Radio Waves-Applied Physics-Lecture Slides, Slides of Applied Chemistry

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Introduction

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Introduction

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Radio Waves Radio waves have the longest wavelengths in the electromagnetic spectrum. These waves can be longer than a football field or as short as a football

They are produced by oscillating electrons in wires of electric circuits, They also carry signals for your television and cellular phones.

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Microwaves

Microwaves have wavelengths in the range of 1mm to 1m. They are commonly produced by an electromagnetic oscillator in electric circuits. The longer microwaves are using in heating our food in a microwave oven. Microwaves are good for transmitting information from one place to another because microwave energy can penetrate haze, light rain and snow, clouds, and smoke. These are also often used to transmit telephone conversations. Shorter microwaves are used in remote sensing, and in radars

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Visible light

Visible light waves are the only electromagnetic waves we can see. The limits of the wavelength of the visible region are from 400nm(voilet) to 700nm (red). Light is often emitted when valence electrons in an atom change their state of motion. That’s why such transitions in the state of electrons are called optical transitions. The study of the light emitted from the Sun and distant stars gives information about their composition.

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Ultraviolet radiations

The radiations of wavelengths shorter than the visible ( 1nm to 400nm ) are called ultraviolet radiations. They can be produced in atomic transitions of the outer electrons as well as in radiations from thermal sources such as sun. They have three types, the near ultraviolet, (NUV), extreme ultraviolet (EUV), and the far ultraviolet (FUV). Our atmosphere (Ozone) strongly absorbs UV radiations, but a little of these radiations from the sun reach the ground, which are very hazardous and may cause skin cancer

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Gamma radiations Gamma-rays have the smallest wavelengths (less than 10 - (^15) m ). They are the most energetic and penetrating of the

electromagnetic spectrum. These waves are generated by radioactive atoms and in a nuclear explosions. They can kill living cells, thus exposure to gamma radiations have harmful effect on the human body.

http://imagers.gsfc.nasa.gov/ems/ems.html docsity.com

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Generating an electromagnetic wave

 An electric charge generates an electric field

 A moving electric charge generates an electric field and a magnetic field

 A current is a flow of charges (a stream of moving electric charges)

 A current produces a magnetic field and an electric field

 An oscillating (back and forth moving) electric field produces an oscillating magnetic field.

 An oscillating (back and forth moving) magnetic field produces an oscillating electric field.

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Generating an electromagnetic wave

 To generate an electromagnetic wave current in wire must be vary with time.

 An oscillating RLC circuit with an external source can be used to generate electromagnetic wave

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Generating an electromagnetic wave

The current in the circuit varies sinusoidally with the resonant circular frequency ω = 1/(LC)1/

 The oscillator is coupled through a transformer to a transmission line, which carries the current to the antenna.

 This will be a dipole (having opposite charges) antenna.

 Antenna consists of simply two conductors

 The charges vibrate back and forth in these two conductors at the frequency ω driven by oscillator

 One branch of antenna carries charge q while the other carries charge – q

 The charge q varies sinusoidally with time every half cycle

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Generating an electromagnetic wave

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Traveling waves and Maxwell’s equations

 At a large distance away from the oscillating dipole, the wave fronts passing through a point are planes

 The lines of E are parallel to y-axes and the lines of B are parallel to Z- axes.

 The E and B fields can be written in the mathematical form as

 E(x,t) = Em sin(kx - ωt)

 B(x,t) = Bm sin(kx - ωt)

 Where ω is the angular frequency associated with the oscillating dipole

 The three dimensional snapshot is shown as

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Traveling waves and Maxwell’s equations

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Traveling waves and Maxwell’s equations

 The electric and magnetic energy densities are given as

2

uB B

m

2 2 0

u^1 E

E =^ e (3)

In electromagnetic wave the magnitude of the above two values remains equal, thus

uB = uE

0

2

0

2 e

m

B =^ E