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Electromagnetic energy is a type of energy that moves in waves through space.
- It is produced by accelerated charged particles such as electrons and protons produces it.
- Because electromagnetic radiation can flow through a vacuum, it does not require matter for propagation.
- Electromagnetic energy is classified according to its frequency, wavelength, or energy.
- The number of waves that travel through a certain place in space per second is defined as frequency.
- The distance between two successive peaks of a wave is defined as its wavelength.
- The energy of the wave is the amount of work it can do.
- The frequency and wavelength of electromagnetic radiation determine its various types.
- Gamma radiations have the highest frequency and shortest wavelength, while radio waves have the lowest frequency and largest wavelength.
- Visible light is just a small portion of the electromagnetic spectrum that the human eye can perceive.
| Table of Content |
Key Terms: Electromagnetic wave, Energy density, Frequency, Wavelength, Electromagnetic spectrum, Electric field, Magnetic field
Electromagnetic Energy
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The term "electromagnetic energy" refers to the various energies that flow across space as wavelengths at the speed of light.
- Electromagnetic radiation has neither mass nor charge. Rather, it travels in the form of photons, which are light energy bundles.
- One of the four fundamental forces of nature, along with the strong, weak, and gravitational forces, is electromagnetic energy.
- These forces vary in strength and operate in a variety of ranges.
- For example, both electromagnetic and gravitational forces have an unlimited range, yet gravity is the weakest.
Electromagnetic spectrum
There is a lot of uncertainty over who discovered electromagnetic waves.
- However, the oldest records show that electromagnetic was discovered in 1820 by Hans Christian Oersted, a Danish scientist and chemist.
- He discovered that variations in the electric current deviated the compass needle from its magnetic north when he switched the battery on and off.
- Oersted was relieved when he discovered that electric current could produce a magnetic field, demonstrating that there was a logical connection between electric current and magnetism, which became known as electromagnetism.
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Electromagnetic Energy Mechanism
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The consequence of changing electric and magnetic fields is electromagnetic energy.
- The electric field changes when a charged particle is triggered by pushing it up and down.
- As a result, the alternating electric field mentioned above develops. Magnetic fields are produced by electric fields.
- When magnetic current changes uniformly, the magnetic field fluctuates.
- Electric and magnetic fields are mutually connected. When one quantity changes, the other follows the same.
- The electric and magnetic fields move perpendicular to one other, allowing electromagnetic fields to be polarised.
- A changing magnetic field causes a changing electric field, which causes a changing magnetic field.
- A continuous chain phenomenon results in transverse electromagnetic waves as a result.
- EM waves travel in carriers that include particles known as photons. They have no rest mass or charge and travel at the speed of light.
- The movement of electromagnetic radiation is caused by the interaction of moving photons and electromagnetic coupling.
- Different photon states and wave characteristics result in many forms of electromagnetic waves.
Energy Density of Electromagnetic Waves
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The energy density of an electromagnetic wave is defined as the energy per unit volume of the space in which it travels.
Electromagnetic waves carry energy and this energy is shared equally by the electric and magnetic fields.
We know, in free space, the energy density of an electric field is given by
UE = ∈0E2/2
Also, in free space, the energy density of the magnetic field is given by
UB = B2/2µ0
The total energy density of electric and magnetic fields in free space is given by
U = UE + UB
⇒ U = ∈0E2/2 + B2/2µ0
However, in electromagnetic waves, both electric and magnetic fields are time-varying i.e. they both vary sinusoidally in space and time. Therefore average energy density of electromagnetic waves can be obtained as
Uav = ∈0E2rms/2 + B2rms/2µ0
Now Erms = E0/√2 and Brms = B0/√2, therefore
Uav = ∈0E02/4 + B02/4µ0……..(i)
Also, we have B0 = E0/c
Where c is the speed of the electromagnetic wave in air or vacuum.
⇒ Uav = ∈0E02/4 + E02/4µ0c2
Also, we have c2 = 1/(µ0∈0), therefore
⇒ Uav = ∈0E02/4 + ∈0E02/4
⇒ Uav = ∈0E02/2 = ∈0E2rms
Put E0 = cB0 in equation (i), we get
Uav = B02/2µ0 = B2rms/µ0
Hence the average energy density of electromagnetic waves is
Uav = ∈0E02/2 = ∈0E2rms and Uav = B02/2µ0 = B2rms/µ0
Electromagnetic Spectrum
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The electromagnetic spectrum is a set of frequencies and wavelengths of electromagnetic radiation.
- Each wavelength and frequency combination produces a unique form of energy.
- The frequency of the electromagnetic waves is equal to the number of waves that reach a certain point each second.
- The unit of frequency is Hertz (Hz). The frequency of 1 Hz measures one wave cycle per second.
The electromagnetic spectrum consists of
- Radio Waves: Radio waves are electromagnetic waves with a low frequency. It is found at the beginning of the electromagnetic spectrum. Longer wavelength EM waves have the lowest frequency and hence the lowest energy. Radio waves offer a wide range of practical uses, including media transmission, wireless communication, radio telescopes, and so on.
- Microwaves: Microwaves are electromagnetic waves with wavelengths ranging from one meter to one millimeter. The equivalent frequencies vary from 300 MHz to 300 GHz. Wireless networks, satellite and spacecraft communications, microwave radio relay networks, medical treatment, remote sensing, particle accelerators, radio astronomy, spectroscopy, and other applications make significant use of microwaves.
- Infrared Waves: An infrared wave, often known as IR radiation, is a part of the electromagnetic spectrum with wavelengths ranging from 700 nanometers to one millimeter. Infrared waves have far longer wavelengths than visible light but much shorter wavelengths than radio waves. A wide spectrum of infrared radiation can be perceived as heat. Infrared is further subdivided into far-infrared, mid-infrared, and near-infrared. Thermal infrared is another name for far infrared.
- Visible Light: Visible light is present in the center of the electromagnetic spectrum. It is the portion of the electromagnetic spectrum that can be seen with the human eye. Every type of electromagnetic energy is considered light, but the EM radiation that can be visible to human eyes is referred to as visible light. A rainbow is a form of visible light with various wavelengths for each rainbow color. The longest wavelength is red, while the shortest wavelength is violet.
- Ultraviolet (UV) Waves: Ultraviolet waves have wavelengths ranging from 10 nanometers (30 PHz) to 400 nanometers (750 THz). They are significantly shorter than visible light and much longer than X-rays. Sunlight contains UV waves. They also account for around 10% of the total EM radiation emitted by the Sun. UV rays have a shorter wavelength and more energy.
- X-rays: X-rays, often known as X-radiation, are a type of high-energy electromagnetic radiation. They have wavelengths ranging from 10 picometers to 10 nanometers and frequencies ranging from 30 petahertz to 30 exahertz. The inherent energy varies between 145 eV to 124 keV. The wavelengths of X-rays are significantly shorter than those of UV light and generally longer than those of gamma radiation.
- Gamma Rays: A gamma-ray (gamma radiation) is a type of penetrating electromagnetic radiation produced by the radioactive disintegration of subatomic nuclei. It is composed of the shortest wavelength electromagnetic waves with a frequency of more than 30 exahertz. It is at the far extreme of the spectrum. The highest energy frequency is gamma-ray. Gamma radiations are the most powerful EM waves because of their properties. Nuclear reactions, nuclear decays, star explosions, and other processes all emit gamma radiation.
Things to Remember
- Electromagnetic energy includes all types of energy of electromagnetic waves that flow through space at the speed of light.
- The electromagnetic spectrum is a set of frequencies and wavelengths of electromagnetic radiation.
- The energy density of an electromagnetic wave is defined as the energy per unit volume of the space in which it travels.
- A changing magnetic field causes a changing electric field, which causes a changing magnetic field.
- Electromagnetic waves are generated due to changes in electric and magnetic fields associated with oscillating charges.
- The movement of electromagnetic radiation is caused by the interaction of moving photons and electromagnetic coupling.
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Sample Questions
Ques. The amplitude of an oscillating magnetic field in an electromagnetic wave is 2 x 10-6 T. What will be the amplitude of the oscillating electric field? (5 Marks)
Ans. The amplitude of an oscillating electric field in an electromagnetic wave is related to the amplitude of the oscillating magnetic field as
E0 = cB0
Where
- E0 is the amplitude of the electric field
- B0 is the amplitude of the magnetic field
- c is the speed of the speed of electromagnetic wave
Given the amplitude of an oscillating magnetic field, B0 = 2 x 10-6 T
On substituting the values, we get
Magnitude of electric field, E0 = 3 x 108 x 2 x 10-6
⇒ E0 = 600 V/m
Ques. What is electromagnetic radiation? (2 Marks)
Ans. Electromagnetic radiation is a form of energy that is generated by the movement of electrically charged particles or the oscillation of electric and magnetic fields. Electromagnetic waves include radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
Ques. Which of the following has the maximum value in an electric or magnetic field? (2 Marks)
(a) Amplitude
(b) Wavelength
(c) Frequency
(d) Focal length
Ans. The correct answer is a. Amplitude
Explanation: The electric and magnetic fields are present in an electromagnetic field as waves in horizontal and vertical patterns. It is required to have maximum amplitude in order to be present in that pattern.
Ques. How does electromagnetic energy interact with matter? (3 Marks)
Ans. Electromagnetic waves may interact with matter in a variety of ways depending on their wavelength.
- Microwaves and radio waves can travel through a number of materials, including paper and wood.
- As a result, they may be utilized for both communication and remote sensing.
- Infrared light may be absorbed by molecules, causing them to vibrate. Infrared radiation can therefore be used to heat and cook.
- Matter has the ability to reflect, refract, and absorb visible light. This is how we see colors and things.
- Ultraviolet light can ionize atoms, removing electrons from them. As a result, UV radiation may be used to sterilize medical equipment while also causing sunburns.
Ques. A plane electromagnetic wave of frequency 25 MHz travels in free space along the x-direction. At a particular point in space and time, E = 6.3 V/m. What is B at that point? (5 Marks)
Ans. Given
- Frequency of the electromagnetic wave, f = 25 MHz
- The amplitude of the electric field, E = 6.3 V/m
The amplitude of the electric field in an electromagnetic wave is related to the amplitude of the magnetic field as
E = cB
Where c is the speed of the electromagnetic wave
From the above equation, we get
B = E/c
On substituting the values, we get
Magnitude of magnetic field, B = 6.3/(3 x 108) = 2.1 x 10-8 T
Ques. If the intensity of wavelength decreases, the energy released will (2 Marks)
(a) Remain the same
(b) Decrease
(c) Increase
(d) Zero
Ans. The correct answer is c. Increases
Explanation: The relationship between wavelength and energy released is inversely proportional, which means that as wavelength increases, so does the energy produced by the body. This can is given by the plank’s theory.
Ques. The unified theory of electromagnetism was explained by (2 Marks)
(a) Albert Einstein
(b) Kelper
(c) James Clerk Maxwell
(d) Faraday
Ans. The correct answer is c. James Clerk Maxwell
Explanation: In 1873, James Clerk Maxwell described relationships between the mobility of charged particles in electric and magnetic fields and the behavior of electric and magnetic fields in his unified theory of electromagnetism.
Ques. What is radiation? (1 Mark)
Ans. Radiation is the emission or transmission of energy in the form of waves or particles over space or a material medium.
Ques. What is the electromagnetic spectrum? (2 Marks)
Ans. The electromagnetic spectrum comprises all electromagnetic radiation, from high-energy gamma rays to low-energy radio waves, with just a small portion in its middle containing visible light. The electromagnetic spectrum is essential for understanding and utilizing various types of electromagnetic energy.
Ques. A source of light has an energy flux of 10 watts/m2. The light falls perpendicular to the surface of an area of 5 cm2. If the surface completely absorbs the incident light, then find (5 Marks)
(a) The momentum delivered to the surface
(b) Force exerted on the surface in 10 minutes
Ans. Given
- Energy flux or intensity of light, I = 10 watts/m2
- Area of the surface, A = 5 cm2 = 5 x 10-4 m2
- Time, t = 10 minutes = 600 seconds
The total energy incident on the surface is given by
E = Intensity x Area x Time
⇒ E = 10 x 5 x 10-4 x 600 = 3 J
- Momentum delivered to the surface is given by
p = E/c
Where c is the speed of the light
⇒ p = 3/(3 x 108) = 10-8 kgm/s
- The force exerted on the surface is given by
F = p/t
⇒ F = 10-8/600 = 1.6 x 10-11 N
Ques. Microwaves can’t be used for long-distance transmission of signals. (2 Marks)
(a) True
(b) False
Ans. The correct answer is b. False
Explanation: Microwaves are not bent by objects of normal dimensions due to their shorter wavelength. As a result, they can be used to direct a signal in a certain direction. As a result, microwaves can be used for long-distance signal transmission.
Ques. What are the characteristics of electromagnetic radiation? (5 Marks)
Ans. The following are the characteristics of electromagnetic radiation
- Wavelength: The distance between successive crests or troughs of a wave is represented by its wavelength (λ). It is the length of one complete oscillation cycle.
- Period: The period (T) of a wave is the amount of time it takes to complete one full cycle or traverse one wavelength. It is given by the reciprocal of frequency i.e. T = 1/f. The time period of a wave tells us about the time characteristics of a wave.
- Amplitude: The amplitude of a wave is defined as its maximum displacement from its equilibrium position. It represents the distance between the wave's center and the peak or trough.
- Frequency: Frequency is defined as the number of wave cycles that occur per second, measured in Hertz (Hz) or cycles per second. It defines the number of times a wave oscillates in a certain time period.
- Velocity: Velocity is defined in electromagnetic radiation as the product of wavelength and frequency, which is given by Velocity = λf. In a vacuum, the velocity of electromagnetic waves is equal to the speed of light, which is about 186,282 miles per second or 2.99 x 108 meters per second (m/s).
Ques. A short wavelength indicates that the frequency will be (2 Marks)
(a) Lesser
(b) Zero
(c) Higher
(d) None of the above
Ans. The correct answer is c. Higher
Explanation: A shorter wavelength indicates a higher frequency since one cycle may pass in a shorter amount of time.
Ques. A laser emits a beam of light of 2 mm in diameter. If the power of the beam is 10 mW, find the intensity of the beam of light. (3 Marks)
Ans. Given
- The diameter of the light beam, D = 2 mm = 2 x 10-3 m
- Power of the beam, P = 10 mW = 10-2 W
Hence area of the beam, A = πr2 = π(D/2)2
On substituting the values, we get
A = 3.14 x (2 x 10-3/2)2 = 3.14 x 10-6 m2
The intensity of the light beam, I = P/A
⇒ I = 10-2/(3.14 x 10-6) = 3.183 x 103 W m-2
Ques. What is the main source of electromagnetic radiation heat on Earth? (2 Marks)
Ans. The main source of electromagnetic radiation on our planet is the sun. The energy from the sun travels to the Earth's surface through space and the Earth's atmosphere. Some of this energy is converted into heat energy, which warms the earth's surface and atmosphere.
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