wordy and confusing to say "The strength of the electric field of the black hole due to the first magnetic field which we considered in equation 3.2 and it's easier to say E_{BH}.

The other tricky thing is that in papers involving electromagnetism, you need to mention whether you are using cgs, SI, or natural units. There are also it's also important to define symbols because sometimes you are using symbols in odd ways. For example one thing that is pretty standard in supernova papers is to use the variable "u" for velocity rather than the variable "v". The reason being that v looks a bit too much like "nu" which is used for neutrinos, and since you want to avoid using "nu", you end up using eta for things involving viscosity which gets you into trouble when when you start using the Riemann eta function to calculate particle distributions.

It's often the situation that you can figure things out from context, but that just puts more work on the reading. Also sometimes you can't figure out things easily from context. When you see u^2, are they talking about u squared or a the second index of a contravariant vector? When something has a prime on it, are they taking a derivative.

The other reason for defining everything is that it makes it more obvious if there is just a typo. If you have a preprint, you can spend an hour staring at an equation, deriving and rederiving it, not sure if you made a mistake, the author made a mistake, and if the mistake is substantive or just a typo. If you say E is an electric field and you mention that you are about to use maxwell's equation, that makes it more obvious that the R in the equation was just a fat finger.

Also, the notation for vectors in the Journal literature is usually $\mathbf{E}$, $\mathbf{H}$, etc., i.e. you use boldface letters.