Chapter 34 Electromagnetic Waves

Currents produce B Change in E produces B Currents produce B Change in E produces B Change in B produces an E charges produce E Magnetic “charges” do not exist (monopole) Magnetic “charges” do not exist (monopole) LAWS of Electromagnetism

Currents produce B Change in E produces B Currents produce B Change in E produces B Change in B produces an E charges produce E Magnetic “charges” do not exist (monopole) Magnetic “charges” do not exist (monopole) LAWS of Electromagnetism  Maxwell’s equations All electromagnetism in just 4 equations

LAWS of Electromagnetism  Maxwell’s equations All electromagnetism in just 4 equations curl: rotation of a vector field divergence: source or sink of a vector field

LAWS of Electromagnetism  Maxwell’s equations All electromagnetism in just 4 equations curl: rotation of a vector field divergence: source or sink of a vector field  large divergence  zero divergence large curl, zero divergence

LAWS of Electromagnetism In empty space (zero charges and currents)

LAWS of Electromagnetism

In empty space (zero charges and currents) LAWS of Electromagnetism =0

For example: =0 equation of a wave that propagates in the x direction with speed

The properties and speed of electromagnetic waves are predicted by Maxwell’s equations c is the speed of light

The Nature of Electromagnetic Waves A straight wire connected to the terminals of an AC generator can create an electromagnetic wave. Only the electric wave traveling to the right is shown here. A straight wire connected to the terminals of an AC generator can create an electromagnetic wave. Only the electric wave traveling to the right is shown here. The oscillating Magnetic Field generates an oscillating Electric field (Faraday’s Law) ….. The wire has oscillating charge, that produces an oscillating E. The oscillating Electric Field generates a oscillating Magnetic Field (Ampere’s- Maxwell’s Law)

The Nature of Electromagnetic Waves This picture shows the wave of the radiation field far from the antenna. The speed of an electromagnetic wave in a vacuum is: Once away from the antenna the Fields generate in vacuum forever.

The Nature of Electromagnetic Waves Properties of the wave E=cB E and B are perpendicular to each other E and B are perpendicular to the direction of propagation The wave is TRANSVERSE (in vacuum)

The Electromagnetic Spectrum Like all waves, electromagnetic waves have a wavelength and frequency, related by:

The Electromagnetic Spectrum EM waves that we can see are called “light”, or visible light The ONLY difference between visible light and the other EM waves is the frequency and wavelength

The Electromagnetic Spectrum ORIGIN OF LIGHT antennas with oscillating charge antennas with oscillating charge excitation and decay of electrons in atoms vibration of atoms or molecules fast stopping of electrons nuclear reactions Stars emit light in all the spectra

The Electromagnetic Spectrum Example 1 The Wavelength of Visible Light Find the range in wavelengths for visible light in the frequency range between 4.0x10 14 Hz and 7.9x10 14 Hz.

The Speed of Light Conceptual Example 3 Looking Back in Time A supernova is a violent explosion that occurs at the death of certain stars. The figure shows a photograph of the sky before and after a supernova. Why do astronomers say that viewing an event like this is like looking back in time?

The Energy Carried by Electromagnetic Waves The total energy density carried by an electromagnetic wave energy stored in an electric field (per volume) energy stored in a magnetic field (per volume) energy stored in a magnetic field (per volume)

The Energy Carried by Electromagnetic Waves The energy travels with the wave. INTENSITY

The Energy Carried by Electromagnetic Waves Electromagnetic waves, like water waves, carry energy. Microwaves penetrate food and deliver their energy to it. The electric field delivers the energy to the water molecules in the food. The oscillating electric field makes the water molecules oscillate with the frequency of the wave (2.4 × 10 9 Hz) Transfer of energy is very efficient, only for water (resonance) In the process, bonds break between neighboring water molecules, energy is released as internal energy. As the internal energy increases, the temperature of the water increases, and the food cooks.

Example An electromagnetic wave that delivers a cellular phone call to a car has an average intensity of 7.4 × W/m 2. The wave passes perpendicularly through an open window, the area of which is 0.31 m 2. How much energy does this wave carry through the window during a 24-s phone call?

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:

Unlike other waves (sound, guitar string…), EM waves require no medium through/along which to travel. EM waves can travel through empty space (vacuum)! Speed of light is independent of speed of observer! You could be heading toward a light beam at the speed of light, but you would still measure c as the speed of the beam! A Most Curious Wave