# What is the difference between cyclotron radiation and Larmor formula?

On Wikipedia, it says

Cyclotron radiation is electromagnetic radiation emitted by accelerating charged particles deflected by a magnetic field.

and

The Larmor formula is used to calculate the total power radiated by a non-relativistic point charge as it accelerates.

Looking only at these two sentences, the only difference seems to be that the description for cyclotron radiation specifically mentions that it is used for accelerating charged particles deflected by a magnetic field.

1. What is the difference between those two formulas?
2. When I'm looking at an accelerating charged particle in a magnetic field, is it more accurate to use cyclotron radiation formula?
3. What does it mean by “total power” in the description for Larmor formula? What's the difference between just saying “power”?

Below is the formula for cyclotron radiation.

$$\frac{-dE}{dt}=\frac{\sigma_t B^2 v^2}{c \mu_0}$$

$$\sigma_t$$ is the total Thomson cross-section, $$B$$ is the magnetic field strength, $$v$$ is the charged particle's velocity perpendicular to the magnetic field, c is the speed of light, and $$\mu_0$$ is the permeability of free space. (Sorry for using words instead of proper notations. I'm not quite familiar with this place yet.)

• One is the Larmor formula for radiated power. Which other formula are you talking about here? @동휘민 – SRS Feb 16 at 15:21
• Sorry, I added the formula for cyclotron radiation in the question. – 동휘민 Feb 16 at 15:30
• Please consider explaining the symbols in order to make the question self-contained. For example, what is $\sigma_t$? For the third question, the Larmor formula $P=\frac{\mu_0q^2a^2}{6\pi c}$ gives the total power (i.e., the power radiated in all directions) radiated by a nonrelativistic point charge $q$ having the instantaneous acceleration $a$. – SRS Feb 16 at 15:33
• Sigma sub t is Thomson cross section, B is the magnetic field strength, v is the charged particle's velocity perpendicular to the magnetic field, c is the speed of light, and mu is the permeability of free space. – 동휘민 Feb 16 at 15:40
• I meant, please consider putting those in the question itself. It helps others to appreciate your question better. – SRS Feb 16 at 15:43

There is no difference between the formulas. The formula in the cyclotron radiation article is just the Larmor formula applied to the specific case of nonrelativistic charged particles moving in circles in a magnetic field.

For such particles, $$F=ma$$ combined with the Lorentz force law $$F=qvB$$ gives the acceleration as

$$a=\frac{qBv}{m}$$

so the Larmor formula

$$P=\frac{q^2a^2}{6\pi\epsilon_0c^3}$$

$$P=\frac{q^2}{6\pi\epsilon_0c^3}\left(\frac{qBv}{m}\right)^2=\frac{q^4B^2v^2}{6\pi\epsilon_0m^2c^3}.$$

To see that this is the same as the cyclotron radiation formula,

$$-\frac{dE}{dt}=\frac{\sigma_tB^2v^2}{c\mu_0},$$

just substitute the formula for the Thomson cross section,

$$\sigma_t=\frac{8\pi}{3}\left(\frac{q^2}{4\pi\epsilon_0mc^2}\right)^2$$

and the formula for the vacuum permeability,

$$\mu_0=\frac{1}{\epsilon_0c^2}$$

to get

$$-\frac{dE}{dt}=\frac{8\pi}{3}\left(\frac{q^2}{4\pi\epsilon_0mc^2}\right)^2\frac{B^2v^2}{c}\epsilon_0c^2=\frac{q^4B^2v^2}{6\pi\epsilon_0m^2c^3}.$$

The total power $$P$$ given by the Larmor formula is the power radiated in all directions. The formula for the power per solid angle radiated in a specific direction is

$$\frac{dP}{d\Omega}=\frac{q^2a^2\sin^2\theta}{16\pi^2\epsilon_0c^3}.$$

Here $$\theta$$ is the angle between the acceleration vector and the direction in which the power is measured.

If you integrate this over all solid angles using $$d\Omega=\sin\theta\,d\theta\,d\phi$$, you'll get the Larmor formula for $$P$$ because

$$\int\sin^2\theta\,d\Omega=2\pi\int_0^\pi \sin^3\theta\,d\theta=2\pi\int_{-1}^1 (1-u^2)\,du=\frac{8\pi}{3}.$$