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Is the additional term in the canonical momentum exactly equal to the momentum of the electromagnetic field?

Physics Asked on March 8, 2021

The canonical momentum of a particle in an electromagnetic field is given by $$textbf{P}=mtextbf{v}+qtextbf{A}$$ Is the term $q textbf{A}$ equal to the momentum of the electromagnetic field (which would be $mathbf{P}_text{field} = int epsilon_0 left(textbf{E}timestextbf{B}right) dV$ ) ?

Otherwise, where is this part of the momentum stored?

When we write the rate of change of momentum in a region in terms of the stress energy tensor, we write: $$frac{d}{d t}left(mathbf{P}_{text {mech }}+mathbf{P}_{text {Field}}right)_{mathrm{alpha}}=oint_{S} sum_{beta} T_{mathrm{alpha} mathrm{beta}} n_{mathrm{beta}} d a$$ (this is equation 6.122 from section 6.7 of Jackson’s electrodynamics book) where $textbf{P}_{text {mech }} = m dot{textbf{v}}$.

This equation does not take into account the change of momentum due to change in $qtextbf{A}$, unless this term is the momentum in the field itself.

One Answer

Yes, you are right. $qA$ is called the field momentum. And it is indeed equal to the momentum carried by the electromagnetic field. It can be derived from the definition of Poynting vector, where you try to express its contribution to the total electromagnetic field energy.

Actually, it can be expressed using a wide variety of expressions, given by

$$mathbf{P_{EM}= qA = int_V rho A , dV = epsilon int (E times B) , dV}$$

You may check this article for details.

Also, as mentioned by @ChiralAnomaly in the comments, this assumes the validity of the Coulomb gauge $mathbf{nabla cdot A =0}$. This renders the vector potential to be a bit unrealistic but it is good enough for semi-classical calculations.

Correct answer by SchrodingersCat on March 8, 2021

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