1. Electromagnetic Waves

Chapter 8: Electromagnetic Waves

Electromagnetic Waves
DISPLACEMENT CURRENT
Displacement current is a quantity appearing in Maxwell’s equations. Displacement current definition is defined in terms of the rate of change of the electric displacement field (D).Apart from conduction current, there is another type of current called displacement current. It does not appear from the real movement of electric charge as is the case for conduction current.

ELECTROMAGNETIC WAVES

  • The Magnetic field is produced by a moving charged particle. A force is exerted by this magnetic field on other moving particles. The force on these charges is always perpendicular to the direction of their velocity and therefore only changes the direction of the velocity, not the speed.
  • So, the electromagnetic field is produced by an accelerating charged particle. Electromagnetic waves are nothing but electric and magnetic fields travelling through free space with the speed of light c. An accelerating charged particle is when the charged particle oscillates about an equilibrium position. If the frequency of oscillation of the charged particle is f, then it produces an electromagnetic wave with frequency f. The wavelength λ of this wave is given by λ = c/f.  Electromagnetic waves transfer energy through space.
  • Electromagnetic wave equation describes the propagation of electromagnetic waves in a vacuum or through a medium.

Maxwell’s Equations of Electromagnetic Waves Maxwell’s equations are the basic laws of electricity and magnetism. These equations give complete description of ail electromagnetic interactions.
There are four Maxwell’s equations which are explained below:


Electromagnetic Spectrum The systematic sequential distribution of electromagnetic waves in ascending or descending order of frequency or wavelength is known as electromagnetic spectrum. The range varies from 10-12 m, to 104 m, i.e. from γ-rays to radio waves.

13. Elementary facts about the uses of electromagnetic waves
Radio waves
(i) In radio and TV communication.
(ii) In astronomical field.
Microwaves
(i) In RADAR communication.
(ii) In analysis of molecular and atomic structure.
(iii) For cooking purpose.
Infrared waves
(i) In knowing molecular structure. (ii) In remote control of TV VCR, etc.
Ultraviolet rays
(i) Used in burglar alarm. (ii ) To kill germs in minerals.
X-rays
(i) In medical diagnosis as they pass through the muscles not through the bones.
(ii) In detecting faults, cracks, etc., in metal products,
γ-rays
(i) As food preservation. (ii) In radiotherapy.

1. Electromagnetic Waves

Introduction

We have studied magnetostatics in the 4th chapter where we have seen the magnetic effect of current. Moving charges or current produces magnetic fields. Later in the 6th chapter, we have seen that changing magnetic fields produce current and this phenomenon is called electromagnetic induction.

What do you think the converse of electromagnetic induction exists?

Can changing electric fields produce magnetic fields?

The answer is Yes!

Maxwell proposed the converse: a changing electric field has a magnetic field associated with it. Maxwell also argued that not only electric current but also a time-varying Electric field generated magnetic field

While applying Ampere’s circuital law to find the magnetic field at a point outside a capacitor connected with a time-varying current. He noticed some inconsistencies with Ampere’s circuital law and suggested the existence of an additional current in addition to the conventional current. He called that additional current the displacement current Id to remove the inconsistency in the Ampere’s Law.

Maxwell formulated a set of four equations involving electric and magnetic fields and their sources, the charges and current sensitises. These equations are called Maxwell’s equations.The most important prediction of Maxwell’s equation is the existence of electromagnetic waves. We will discuss displacement currents and electromagnetic waves in this chapter. Below is the list of four maxwell equations.

Displacement current

I would like you to think something before actually jumping on the discussion of displacement current.

Imagine a  parallel plate capacitor that is charging through a dc battery. If there is nothing but a vacuum between the parallel plates of the capacitor then why doesn't it behave like an open circuit ?. How does the flow of charges through this vacuum happen?

As for conducting current to flow through a wire we need the circuit to be closed.  Just hold on for a while and you will get answers to all the questions above.

Let me first introduce displacement current.

Displacement current is that current that comes into existence in addition to the conduction current, whenever the electric field and hence the electric flux changes with time

Now let's try to understand what happens during the charging/discharging of capacitors.

When the capacitor is charging the charge density on the plates of the capacitor increases with time. The electric field between plates due to charges on the plates also increases with time. So due to this changing electric field between the plates of the capacitor, the displacement current comes into existence between the plates and makes the current continuous.

The displacement current between the plates is equal to the conduction current in the circuit.

When the capacitor is fully charged, the charge density on the plates of the capacitor is constant and therefore the electric field between the plates is also constant. So the displacement current is zero.

Conduction current is also zero when the capacitor is fully charged.

That is why the capacitor blocks DC when it is fully charged.

Note: Displacement current is not due to flow of charges but due to change in electric flux.

Electromagnetic waves

From our previous knowledge, we know that Charges at rest produce electric fields and Charges in motion produce electric current and electric current then produces magnetic fields. Changing magnetic fields produce electric current and this phenomenon is electromagnetic induction. So what would be the source of electromagnetic waves?

By Maxwell's theory, it was suggested that accelerated charges radiate electromagnetic waves.

In 1864, British physicist James Maxwell made the remarkable suggestion that the accelerated electric charges generate linked electric and magnetic disturbance that can travel indefinitely through space. If the charged particle oscillates periodically, the disturbance is waves whose electric and magnetic components are perpendicular to each other and also perpendicular to the direction of propagation.

Consider a charge oscillating with some frequency. An oscillating charge is an example of accelerating charge. This produces , changing electric field in space and this produces an oscillating magnetic field which in turn produces an oscillating electric field and thus oscillating magnetic field. We can conclude that oscillating electric and magnetic fields thus regenerate each other as the waves propagate through space and these waves are called electromagnetic waves.

Characteristic of electromagnetic waves:

Amplitude: The amplitude of an electromagnetic wave determines the maximum intensity of its field quantities. It is the maximum disturbance from the mean position of the waves.

Wavelength: The distance traveled by the wave in one complete cycle. It is the distance between two consecutive crests and troughs. Then S.I.  unit of wavelength is the meter.

Time periodThe time taken by the wave in one complete oscillation.Then S.I.  unit of time period is seconds

FrequencyIt is the complete oscillation of the waves in 1 second.Then S.I.  unit of  frequency is hertz.

 

Nature of electromagnetic waves:

  • Electromagnetic waves do not require any medium to travel through space. It can travel in vacuum also.
  • The speed of the electromagnetic waves in vacuum is 3*108 m/s and is an important fundamental constant ‘C’.
  • The Electric field , magnetic field and the direction of propagation is in mutually perpendicular directions like shown below.

 

  • The speed of the electromagnetic waves changes in different mediums. It depends on electric and magnetic properties of the medium.
  •  Speed or electromagnetic waves are maximum in vacuum. Speed of electromagnetic waves changes when it travel from one medium to another
  • The frequency of the electromagnetic wave remains the same in all mediums.
  • EM waves show phenomena like dispersion, reflection, refraction and polarization just like light waves do. Light is an electromagnetic wave.
  • EM waves carry energy and momentum with it, since it carries momentum, an electromagnetic wave also exerts pressure called radiation pressure.

Radiation pressure P= U ( energy density of EM wave)/ C. When the sun shines on our hand , we feel energy being absorbed as warmth from the electromagnetic wave. Do we feel pressure too? No ! We do not feel the pressure but it is present.

EM waves also transfer momentum to our hands but since ‘c= 3*108 m/s ’ and is very large so the momentum transfer is so small that we cannot feel it.

The great technological importance of electromagnetic waves stems from their capability to carry energy from one place to another. The radio signal and TV signals from broadcasting stations carry energy.  Light carries energy from the sun to the earth and thus makes life possible on the earth.

Electromagnetic spectrum

The orderly distribution of electromagnetic radiation according to their wavelength or frequency is known as the electromagnetic spectrum.

The main parts of EM waves are γ-rays , X-rays , ultraviolet rays, visible light, Infrared , microwave and radio waves

The various regions of the EM spectrum do not have sharply defined boundaries and they overlap. The classification is based roughly on how they are detected or produced.

Radiowave : These are the electromagnetic waves of longest wavelength and minimum frequency. It was discovered  by Marconi.

Source of radio wave of accelerated motion of charges in conducting wires of oscillating circuits.

Wavelength range : 600m and 0.1 m

Frequency range : 500 KHz to 1000 MHz                                  

Use of radio wave :

  1. In radiowave and television communication systems.
  2. In radio astronomy.

Microwave : They are the electromagnetic waves having wavelength next smaller to radio waves. Source of the microwave is oscillating currents in special vacuum tubes like klystrons, magnetrons and Gunn diodes.

It was discovered by Marconi.

Wavelength range : 0.3 m to 10-3 m

Frequency range: 109 Hz to 1012 Hz.

Uses of microwave

  • Mobile uses a microwave for communication.

  • Wifi connection uses Microwave

  • Microwaves are used in microwave ovens for cooking purposes.

Infrared Waves :

These radiations lie close to the low-frequency or long wavelength of the visible spectrum. Infrared waves produce heating effects, so they are also known as heat waves or thermal radiation.

Source of infrared is hot bodies.

Wavelength range- 5×10-3 m to 10-6 m.

Frequency range : 1011 Hz to 5×1014 Hz.

Uses of infrared:

Visible Light :

It is a very small part of the electromagnetic waves spectrum towards which the human retina is sensitive. The visible light emitted or reflected from bodies around us gives information about the world.

Source of visible gas discharge tubes, arcs of iron and mercury and the sun.

It was discovered by Ritter in 1800.

Wavelength Range : 3.5×10-7m to 1.5×10-7

Frequency range : 1016 Hz to 1017 Hz.

Uses of visible light

  • It provides us with information about the world around us.
  • It can cause chemical reactions . Example: Photosynthesis.
  • Some man made applications are light bulb and lasers which have numerous applications​​​​​​​

Ultraviolet light

The region of the electromagnetic spectrum has a shorter wavelength than visible light and can be detected beyond the violet end of the solar spectrum. Source of visible gas discharge tubes, arcs of iron and mercury and the sun.

It was discovered by Ritter in 1800.

Wavelength Range : 3.5×10-7m to 1.5×10-7m.

Frequency range : 1016 Hz to 1017Hz

Uses of Ultraviolet light

  • Treatment of water is done by Ultraviolet light.

  • To kill bacteria in hospitals.
  • Treats jaundice in newborns.

  • Used in detective work to reveal the presence of substance that cannot be seen by visible light

X- Ray

These EM waves have wavelengths just shorter than ultraviolet light. As X-ray can pass through many form of matter, so they have many medical and industrial applications

Source of X-ray is sudden dece;eration of fast moving electron by a target metal. It was discovered by Rontgen in 1895.

Wavelength range : 10-8 m to 10-11m.

Frequency range: 1018 Hz to 1020  Hz

Uses of X-Ray

  • In medical diagnosis because X-ray can pass through flesh but not through bones.

  • In the study of crystal structure because X-ray can be reflected and diffracted by crystals.

  • In engineering for detecting faults, cracks and holes in finished metal products.
  • In radiotherapy to cure untraceable skin disease and malignant growth.

γ-rays

These are the e.m. wave of highest frequency range and lowest wavelength range. These are most penetrating e.m. waves.

Source of γ-ray is radioactive nuclei and nuclear reactions. It was discovered by Henry Becqurel in 1896.

Wavelength range : 10-14 m to 10-10m

Frequency range : 1018 Hz to 1022 Hz.

Uses  of γ-rays

  • Gamma rays are used for treatment of tumors.

  • It is used for treatment of cancerous cells in the human body without the use of surgery. Cancer cells cannot regenerate once destroyed by gamma rays.

  • It is used to sterilize medical equipment.

At a glance