What are Permittivity and Permeability?
In electromagnetic theory, there are two concepts known as permittivity and permeability. The concept related to the formation of an electric field is known as permittivity, and the one related to the magnetic field is known as permeability.
Definition of Permittivity
Permittivity can be explained as the ratio of electric displacement to the electric field intensity. It is the property of a material to measure the opposition generated by the material during the electric current development.
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The permittivity of a material is represented by the symbol ε. The SI unit of permittivity is Farad per metre. The approximate value for permittivity is 8.85 X 10-12 Faraday/metre, which is found in a vacuum medium. The permittivity measures the number of charges needed for generating a unit of electric flux in a specific channel.
Permittivity is expressed in relative terms in engineering applications instead of absolute terms. The permittivity of free space (that is, 8.85 x 10-12 F/m) is represented by eo and the permittivity of substance in question (also represented in farads per metre) is represented by e. Here, the relative permittivity dielectric constant er, is given by:
\[e_r = \frac{e}{e_o} \]
= e (1.13 x 1011)
Definition of Permeability
The property of the material which supports the formation of magnetic flux when passed through a magnetic field is known as permeability. It is affected by the field frequency, temperature, field strength, and humidity. It is represented by μ.
The permeability of the material is defined as the ratio of flux density to the field strength of a material. It is also directly proportional to the conduction of magnetic lines of force. The permeability of free space is also known as the permeability constant and is represented by μ0, which is approximately equal to 4Π X 10-7Henry/metre.
The symbol for permeability is μ. It is also defined as the ratio of the intensity of magnetic field (simple magnetic field) to the magnetic field (i.e., the limit to which the magnetic field can magnetize a material).
\[\mu = \frac{\text{Intensity of Magnetic Field (B)}}{\text{Intensity of Magnetizing Field (H)}}\]
or, \[\mu = \frac{B}{H}\]
Unit of Permeability
The unit of measurement for the intensity of the magnetic field is Tesla (T) or Newton per Ampere metre (N/Am).
Ampere per metre (A/m) is the unit of a magnetizing field.
Magnetic reluctivity is the reciprocal of magnetic permeability. The SI unit of permeability is given as Henries-per-metre (H/m). It is measured in Newtons-per Ampere squared (N/A2)
So as per the above permeability formula, it becomes unit Newton per Ampere square.
\[\mu = \frac{Newton}{\text{ Ampere Square}}\]
Or \[\mu = \frac{N}{A^2} \]
By calculating their equations and units, we will find the dimension of permeability to be
[M1L1T-2I-2].
Difference Between Permittivity and Permeability
The main differences between permittivity and permeability are explained below.
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From the above text, we understand that during the formation of electric fields, the obstruction produced by the material is measured by the permittivity. In contrast, the ability of the material to allow magnetic lines to conduct through it is known as permeability.
FAQs on Permittivity and Permeability
1. Is the speed of light dependent on the permittivity and permeability of space?
This is true.
Taking the inverse square of the product of these two quantities, the speed of light is dependent. As the values of permittivity and permeability are different in different materials, the speed of light is different for those materials.
Considering there is no violation involved, relativity only requires that the speed of light in a vacuum is constant in all reference frames.
2. What is the physical significance of permittivity?
The variation in the force experienced by a charge (test charge) in the medium (where the test charge stays) compared to the force it (test charge) would have experienced in vacuum space is determined by the permittivity of the medium.
In simpler terms, let's assume a force F is experienced by a positive charge in a vacuum space. Now, if the same charge is placed in a medium like water, whose relative permittivity is obtained by the formula Permittivity of water/Permittivity of Vacuum = 80, then the same test charge would experience a force F/80 due to the same electric field.
3. What is the application of permittivity?
When an external electric field is established, the amount of resistance being incurred by the medium as an imposing factor is known as permittivity.
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The above image shows a dielectric medium showing orientation of charged particles creating polarization effects. Compared to a space, such a medium shall have a lower ratio of electric flux to charge.
In electrostatics, the capacitance of a capacitor is determined by the permittivity.
In general, permittivity is a thermodynamic function of the state. It is affected by the frequency, magnitude, and direction of the applied field. Farad per metre (F/m) is the SI unit for permittivity.
4. What is the difference between permeability and relative permeability?
Not much.
The tendency of a vacuum to allow a magnetic field and force to permeate through it is known as permeability and it is denoted by μo.
The neat way to say permeability of an object concerning vacuum is known as relative permeability. (It is written as μr.)
