Electromagnetic Waves is the field of Electrostatistics and Magnetism describes the Nature, Intensity, Energy density of the electromagnetic waves. The weightage of Electromagnetic Waves in JEE Main  Physics is 3.33%. The topic includes questions from few major topics like, Maxwell’s Displacement Currrent, Momentum, Continuity of Electric Current, etc.

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Candidates will have to deal with with questions like:

• To find the intensity of the wave when the magnetic fields and electric fields are given.

• To find the average energy density when the magnetic field is given.

• To find the dimensional formula of quantities like the magnetic field at zero position and questions related to finding the free current at the zero position.

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In this article we have provided the quick notes covering the major topics of Electromagnetic waves, tips to keep in mind, sample questions from electromagnetic waves, atmoshpheric layers etc.

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What are Electromagnetic Waves?

An electromagnetic wave is the one constituted by oscillating magnetic and electric fields which oscillate in two mutually perpendicular planes. The wave itself propagates in a direction perpendicular to both of the directions of oscillations of magnetic and electric fields.

Properties of Electromagnetic Waves

• The direction of oscillations of E and B fields are perpendicular to eachother as well as the direction of propagation. The Electromagnetic waves are transverse in nature. 

• The magnetic and electric fields oscillate in the same place.

• The energy density of magnetic field is and that of electric field is . So the energy density of the electromagnetic wave is .

Where, E and B are the instantaneous values of the magnetic and electric field vectors.

• The ratio

• The electromagnetic waves travel through vacuum with the same speed of light

Displacement Current

Displacement Current is that Current which comes into play in a region, where electri field ( and hence the electric flux) is changing with time.

The displacement is given by

Where, = absolute permittivity (or permmittivity of free space) and

= rate of change of electric flux

In case of a steady electric flux linked with a region, the displacement current is maximum. The current in the electric circuit which arises due to flow of electrons in the connecting wires of the circuit, in a defined closed path is known as conduction current.

Maxwell’s Modification of Ampere’s Circuital Law

In 1864, Maxwell showed that Ampere’s circuital law is logically inconsistent for non-setady currents. Than he modified Ampere’s Law as (Sum of displacement current and conduction current)

.

The Term is displacement current. This current comes into existence, in addition to the conduction current, whenever the electric field and hence the electric flux changes with time.

Maxwell's Equation

Maxwell found that all the nasic principles of electromagnetism can be formulated in terms of four fundamental equations, known as Maxwell’s equation. The equations are mentioned below:

Gauss’s Law of Electrostatistics

According to Gauss’s law, the total electric flux through any closed surface is equal to the charge inside that surface divided by

i.e.

This law relates the electric field to charge distribution, whereas electric field lines originate from (+ve) charge and terminate on negative (-ve) charge. The differential form of this law is given below:

Where p is colume chharge density and is the permittivity of free space.

Gauss’s Law of Magnetism

This law states that the net magnetic flux through a closed surface is zero. It means the number of magnetic field lines that enter a closed volume must be equal to number of field lines leaving that volume,

i.e.

It means the magnetic monopole cannot exist in nature. The differential form of this law is 

Faraday’s Law

Faraday’s Law states that the line integral of the electric field around any closed path (which equals the e.m.f.) equals the rate of change of magnetic flux through any surface area bounded by that path.

i.e.

The consequence of this law is that, if we keep any conducting loop in a time varying magnetic field, then an induced current flows in that conducting loop. The differential form of this law is

Hence this law describes the relationship between an electric field and a changing magnetic flux.

Modified Ampere-Maxwell Law

It states that “the line integral of magnetic field around any closed path is determined by the sum of the net conduction current through that path and the rate of change of electric flux through any surface bounded by that path.

i.e.

Hence, this law describes the relationship between electric and magnetic fields and electric current. The differential form of this law is,

Production of Electromagnetic Waves

An accelerated charge emits electromagnetic waves. An oscillating charge, as in an LC--circuit has non-zero acceleration, it continues to emit electromagnetic waves. The frequency of electromagnetic waves is the same as that of the oscillating charge.

Hertz experiment- In 1888, Hertz succeeded in experimentally confirming the existence of electromagnetic waves. By using oscillator LC-Circuits, he not only produced and detected electromagnetic waves, but also demonstrated their properties of refraction, reflection and interference and established beyond doubt that light radiation has wave nature.

Tips to keep in Mind 

• The velocity of electromagnetic waves does not depend on the amplitude of field vectors.

• The electric vector of an electromagnetic wave is responsible for the optical effects and is called a light vector.

• The amplitude of magnetic and electric fields in free space, in electromagnetic waves are related by or

• In a plane electromagnetic wave, the average energy densities of magnetic and electric fields are equal. In vacuum, the average electric energy density is given by

Also,

Therefore,  

• Poynting vector is when an electromagnetic wave advances, the electromagnetic energy flows in the direction. The total energy flowing perpendicularly per second per unit area into the space in free space is known as the Poynting vector where,

                  The SI unit of S is watt/m2

• Radiation pressure- The pressure exerted by electromagnetic waves is called radiation pressure (P). When an electromagnetic wave with Poynting vector is incident on a perfectly absorbing surface, then radiation pressure on surface is .

• The intensity of a sinusoidal plane electromagnetic wave is defined as the average value of poyting vector taken over one cycle. Thus, 

Electromagnetic Spectrum

The arranged array of electromagnetic radiations in the sequence of their frequency or wavelength is known as electromagnetic spectrum. Also, there is no sharp division between one kind of wave and the next. The classification has much more to do with the way these waves are produced and detected. Different regions are known by different names such as X-rays, g-rays, Ultraviolet rays, infrared rays, visible raysradio waves and microwaves. 

Propagation of Electromagnetic Spectrum

In radiowave communication between two places, the electromagnetic waves are radiated out by the transmitter antenna at one place which travel through the space and reach the receiving antenna at the othe place.

Uses of Electromagnetic Waves

Some of the uses of electromagnetic waves are mentioned below:

• Radio waves are used in radio and T.V communication systems.

• X-rays are used in the study of Crystal structures. They can pass through flesh and blood but not through bones. It is also used in medical diagnosis after X-rays photographs are made. 

• Microwaves are used in microwave ovens.

• Infrared Radiations are used in (a) greenhouse to keep plants safe (b) revealing the secret writings on the ancient walls (c) weather forecasting through infrared photography (d) treat muscular strain.

• Ultraviolet Radiations are used in the study of molecular structure, in the detection of forged documents, fingerprints in forensic laboratories and to preserve the food stuff.

• y-rays are used for the study of nuclear structure or provide the valuable information about the structure of the atomic nuclie.

• Super high frequency electromagnetic waves (3000 to 30,000 MHz) are used in radar and satellite communications.

• Electromagnetic waves (frequency 30 to 60 Hz) are used for lighting. These are weak waves having wavelength m and can be produced from A.C.circuits.

Point to Remember:

Greenhouse effect is the phenomenon which keeps the earth’s surface warm at night. The earth absorbs solar radiation and reflects back only infrared rays due to its low temperature. These rays are reflected back by the clouds and the gas molecules of the lower atmosphere. This keeps the earth's surface warm at night.

Quick Notes on types of atmospheric layer

Thermosphere

The first layer Thermosphere is a thermal classification of the atmosphere. In the thermosphere, temperature increases with altitude. It includes the exosphere and part of the ionosphere.

Exosphere

The Exosphere is the outermost layer of the Earth’s atmospheric layer. This layer goes from about 400 miles (640 km) high to about 800 miles (1,280 km). The lower boundary of the exosphere is known as the critical level of space, where atmospheric pressure is very low (the gas atoms are widely spaced) and the temperature is very low.

Mesosphere

Next atmospheric layer is the mesosphere which is characterized by temperatures that quickly decrease as height increases. This layer extends from between 31 and 50 miles (17 to 80 kilometers) above the earth’s surface. 

Stratosphere

The Stratosphere is described by a slight temperature increase with altitude and the absence of clouds. This layer extends between 11 and 31 miles (17 to 50 kilometers) above the earth’s surface. The earth’s ozone layer is located in the stratospheric layer. Ozone, a form of oxygen, is important to our survival, this layer absorbs a lot of ultraviolet solar energy. Only the higher clouds (cirrus, cirrostratus, and cirrocumulus) are in the lower stratosphere.

Tropopause

The Tropopause is the boundary zone (or transition layer) between the troposphere and the stratosphere layers. The tropopause is described by little or no change in temperature altitude increases.

Troposphere

This layer is the lowest region in the Earth’s (or any planet’s) atmosphere. On the Earth, the layer goes from ground (or water) level up to about 11 miles (17 kilometers) high. The weather and clouds occur in the troposphere. In this layer, the temperature generally decreases as altitude increases.

Inosphere

The Ionosphere starts at about 43-50 miles (70-80 km) high and remains for hundreds of miles (about 400 miles - 640 km). It contains many ions and free electrons (plasma). The ions are generated when sunlight hits atoms and tears off some electrons. Auroras occur in the inosphere. 

• D-layer is at a virtual height of 65 Km from surface of earth and having electron density of ~

• E-layer is at a virtual height of 100 km, from the surface of earth and having electron density ~

• F1-layer is at a virtual height of 180 km, from the surface of earth and having electron density ~

• F2-layer is at a vertical height of about 300 km in night time and about 250 to 400 km in day time. The electron density of the F-2 layer is ~ 8. 

Sample Questions on Electromagnetic Wave

Question: The decreasing order of wavelength of microwave, ultraviolet,infrared, and gamma rays is

(A) Microwave, Infrared, Ultraviolet, Gamma rays

(B) Infrared, Microwave, Ultraviolet, Gamma rays

(C) Microwaves, Gamma rays, Infrared, Ultraviolet

(D) Gamma rays, Ultraviolet, Infrared, Microwaves

Answer: (A) Microwave > Infrared > Ultraviolet > Gamma rays

Question: The electric and magnetic field of an electromagnetic waves are:

(A) In the opposite phase and parallel to each other.

(B) In Phase and perpendicular to eachother

(C) In opposite phase and perpendicular to each other

(D) In phase and parallel to each other

Answer: (B) In Phase and perpendicular to eachother

Question: X-rays are

(A) Stream of electron

(B) Stream of uncharged particle

(C) Electromagnetic radiation

(D) Stream of proton

Answer: (C) Electromagnetic radiation

X-rays consist of oscillating electric and magnetic field at right angles of eachother and in the direction of propagation of the x-rays. Therefore, x-rays are electromagnetic radiations.

Question: In Gamma ray emission from a nucleus 

(A) Only the proton number changes

(B) Only the neutron number changes

(C) There is no change in the neutron number and the proton number

(D) Both the proton number and the neutron number changes

Answer: (C) There is no change in the proton number and the neutron number

Question: If there were no atmosphere, the average temperature on earth surface would be

(A) Higher

(B) Same

(C) Lower

(D) 0 degree C

Answer: (C) Lower

As the atmosphere is the blanket of glasses that restrain the heat to escape in outer space. If there was no atmosphere, the earth’s temperature would rise in the daytime and it would reach the freezing point during the night. Hence, it would affect the survival of all living organisms of the earth.


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