Fine Structure Constant

In advanced physics, the fine structure constant is synonymous with Sommerfeld’s constant. It is denoted by the Greek letter 𝜶 (alpha). It is a dimensionless physical constant. Fine-Structure Constant characterises the strength of electromagnetic interactions between any two elementary charged particles.

Table of Contents:

Measurement

Being a dimensionless quantity, Fine-Structure Constant has the same numerical value in all system of units. Which is approximately

\(\begin{array}{l}\alpha =\frac{1}{137}\end{array} \)
= 0.0072973525664(17).

Definition

Fine-Structure Constant characterises the strength or magnitude of coupling of any elementary charged particle(e) with the given electromagnetic field. Mathematically 𝜶 can be expressed in terms of other fundamental physical constants as follows-

\(\begin{array}{l}\alpha =\frac{1}{4\pi \epsilon _{0}}\frac{e^{2}}{\left ( \frac{h}{2\pi } \right )c}=\frac{\mu _{0}}{4\pi }\frac{e^{2}c}{\left ( \frac{h}{2\pi } \right )}=\frac{k_{e}e^{2}}{\left ( \frac{h}{2\pi } \right )c}=\frac{c\mu _{0}}{2R_{K}}=\frac{e^{2}}{4\pi }\frac{Z_{0}}{\left ( \frac{h}{2\pi } \right )}\end{array} \)

Where,

  • \(\begin{array}{l}\bar{h} = \frac{h}{2\Pi }\end{array} \)
    is the reduced Planck’s Constant
  • e is the elementary charge (e=1.602176634 × 10-19C)
  • c is the velocity of light in vacuum (c=299792458 m/s)
  • ε0 is the permittivity of free space.
  • µ0 is the Permeability of free space.
  • Ke is the Coulomb constant.
  • RK is the von Klitzing constant.
  • Z0 is the impedance of free space.

In non-SI units

The value of this dimensionless physical quantity can be expressed in many units.

Electrostatic CGS unit

In electrostatic CGS units, electric charge is measured using statcoulomb and is defined assuming permittivity factor 4πε0 or Coulomb constant ke is 1 and is dimensionless. Thus, the value of fine structure constant turns out to be-

\(\begin{array}{l}\alpha =\frac{e^{e}}{\left ( \frac{h}{2\pi } \right )c}\end{array} \)

Natural unit

High energy physics commonly use natural units, where ε0 = c = ħ = 1. here, ε0 is the permittivity of free space. Thus the value of fine structure constant turns out to be-

\(\begin{array}{l}\alpha =\frac{e^{e}}{4\pi }\end{array} \)

Atomic units

According to atomic units,

\(\begin{array}{l}\epsilon _{0} =\frac{1}{4\pi }\end{array} \)
and e = me = ħ = 1, thus the fine structure constant turns out to be-

\(\begin{array}{l}\alpha =\frac{1}{c}\end{array} \)

Physical interpretations

The physical interpretation of Fine Structure Constant is plenty. Some are listed below-

  • In quantum electrodynamics, Fine Structure Constant 𝜶 is directly related to coupling constant.
  • In solid state physics and electrical engineering, 𝜶 is one-fourth product of the characteristic impedance of free space.
  • Within the Bohr model, 𝜶 gives the maximum positive charge of an atomic nucleus that will allow a stable-orbit around it.
  • 𝜶 gives the probability of emission or absorption of an electron by a photon.
  • It also has got wide applications in low energy physics, particularly in electroweak theory. Also, some properties of subatomic particles exhibit a strong correlation with 𝜶.

Hope you have understood about fine structure constant(𝜶). Including, definition, formula, terms along with units, values in non-SI units and physical interpretation of alpha.

Physics Related Topics:

Mu Naught Value
Band Theory Of Solids
Value Of Planck’s Constant
Permittivity And Permeability

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Frequently Asked Questions – FAQs

Q1

What is permittivity of free space?

The permittivity of free space(ε0) is the capability of the classical vacuum to permit the electric field. It as the definite defined value which can be approximated to
ε0 = 8.854187817 × 10-12. F.m-1 (In SI Unit)
Q2

What is electromagnetic force ?

The electromagnetic force is a type of physical interaction that occurs between electrically charged particles. It acts between charged particles and is the combination of all magnetic and electrical forces. The electromagnetic force can be attractive or repulsive.
Q3

State Faraday’s first law of electromagnetic induction.

Faraday’s first law of electromagnetic induction states that: Whenever a conductor is placed in a varying magnetic field, an electromotive force is induced. If the conductor circuit is closed, a current is induced, which is called induced current.
Q4

Define fine-structure constant.

Fine-Structure Constant characterises the strength or magnitude of coupling of any elementary charged particle(e) with the given electromagnetic field.
Q5

What is electromagnetism ?

Electromagnetism is a process where a magnetic field is created by introducing the current in the conductor.
Q6

Define Fine Structure Constant

Fine-Structure Constant characterises the strength or magnitude of coupling of any elementary charged particle(e) with the given electromagnetic field.
Its numerical value is:
\(\begin{array}{l}\alpha = \frac{1}{137}\end{array} \)
\(\begin{array}{l}= 0.0072973525664(17)\end{array} \)
Q7

What is the value of Fine Structure Constant in Atomic Unit?

In Atomic units, the value of the Fine Structure Constant is:
\(\begin{array}{l}\alpha = \frac{1}{c}\end{array} \)
Test Your Knowledge On Fine Structure Constant!

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