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Questions and Answers
Electromagnetic Waves
What is the difference between electromagnetic wave and electromagnetic field?
Answer 1: "Electromagnetic field" just refers to the field created by static (electric field) or moving (magnetic field) charges. An electromagnetic wave is a certain configuration of the electromagnetic field. The key is that an electromagnetic wave is not constant--it oscillates with time. You can have a constant electric or magnetic field filling a space (for instance, the inside of a solenoid coil has a spatially and temporally constant magnetic field); this is not a wave. However, when an electric (or magnetic) field oscillates, it generates an oscillatory magnetic (electric) field. This oscillatory magnetic (electric) field then generates its own electric (magnetic) field, and back and forth they go until the energy in the field is absorbed by something. This oscillatory electric-magnetic field is an electromagnetic wave. An EM wave can be traveling (e.g. the radiation from your cell phone) or it can be confined in what is called a standing wave (e.g. the radiation inside your microwave oven). It is the oscillation that makes it a wave.
On a more formal level, an EM wave is any EM field that obeys the differential equations governing waves. Technically ALL EM fields obey this equation, so we usually restrict the definition to fields which have a non-zero frequency component--that is, fields that oscillate.
Answer 2:
Any changing or dynamic em field may induce electric and magnetic flux densities in either free space or any material, which in turn creates electric and magnetic flux densities and fields within which energy imparted from the source of change may be stored within the em fields themselves, and this process allows the em field to propagate through any material or free space as an em wave.
This is how a radio antennae works and where its changing em field induces electric (ϵoϵo) and magnetic flux densities (μoμo) which allow em waves to self-form and self-propagate through space
Why don't electromagnetic waves require a medium to travel?
Electromagnetic waves are predicted by Maxwell’s equations, which also show that a changing magnetic field gives rise to an electric field, and a changing electric field gives rise to a magnetic field. In this way, the wave gives rise to fields that continue its propagation in a vacuum. That said, the vacuum does have properties of permittivity and permeability of free space, which are related to the speed of light in a vacuum.
Dielectrics
1. Is Insulator and Dielectric are same? What makes them different?
Answer 1: While the term insulator implies low electrical conduction, dielectric typically means materials with a high polarizability. The latter is expressed by a number called the relative permittivity (also known in older texts as dielectric constant). The term insulator is generally used to indicate electrical obstruction while the term dielectric is used to indicate the energy storing capacity of the material (by means of polarization). A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor. The polarization of the dielectric by the applied electric field increases the capacitor's surface charge for the given electric field strength.
Answer 2: The term insulator is generally used to indicate electrical obstruction while the term dielectric is used to indicate the energy storing capacity of thematerial(by means of polarization). A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor.
Dielectrics are also insulators. But, more specifically, they are materials which can be polarized. In dielectric materials, the electrons are bound to the nucleus and have limited movement. When an external voltage is applied to the dielectric, the nucleus of the atoms is attracted to the negative side and the electros are attracted to the positive side. Hence, the material gets polarized. This is a key feature of a dielectric.
Thus a dielectric can be defined as an insulator that can be polarized. Thus all dielectrics are insulators, but all insulators are not dielectrics. A dielectric can thus store charge. This characteristic makes it very useful in the form of capacitors.
Dielectric substances conduct very little electricity but are good supporters of electric fields. They also dissipate very less energy, i.e. have low dielectric loss.
https://www.electrikals.com/
Answer 3:
The current JJ in a material is a combination of conduction current and displacement current. These can be written as
J=σE+jωεEJ=σE+jωεE ; where σσ is the conductivity of the material, εε is the permittivity of the material, and EE is the electric field in the material.
For good conductors, σ>>jωεσ>>jωε i.e conduction current dominates displacement current.
For very good dielectrics, σ<<jωεσ<<jωε i.e displacement current dominates conduction current.
For insulators, both σσ and εε are close to zero.
Answer 4:
Actually everything is a conductor at some point of temperature or electric field. Air is insulator we know it very well and we call it dielectric when we study capacitors. Only name changes function remains the same. You might know about electric breakdown of air when lightning comes to earth because the air which was acting as a dielectric between the two plates( clouds and earth) of this natural capacitor is now conducting because of high electric field between the plate (cloud and earth).
Source: https://www.quora.com/What-is-difference-between-insulator-and-dielectric-substance
2. What is dielectric loss?
Answer 1:
- Definition : Its the loss of electromagnetic energy propagating inside a dielectric.
-Why it happens? well two reasons, one is the relaxation effects, basically the oscillating electric field polarize the dielectric (that is why its "di"electric) and this creates a dipole that is also oscillating, this dipole absorbs and then rescatters the electric field . Of course this response is not instantaneous , the dipole's response is always delayed by some phase with respect to the field. Now what if the oscillation frequency increases to the extent that by the time the dipole starts to respond in one direction the field switch its direction, well the momentum gained will be lost in the switch, imagine stirring a cup clockwise and then suddenly stir it anti-clockwise.
The second reason is resonance, when the electric field's frequency resonates with the electronic/vibrionic/rotational transitions of the dielectric , the dielectric will "absorb" the energy in order to perform such transition and the energy will dissipate and you get losses (this is the physics behind the controversial issue of global warming :D seriously)
Source: https://www.quora.com/What-is-dielectric-loss