ELECTROMAGNETIC WAVES

• ​Find the similarities between pictures beside?

• ​What is the difference betw​een each pictures beside?

• ​So, what is the difference between electromagnetic waves and other waves?

• Electromagnetic generation explained:

​https://www.youtube.com/watch?v=FWCN_uI5ygY

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• ​Hertz experiment:

https://www.youtube.com/watch?v=9gDFll6Ge7g

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• ​Simulation: https://javalab.org/en/radio_wave_communication_en/

• ​Accelerating charges produce electromagnetic waves.

• ​Electromagnetic radiation, consist of oscillating electric and magnetic fields that travel away from the accelerating charges.

Speed of EM Wave in Vacuum

1. ​Unlike mechanical waves, electromagnetic waves can travel through vacuum; they do not require a material medium.

2. ​Looking back at the laws that describe electric and magnetic fields, we find two universal constants. One of them is the permittivity of free space, found in Coulomb’s law and Gauss’s law; it is associated with the electric field. The second is the permeability of free space, found in Ampère’s law; it is associated with the magnetic field

Speed of EM Wave in Vacuum

​When an EM wave passes from one medium to another, the frequency and wavelength cannot both remain unchanged since the wave speed changes and v = fλ.

As is the case with mechanical waves, it is the wavelength that changes; the frequency remains the same.

Instead of specifying the speed, it is common to specify the index of refraction n.

Index of refraction: n = c/v

Since the frequencies are equal,

1. ​Speed of EM waves in a material medium does depend on frequency.

2. ​The index of refraction is not a constant for a given material, it is a funcion of frequency.

3. ​Variation of the speed of a wave with frequency is called dispersion.

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Look at this three simulation below:

https://ophysics.com/em3.html

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https://javalab.org/en/electromagnetic_wave_en/

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https://javalab.org/en/radio_wave_communication_en/

• ​EM waves in vacuum travel at speed c = 3.00 × 10⁸ m/s, independent of frequency. The speed is also independent of amplitude

• The electric and magnetic fields oscillate at the same frequency. Thus, a single frequency f and a single wavelength λ= c/f pertain to both the electric and magnetic fields of the wave. 

• The electric and magnetic fields oscillate in phase with one another. That is, at a given instant, the electric and magnetic fields are at their maximum magnitudes at a common set of points. Similarly, the fields are both zero at a common set of points at any instant.

• The amplitudes of the electric and magnetic fields are proportional to one another. The ratio is c: Em = c Bm

• Since the fields are in phase and the amplitudes are proportional, the instantaneous magnitudes of the fields are proportional at any point.

• ​The EM wave is transverse; that is, the electric and magnetic fields are each perpendicular to the direction of propagation of the wave. 

• The fields are also perpendicular to one another. Therefore, E, B, and the velocity of propagation are three mutually perpendicular vectors.

• At any point, E x B is always in the direction of propagation 

  The electric energy density is equal to the magnetic energy density at any point.The wave carries exactly half its energy in the electric field and half in the magneticfield.