Lecture 38: MON 24 NOV Lecture 38: MON 24 NOV Ch.33 Electromagnetic WavesCh.33 Electromagnetic Waves

Heinrich Hertz

(1857–1894)

Physics 2113

Jonathan Dowling

Maxwell Equations in Empty Maxwell Equations in Empty Space:Space:

Fields withoutsources?

Changing E gives B.Changing B gives E.

A solution to the Maxwell equations in empty space is a “traveling wave”…

The electric-magnetic waves travel at the speed of light?

Light itself is a wave of electricity and magnetism!

Maxwell, Waves, and LightMaxwell, Waves, and Light

electric and magnetic fields can travel in EMPTY SPACE!

First person to use electromagnetic waves for communications:Guglielmo Marconi (1874-1937), 1909 Nobel Prize

(first transatlantic commercial wirelessservice, Nova Scotia, 1909)

Electromagnetic wavesElectromagnetic wavesFirst person to prove that electromagnetic waves existed:

Heinrich Hertz (1875-1894)

A solution to Maxwell’s equations in free space:

Visible light, infrared, ultraviolet,radio waves, X rays, Gammarays are all electromagnetic waves.

Electromagnetic WavesElectromagnetic Waves

33.3: The Traveling Wave, Qualitatively:

Figure 33-4 shows how the electric field and the magnetic field change with time as one wavelength of the wave sweeps past the distant point P in the last figure; in each part of Fig. 33-4, the wave is traveling directly out of the page.At a distant point, such as P, the curvature of the waves is small enough to neglect it. At such points, the wave is said to be a plane wave.

Here are some key features regardless of how the waves are generated:

1. The electric and magnetic fields and are always perpendicular to the direction in which the wave is traveling. The wave is a transverse wave.2. The electric field is always perpendicular to the magnetic field.3. The cross product always gives the direction in which the wave travels.4. The fields always vary sinusoidally. The fields vary with the same frequency and are in phase with each other.

Radio waves are reflected by the layer of the Earth’s atmosphere called the ionosphere.

This allows for transmission between two points which are far from each other on the globe, despite the curvature of the earth.

Marconi’s experiment discovered the ionosphere! Experts thought he was crazy and this would never work.

Electromagnetic Waves: Electromagnetic Waves: One Velocity, Many Wavelengths!One Velocity, Many Wavelengths!

with frequencies measured in “Hertz” (cycles per second)and wavelength in meters.

http://imagers.gsfc.nasa.gov/ems/http://www.astro.uiuc.edu/~kaler/sow/spectra.html

33.2: Maxwell’s Rainbow: Visible Spectrum:

Fig. 33-1

The wavelength/frequency range in which electromagnetic (EM) waves (light) are visible is only a tiny fraction of the entire electromagnetic spectrum.

Maxwell’s Rainbow

Fig. 33-2

(33-2)

An LC oscillator causes currents to flow sinusoidally, which in turn produces oscillating electric and magnetic fields, which then propagate through space as EM waves.

Fig. 33-3

Oscillation Frequency:

Next slide

The Traveling Electromagnetic (EM) Wave, Qualitatively

(33-3)

33.3: The Traveling Wave, Quantitatively:

The dashed rectangle of dimensions dx and h in Fig. 33-6 is fixed at point P on the x axis and in the xy plane.

As the electromagnetic wave moves rightward past the rectangle, the magnetic flux B through the rectangle changes and—according to Faraday’s law of induction—induced electric fields appear throughout the region of the rectangle. We take E and E + dE to be the induced fields along the two long sides of the rectangle. These induced electric fields are, in fact, the electrical component of the electromagnetic wave.

33.4: The Traveling Wave, Quantitatively:

Fig. 33-7 The sinusoidal variation of the electric field through this rectangle, located (but not shown) at point P in Fig. 33-5b, E induces magnetic fields along the rectangle.The instant shown is that of Fig. 33-6: is decreasing in magnitude, and the magnitude of the induced magnetic field is greater on the right side of the rectangle than on the left.

33.3: The Traveling Wave, Qualitatively:

We can write the electric and magnetic fields as sinusoidal functions of position x (along the path of the wave) and time t :

Here Em and Bm are the amplitudes of the fields and, and k are the angular frequency and angular wave number of the wave, respectively.

The speed of the wave (in vacuum) is given by c.

Its value is about 3.0 x108 m/s.

Fig. 33-5

Mathematical Description of Traveling EM Waves

Electric Field:

Magnetic Field:

Wave Speed:

Wavenumber:

Angular frequency:

Vacuum Permittivity:

Vacuum Permeability:

All EM waves travel a c in vacuum

Amplitude Ratio: Magnitude Ratio:

EM Wave Simulation

(33-5)

Example 1 : Tuning a Radio ReceiverExample 1 : Tuning a Radio Receiver

The inductor and capacitor in my car radio have one program at L = 1 mH & C = 3.18 pF. Which is the FM station?

(b) WRKF 89.3

What is wavelength of radio wave?

How about for WJBO 1150 AM?

FM radio stations: frequency is in MHz.AM in KHz