Larmor formula

Explore the Larmor formula, its significance, derivation, and applications in classical electromagnetism, with a calculation example.

Larmor Formula: A Comprehensive Overview

The Larmor formula, named after physicist Joseph Larmor, is a fundamental equation in classical electromagnetism. This formula calculates the power radiated by a charged particle as it accelerates or decelerates. In this article, we delve into the significance and applications of the Larmor formula, as well as its derivation.

Significance of the Larmor Formula

The Larmor formula is significant because it allows us to determine the energy loss of an accelerating charged particle due to electromagnetic radiation. This energy loss is responsible for various phenomena observed in physics, such as synchrotron radiation and bremsstrahlung. By providing a quantitative understanding of energy loss, the Larmor formula plays a crucial role in the study and design of particle accelerators, astrophysics, and other areas where charged particles undergo acceleration.

Derivation of the Larmor Formula

The derivation of the Larmor formula is rooted in classical electromagnetism, using Maxwell’s equations and the Lorentz force. First, the power radiated by an accelerating charged particle is calculated by determining the Poynting vector, which represents the energy flux of the electromagnetic field. Next, this energy flux is integrated over a sphere surrounding the particle, taking into account the retarded potentials and the acceleration of the charged particle. Finally, after some mathematical manipulations, the Larmor formula is obtained.

Expression of the Larmor Formula

The Larmor formula can be expressed as:

P = (2/3) * (q2 * a2) / (c3 * 4πε0)


  • P is the power radiated by the accelerating charged particle.
  • q is the charge of the particle.
  • a is the magnitude of the acceleration of the particle.
  • c is the speed of light in vacuum.
  • ε0 is the vacuum permittivity.

It is important to note that the Larmor formula is only valid for non-relativistic scenarios, as it does not take into account the effects of special relativity. For relativistic cases, the Larmor formula must be modified accordingly.

Applications of the Larmor Formula

Some of the key applications of the Larmor formula include:

  1. Estimating energy loss in particle accelerators, which helps to optimize their design and operation.
  2. Understanding astrophysical phenomena, such as the radiation emitted by electrons spiraling in magnetic fields of neutron stars and black holes.
  3. Studying atomic and molecular processes, such as collisions between charged particles and atoms or molecules, where energy is radiated as bremsstrahlung radiation.
  4. Designing antennas and transmitters in communication systems, by estimating the radiation efficiency and power output.

In conclusion, the Larmor formula is a vital tool in classical electromagnetism, offering insights into the power radiated by accelerating charged particles. Its applications span a wide range of disciplines, making it an indispensable equation in modern physics.

Example of a Calculation Using the Larmor Formula

Let’s consider an electron that is undergoing constant acceleration. We will calculate the power radiated by the electron due to this acceleration.


  • Charge of the electron (q) = -1.6 x 10-19 C
  • Acceleration of the electron (a) = 5.0 x 1015 m/s2
  • Speed of light in vacuum (c) = 3.0 x 108 m/s
  • Vacuum permittivity (ε0) = 8.85 x 10-12 C2/N·m2

We will use the Larmor formula:

P = (2/3) * (q2 * a2) / (c3 * 4πε0)

First, we calculate the square of the charge and the square of the acceleration:

q2 = (1.6 x 10-19 C)2 = 2.56 x 10-38 C2

a2 = (5.0 x 1015 m/s2)2 = 2.5 x 1031 m2/s4

Next, we calculate the cube of the speed of light:

c3 = (3.0 x 108 m/s)3 = 2.7 x 1025 m3/s3

Now, we can substitute these values into the Larmor formula:

P = (2/3) * (2.56 x 10-38 C2 * 2.5 x 1031 m2/s4) / (2.7 x 1025 m3/s3 * 4π * 8.85 x 10-12 C2/N·m2)

After calculating the expression, we get:

P ≈ 1.31 x 10-11 W

Thus, the power radiated by the accelerating electron is approximately 1.31 x 10-11 watts.

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