Subfield: Electromagnetism

## Electroneering



An electron is a particle with charge $-q$, mass $m$, and magnetic moment $\mu$. In this problem we will explore whether a classical model consistent with these properties can also explain the rest energy $E_{0}=m c^{2}$ of the electron.



Let us describe the electron as a thin spherical shell with uniformly distributed charge and radius $R$. Recall that the magnetic moment of a closed, planar loop of current is always equal to the product of the current and the area of the loop. For the electron, a magnetic moment can be created by making the sphere rotate around an axis passing through its center.
Context question:
a. If no point on the sphere's surface can travel faster than the speed of light (in the frame of the sphere's center of mass), what is the maximum magnetic moment that the sphere can have? You may use the integral:



$$

\int_{0}^{\pi} \sin ^{3} \theta d \theta=\frac{4}{3}

$$
Context answer:
\boxed{$\mu=q c R / 3$}


Context question:
b. The electron's magnetic moment is known to be $\mu=q \hbar / 2 m$, where $\hbar$ is the reduced Planck constant. In this model, what is the minimum possible radius of the electron? Express your answer in terms of $m$ and fundamental constants.
Context answer:
\boxed{$R=\frac{3}{2} \frac{\hbar}{m c}$}


Context question:
c. Assuming the radius is the value you found in part (b), how much energy is stored in the electric field of the electron? Express your answer in terms of $E_{0}=m c^{2}$ and the fine structure constant,



$$

\alpha=\frac{q^{2}}{4 \pi \epsilon_{0} \hbar c} \approx \frac{1}{137}

$$
Context answer:
\boxed{$U_{E}=\frac{1}{3} \alpha E_{0}$}


Question: d. Roughly estimate the total energy stored in the magnetic field of the electron, in terms of $E_{0}$ and $\alpha$. (Hint: one way to do this is to suppose the magnetic field has roughly constant magnitude inside the sphere and is negligible outside of it, then estimate the field inside the sphere.)

Solution:
Following the hint, we can estimate



$$

U_{B} \sim \frac{B_{0}^{2}}{2 \mu_{0}}\left(\frac{4}{3} \pi R^{3}\right)

$$



where $B_{0}$ is the typical magnetic field inside the sphere. Actually finding the value of $B_{0}$ would require doing some complicated integrals. To get a rough estimate, note that if we replaced the sphere with a ring of charge, then at the center of the ring,



$$

B_{0} \sim \frac{\mu_{0} I}{R} \sim \frac{\mu_{0} q c}{R^{2}}

$$



Thus, we have



$$

U_{B} \sim \frac{1}{\mu_{0}}\left(\frac{\mu_{0} q c}{R^{2}}\right)^{2} R^{3} \sim \frac{\mu_{0} q^{2} c^{2}}{R} \sim \frac{\mu_{0} m c^{3} q^{2}}{\hbar}

$$



To get this in terms of the fine structure constant, we use $c^{2}=1 / \mu_{0} \epsilon_{0}$, giving



$$

U_{B} \sim m c^{2} \frac{q^{2}}{\epsilon_{0} \hbar c} \sim \alpha E_{0}

$$



An even faster way to get this result is to note that in general, the energy stored in magnetic fields tends to be a factor of order $(v / c)^{2}$ smaller than the energy stored in electric fields, where $v$ is the speed of the charge. In this problem the charge is all moving relativistically, so we must have $U_{B} \sim U_{E}$.

Final answer: $\alpha E_{0}$