Reference of Feynman's "bullet analogy" to explain the notion of electric flux?

Question

In this answer, user36790, who is unfortunately no longer a member of SE, alludes to an analogy from R. Feynman that correctly describes and explains the notion of electric flux. Does anyone have the reference from Feynman ? user36790 called it the "bullet analogy".

Not sure if, only Feynman's analogy describes the notion of electric flux, or if there isn't any viable analogy to electric flux (i.e. it is only an abstract mathematical definition).

Also, another answer of the same post proposed an analogy with the rain (whose rain intensity $I$ is in $[L/m^2]$) and a vessel, saying that the flux is the total volume of water which goes through the open area of the vessel at any moment. I didn't get what was meant by "at any moment":

• was he meaning at a time $t_1$, in which case it is nonsense, since no raindrop is exactly on the plane formed by the vessel's open area at $t_1$.
• Was he meaning during the whole experiment session? In which case, I would say that a $\Delta t$ should be introduced in the equations.

In either case, I don't get his analogy.

Anyway, if anyone has the reference of Feynman's analogy, it'd be great. If anyone has an analogy in order to understand what is electric flux, it'd still be great!

Answer 1

A flux is just an amount of something that is crossing a surface at any given moment in time $t$. The bullet analogy is similar to the explanation using electric field lines, where the flux is proportional to the amount of field lines that cross a surface. This explanation is qualitatively useful but it's not quantitatively correct since there is no fixed amount of electric field lines you can draw, it's arbitrary. That's why you can only say it's "proportional".

Regarding your comments on the water analogy, the first way of thinking about it is the correct one. A fixed amount of rain drops are intersecting the plane at any time $t$ since they are not point particles, they occupy a volume. You can define the flux as the number of rain drops intersecting this plane. You could even go further and take into account the intersection area such that rain drops that are intersecting a larger area contribute more to the flux.