2.1 Introduction
2.2 Generation of Electromagnetic Waves
2.2.1 Speed of Electromagnetic Waves
2.2.2 Direction of Propagation of Electromagnetic Waves
2.2.3 Doppler Effect of Electromagnetic Waves
2.3 The Electromagnetic Spectrum
2.4.1 Energy Carried by Electromagnetic Waves
2.4.2 Radiation Pressure
2. Summary
Unit 2 - Multiple Choice - Electromagnetic Waves Questions
Unit 2 - Multiple Choice - Electromagnetic Waves Answers
Unit 2.1 Multiple Choice Extended Questions - Waves
Unit 2.1 Multiple Choice Extended Answers-Waves
 

Unit 2 Electromagnetic Waves

2.1 Existence of Electromagnetic Waves

2.1.2 Generation of Electromagnetic Waves

Maxwell’s idea was that a magnetic field can be caused not only by a normal electric current, but also by a changing electric field or changing electric flux.

In 1887, practical confirmation of this theoretical prediction was demonstrated by Heinrich Hertz when he actually generated and detected EM waves .

hertz  
Fig 2.1.2.1
 

The transmitter made up of two spherical electrodes separated by a narrow gap and connected to an induction coil. When current flows the circuit a magnetic filed is setup up in the coils. This attracts the ferromagnetic contact at A and breaks the circuit. The high voltage induced in the secondary coil makes one electrode at X positive and the other negative. A spark is generated between the spheres when the electric field near either electrode ionizes the air. This ionization provides more electrons, which can accelerate and cause further ionizations. As the air in the gap is ionized, it becomes a much better conductor and the discharge between the electrodes. Very high frequency oscillations are setup. This apparatus is equivalent to an LC circuit in which the the coil has inductance and the spherical electrodes capacitance.

Using small values for L and C (recall ω = 1/v[LC] ), the frequency can be of the order of 100MHz. By adjusting the receiving frequency of the electrodes, Hertz was able to induce a spark in the receiver which was several meters away. His experiment is similar to the mechanical phenomenon in which a tuning fork vibrates in resonance to sound vibrations produced by an identical vibrating tuning fork placed nearby

It soon became obvious that these waves were similar to light waves in speed but of a different frequency and wavelength.

Maxwell predicted that if there is a changing electric field in empty space it will produce a magnetic field. Now by the symmetry argument, that changing magnetic field will produce a changing electric field... and it continues...

As a result, a wave of interacting electric and magnetic fields propagates itself through space.

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Consider what happens when the switch in fig 2.12.2 is turned on.

The plate on the right becomes negatively charged and the one on the left positive as a a charge flows and creates a temporary current (green arrows).

Fig 2.1.2.2
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The electric filed lines (red lines) flow from the positive plate to the negative plate in the plane of the paper. A magnetic field (blue lines) is also produced as the current flows going into the screen above the plates and coming out of the screen below the plates (use the right hand grip rule - Fig 2.1.2.3A). The magnetic field is perpendicular to the plane of the electric field,

The fields quickly appear near the plates but takes time to extend outward.

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Fig 2.1.2.3
Fig 2.1.2.3A
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Let us replace the cell with an ac generator.
Fig 2.1.2.4
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If we use an ac generator when the polarity reverses as the fields reverse direction the old electric field lines are collapsing slower so they fold on the new lines of the emerging field to form closed loops.

The old magnetic field lines also fold back on the new field lines to form closed loops perpendicular to your screen as they collapse.

As Maxwell predicted, the varying electric field induces a magnetic field and and vice-versa. A self sustaining wave is producef with oscillating electric and magnetic filed at right angles to each other and also at right angles to the direction of the their outward movement.

The plates are an antenna and the loops that have formed are now self sustaining as they extend outwards.

As the wave moves further from the antenna it has wave fronts emof alternating electric and magnetic filed's which are at right angles to each other.

 

 

They are called plane waves because the electric and magnetic fields are confined to a plane and uniform in that plane at all times.

Fig 2.1.2.5
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The fields are strongest perpendicular to the antenna, and drop to zero in the direction of the antenna.

Like any wave, the energy carried by the wave decreases according to the inverse square law.

I ∝ 1/r2

 

Fig 2.1.2.6
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The Electric field (red lines) oscillates in the plane of the screen, while the magnetic field oscillates perpendicular to the the plane of the screen as they propagate in the forward (x) direction.

Note that they are in phase.

   

 

Points to Note:

1. The electric and magnetic fields are perpendicular to each other and to the direction of travel.

2. The electric and magnetic fields are in phase.

3. Very far from the antenna, the field lines are flat over reasonably large areas and are called plane waves.

4. They are transverse waves.

5. They are field waves NOT matter waves and so can propagate in empty space.

 

     
Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.