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NEET Important Chapter - Electromagnetic Waves

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Important Concepts of Electromagnetic Waves

Important Concepts of Electromagnetic Waves

In the previous chapters, we have learnt  that electric current produces magnetic fields and changing magnetic fields produce electric fields.

In this chapter, we will discuss whether changing electric fields develops magnetic fields or not. James clerk Maxwell explained that not only electric current but also varying electric fields, with time, also generate magnetic fields. He introduced displacement current along with conduction current.

We will also discuss Maxwell's equations in this chapter, which proves the existence of electromagnetic waves. Maxwell corrected the ampere law by adding the effect of displacement current  and gave Ampere - Maxwell's law.

We will also get to know what is the source of electromagnetic waves and their nature, speed,  frequency,  wavelength and intensity in this chapter, as well as, know about the relationship between electric field and magnetic field.

This chapter also discusses the spectrum of electromagnetic waves, which is an electromagnetic spectrum that contains a list of waves like radio wave, microwave, infrared wave, visible wave, and ultra visible rays.

We will also deal with light, which is an electromagnetic wave and find the relationship between electric field and magnetic field.

Now, let's move onto the important concepts and formulae related to NEET exams along with a few solved examples and previous year questions.

Important Topics of Electromagnetic Waves

  • Revision Notes of Electromagnetic waves

  • Electromagnetic Spectrum

  • Electromagnetic Waves - Definition, Equation and Properties

  • NCERT Solutions - electromagnetic  wave

  • Displacement equation 

  • Maxwells Equations

  • Radio Waves

  • The Electromagnetic Spectrum Visible Light

  • Velocity of Electromagnetic waves


Electromagnetic Waves Important Concept for NEET

Name of the Concept

Key Points of the Concepts

  1. Conduction current 

  • The current carried by a conductor due to the flow of charges is called conduction current.

  • Outside the capacitor plate, we only have conduction current.

  1. Displacement current 

  • The current which is due to the changing electric field is called displacement current.

  • It is due to the time varying electric field.

  • Inside the capacitor, there is no conduction current, there is only displacement current.

  1. Source of Electromagnetic waves

  • Accelerated charges radiate electromagnetic waves. The frequency of electromagnetic waves naturally equals the frequency of oscillation of charge.

  1. Nature of Electromagnetic waves

  • Electromagnetic waves propagate in the direction of a propagation vector that is k. It is perpendicular to both electric and magnetic fields. Electric fields and magnetic fields are also perpendicular to each other.

K = $\dfrac{2\pi}{\lambda}$

Here ${\lambda}$ is the wavelength of em wave.


Source of Electromagnetic waves

  • Electromagnetic waves are transverse in nature.

  1. Intensity


  • Power per unit area or energy crossing per unit area per unit time perpendicular to the direction of propagation is called intensity of a wave.

  1. Electromagnetic spectrum

  • According to frequency, all electromagnetic waves have to be  classified. This is known as the electromagnetic spectrum.

  • In the Electromagnetic spectrum, all the different regions are known by different names like gamma -rays, X-rays, ultraviolet rays, visible rays, infrared rays, microwaves and radio waves.

  • The range of wavelengths in the visible ray is 400 nm to 700 nm. Electromagnetic spectrum

  1. Electromagnetic pressure 

  • As we know, electromagnetic waves carry energy and momentum. An em wave exerts pressure, called radiation pressure. 


List of Important Formulae for Electromagnetic Waves

S.No.

Name of the Concept

Formula


Ampere’s circuital law

$\oint\overleftarrow{B}.\overrightarrow{dl} =\mu_{o} i (t)$

Here  i (t) is the time-dependent current which is flowing through the circuit.

$\mu_{o}$ is permeability in vacuum.


Displacement current

$i=\varepsilon_{o}\times\dfrac{\text{d}\phi_{E}}{\text{d}t}$

Here $\phi_{E}$ is electric flux and $\varepsilon_{o}$ is permittivity in vacuum.


Speed of propagation of wave 

$\dfrac{\omega}{K}$

Here ${\omega}$ is angular frequency and k is propagation vector K = $\dfrac{2\pi}{\lambda}$

Here ${\lambda}$ is the wavelength of em wave.

Angular frequency=${\omega}= 2\pi\upsilon$

Here $\upsilon$ is the frequency of wave 

4.

Relation between frequency and wavelength

c = $\upsilon \times\lambda$

Here c is the velocity of light.

5.

Relation between electric and magnetic field

$B = \dfrac{E}{c}$

Here c is the velocity of light that has a constant value

$c = 3\times10^{8}$ m/s

6. 

First Maxwell’s equation (Gauss law for electricity)

$\oint\overleftarrow{E}.\overrightarrow{dA} =\dfrac{Q}{\varepsilon_{o}}$

7.

Second Maxwell’s equation (Gauss law forMagnetim)

$\oint\overleftarrow{B}.\overrightarrow{dA} =0$

8.

Third Maxwell’s equation (Faraday’s law)

$\oint\overleftarrow{E}.\overrightarrow{dl} = -\dfrac{\text{d}\phi_{B}}{\text{d}t}$

9. 

Fourth Maxwell’s equation (Ampere maxwell law)

$\oint\overleftarrow{B}.\overrightarrow{dl} =\mu_{o} i_c+\mu_{o} \varepsilon_{o} \dfrac{\text{d}\phi_{E}}{\text{d}t}$

Here $\varepsilon_{o}$ is permittivity in vacuum. And $ \mu_{o} $ is the permeability in vacuum. 

10.

Velocity of light in terms of permittivity and permeability

$c=\dfrac{1}{\sqrt{\varepsilon_{o}\mu_{o}}}$

11.

Electromagnetic pressure

P = U/c

Here U = total energy and c is light velocity


Solved Examples 

1. A plane electromagnetic wave of frequency 4 Mhz travels in free space along the x-direction. At a particular point in space and time,  E = 6.3 $\widehat{j} \dfrac{V}{m}$. What is B at this point?

Sol: 

Given:

 Electric field = E = 6.3 $\widehat{j} \dfrac{V}{m}$

We know that velocity of light = $c = 3\times10^{8}$ m/s

The relation between electric and magnetic fields is $B = \dfrac{E}{c}$

B = $\dfrac{6.3}{3\times10^{8}}$ = ${2.1\times10^{-8}} T$

For direction - As per question, the electric field is in y-direction and waves propagate in x-direction. So the magnetic field is perpendicular to both the wave vector (K) and the electric field (E).

Therefore, the magnetic field is along the z-direction 

$\overrightarrow{B} =2.1\times10^{-8}  \widehat{k} T$

Key Point - Keep in mind the relation between electric and magnetic fields.

2. A charged particle oscillates about its mean equilibrium position with a frequency of $10^9$ Hz. What is the frequency of the electromagnetic waves produced by the oscillator?

Sol:  

The frequency of the electromagnetic wave is the same as that of an oscillating charged particle about its equilibrium position, which is $10^9$ Hz.

Key Point: As accelerated charges radiate electromagnetic waves, the frequency of electromagnetic waves naturally equals the frequency of the oscillation of charge.

Previous Year Questions From NEET Paper

1. The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is (c is speed of electromagnetic wave ) (NEET 2021)

  1. C : 1

  2. 1 : 1

  3. 1 : c

  4. $1 : c^{2}$

Sol: 

The intensity of an electromagnetic wave is given by: $I = U_{av} \times c$

The average electric energy = $\dfrac{1}{2} \times \varepsilon_{o} \times E^{2}$

The average magnetic energy = $\dfrac{\ B^{2}}{2\mu_{o}}$

We know that the relation - $c=\dfrac{1}{\sqrt{\varepsilon_{o}\mu_{o}}}$

$B = \dfrac{E}{c}$

Here, c is the velocity of light that has a constant value.

The average electric energy is $\dfrac{1}{2}\times\varepsilon_{o}\times\dfrac{B^{2}}{2\mu_{o}\varepsilon_{o}}$

= $\dfrac{\ B^{2}}{2\mu_{o}}$

As we can see that the energy due to electric fields and magnetic fields is the same, therefore they make the same contribution in intensity.

Trick - Average  energy due to electric field and magnetic field are  equal.

2. For a plane electromagnetic wave propagating in the x-direction, which one of the following combinations give the correct possible direction for electric field (E) and magnetic field (B), respectively ?(NEET 2021)

  1. $\widehat{j} + \widehat{k} , -\widehat{j} - \widehat{k}$

  2. $-\widehat{j} + \widehat{k} , -\widehat{j} + \widehat{k}$

  3. $\widehat{j} + \widehat{k} , \widehat{j} + \widehat{k}$

  4. $-\widehat{j} + \widehat{k} , -\widehat{j} - \widehat{k}$

Sol:  

Option (4) is correct.

As per the question, the waves propagate in the x-direction. So the wave vector (K) is also in  x-direction. The magnetic field and electric field both are perpendicular to the wave vector (K).

Direction of propagation of electromagnetic waves or k vector is along the product of electric or magnetic field.

$\overleftarrow{K} = \overleftarrow{E}\times \overrightarrow{B}$

By checking all the options, it can be seen that only option (c)  follows the condition.

$\overleftarrow{K} = (-{\widehat{j} + \widehat{k})}\times {(-\widehat{j} - \widehat{k})} = 2\widehat{i}$

Trick - Electromagnetic waves propagate in the direction of the propagation vector and perpendicular to magnetic field and electric field.

Practice Questions 

1. A plane electromagnetic wave travels in vacuum along the z-direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave is 300 MHz, what is its wavelength? (Ans: 1m)

2. A radio can tune into any station in the 7.5MHz to 12MHz band. What is the corresponding wavelength band? (Ans: 40m  to 25m )

Conclusion

In this article, we have provided important information regarding the chapter, such as important topics, concepts, and formulae, etc. Students should work on more solved examples for securing good marks in the NEET 2022 exams. Try to remember the relationship between all the parameters that are required to understand  the electromagnetic wave.

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NTA is responsible for the release of the NEET 2022 cut off score. The qualifying percentile score might remain the same for different categories. According to the latest trends, the expected cut off mark for NEET 2022 is 50% for general category candidates, 45% for physically challenged candidates, and 40% for candidates from reserved categories. For the general category, NEET qualifying marks for 2021 ranged from 720-138 general category, while for OBC/SC/ST categories, they ranged from 137-108 for OBC, 137-108 for SC and 137-108 for ST category.
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FAQs on NEET Important Chapter - Electromagnetic Waves

FAQ

1. What is the weightage of the chapter  Electromagnetic Waves the NEET exam?

In the NEET exam, there are 45 questions asked in the Physics section. Out of 45, there are 1 question on average that comes under this chapter, which is nearly 1 - 2% of the  Physics section in the exam.

2. How can we expect full marks for questions from the chapter Electromagnetic Waves NEET exam?

This chapter Electromagnetic Waves is a very simple chapter in Physics. Try to remember all the properties of electromagnetic waves and relationship between parameters like wavelengths, frequency, wave vector, velocity of light and angular velocity. If you practice Previous Year Question Paper,  and reference books then you can easily get full marks from this chapter in the exam.

3. What is the difficulty level of questions in the NEET exam from the chapter Electromagnetic Waves ?

As far as NEET is concerned, the difficulty level is medium for the chapter Electromagnetic Waves. Go through the previous year's question papers, and try to solve it. Most of the questions are formula based questions from this chapter and learn all the concepts and formulae related to it.