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Looking at Konopinski's formula for conjugate momentum (in the comment after equation 3 of "What the Vector Potential Describes"):

$\mathbf{p}= M \mathbf{v} + q\mathbf{A} /c$

it is plain enough that $M \mathbf{v}$ is the momentum, but if we naively take the usual notion of the magnetic vector potential $\mathbf{A}$to have dimensions of magnetic flux/length or, equivalently, momentum/charge (weber/meter = (kg $\cdot$ m/s)/coulomb) the following dimensional expression obtains for the right hand side of the $+$:

charge $\cdot$ ((mass $\cdot$ velocity)/charge)/velocity

So in a naive dimensional analysis this simplifies to mass; but this is not commensurable with momentum (mass $\cdot$ velocity) on the left side of the $+$.

Likewise, in that same comment, Konopinski describes the "interaction energy" as:

$$q[\phi-\mathbf{v}\cdot\mathbf{A}/c]$$

a naive dimensional analysis would notice a similar discrepancy -- this time in that the electric scalar potential, phi is energy/charge whereas the dimensional expression of the right hand side of the $-$ is:

velocity $\cdot$ (momentum/charge)/velocity

which, again, naively, simplifies to momentum/charge rather than the required energy/charge.

Obviously, naive dimensional analysis doesn't work here.

Sebastiano
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James Bowery
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2 Answers2

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You seem to be doing dimensional analysis in SI units. The paper seems to using Gaussian units. The magnetic field differs between these units by a factor $c$. In SI units we have $$\mathbf F = q(\mathbf E + \mathbf v \times \mathbf B \tag{SI})$$ but in Gaussian units $$\mathbf F = q(\mathbf E + \frac{\mathbf{v}}{c} \times \mathbf B \tag{G}).$$ The latter definition makes electric and magnetic fields have the same unit and is the form used in the paper you linked.

Robin Ekman
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Here's the conversion. I will write Gaussian quantities as $$\hat{φ}, , , , , , , \hat{ρ}, q, \hat{ε}, \hat{μ}$$ and the corresponding SI quantities as $$φ, , , , , , , ρ, e, ε, μ.$$

Kinematic and mechanical quantities $$ (velocity), $$ (momentum), $m$ (rest mass), $M$ (moving mass), $E$ (energy), $$ (force), $P$ (power) do not differ between the two. I will also use $$ to denote the Maxwell-Lorentz Lagrangian density, which is also the same for the two.

Corresponding to the equations in Gaussian form: $$ = ∇×, \hspace 1 em = -∇\hat{φ} - \frac{1}{c} \frac{∂}{∂t}, \hspace 1em ∇· = 0, \hspace 1em ∇× + \frac{1}{c}\frac{∂}{∂t} = , \\ ∇· = 4π\hat{ρ}, \hspace 1em ∇× - \frac{1}{c}\frac{∂}{∂t} = \frac{4π}{c}, \hspace 1em ∇· + \frac{∂\hat{ρ}}{∂t} = 0, \\ = \frac{||^2 - ||^2}{8π}, \hspace 1em = \hat{ε}, \hspace 1em = \hat{μ}, \\ = q\left( + \frac{×}{c}\right), \hspace 1em P = q· $$ are the equations in SI form: $$ = ∇×, \hspace 1 em = -∇φ - \frac{∂}{∂t}, \hspace 1em ∇· = 0, \hspace 1em ∇× + \frac{∂}{∂t} = , \\ ∇· = ρ, \hspace 1em ∇× - \frac{∂}{∂t} = , \hspace 1em ∇· + \frac{∂ρ}{∂t} = 0, \\ = \frac{ε_0||^2}{2} - \frac{||^2}{2μ_0}, \hspace 1em = ε, \hspace 1em = μ, \\ = e\left( + ×\right), \hspace 1em P = e·, $$ with $c = 1/\sqrt{ε_0μ_0}$.

The conversions are: $$ (\hat{φ}, , , ) = \sqrt{4πε_0} (φ, c, c, ), \\ (, , , \hat{ρ}, q) = \frac{1}{\sqrt{4πε_0}}\left(4π, \frac{4π}{c}, , ρ, e\right), \\ (\hat{ε}, \hat{μ}) = \left(\frac{ε}{ε_0}, \frac{μ}{μ_0}\right).$$ In the reverse direction, they are $$ (φ, , , ) = \frac{1}{\sqrt{4πε_0}}\left(\hat{φ}, \frac{}{c}, \frac{}{c}, \right), \\ (, , , ρ, e) = \sqrt{4πε_0}\left(\frac{}{4π}, \frac{c}{4π}, , \hat{ρ}, q\right), \\ (ε, μ) = (ε_0 \hat{ε}, μ_0 \hat{μ}). $$ There is no direct mention of $ε_0$ or $μ_0$ in the Gaussian version, only of $c$; while in the SI version, there is no direct mention of $c$, only of $ε_0$ and $μ_0$ and it's confined to the (vacuum version of the) constitutive relations and Lagrangian density, nowhere else. All the other equations are $c$-independent and (in fact) non-metrical. They live at a deeper level of geometry where there is no concept of a metric, parallelism, orthogonality, congruence, speed, or any distinction between space-like and time-like.

If the dimensions are denoted $Q$, $P$, $M$, $L$ and $T$, respectively for electric charge/flux, magnetic charge/flux, mass, length and time duration, then the dimensions for the various quantities in SI will be $$[\left(φ, , , , , , , ρ, e, ε, μ\right)] = \left(\frac{P}{T}, \frac{P}{L}, \frac{P}{L^2}, \frac{P}{LT}, \frac{Q}{L^2}, \frac{Q}{LT}, \frac{Q}{L^2T}, \frac{Q}{L^3}, Q, \frac{QT}{PL}, \frac{PT}{QL}\right).$$ The dimensions of $P$ and $Q$ are conjugate and multiply out to those of action: $$PQ = \frac{ML^2}{T}.$$

From this, you can determine what the dimensions for the Gaussian quantities are: $$[\left(\hat{φ}, , , , , , , \hat{ρ}, q, \hat{ε}, \hat{μ}\right)] = \left(\frac{G}{L}, \frac{G}{L}, \frac{G}{L^2}, \frac{G}{L^2}, \frac{G}{L^2}, \frac{G}{L^2}, \frac{G}{L^2T}, \frac{G}{L^3}, G, 1, 1\right), \hspace 1em G = \sqrt{\frac{ML^3}{T^2}}.$$ They are, needless to say, awkward - square roots of dimensions involving $M$, $L$ and $T$. But at least they got rid of the $P$'s and $Q$'s. You might say that they paid no mind to the $P$'s and $Q$'s.

Finally, the kinematic and mechanical dimensions are independent of the system: $$[(, P, E, , m, M, c, , )] = \left(\frac{ML}{T^2}, \frac{ML^2}{T^3}, \frac{ML^2}{T^2}, \frac{ML}{T}, M, M, \frac{L}{T}, \frac{L}{T}, \frac{M}{LT^2}\right). $$

NinjaDarth
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