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W13D2: Maxwell ’ s Equations and Electromagnetic Waves

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### W13D2:Maxwell’s Equations and Electromagnetic Waves

### Announcements

### Do Problem 1In this Java Applet http://web.mit.edu/8.02t/www/applets/superposition.htm

### Traveling Plane Sinusoidal Electromagnetic Waves

### Group Problem: 1 Dim’l Sinusoidal EM Waves

### Group Problem: Poynting Vector

### Standing Waves Do Problem 2 In the Java Applet http://web.mit.edu/8.02t/www/applets/superposition.htm

Today’s Reading Course Notes: Sections 13.5-13.7

PS 10 due Week 14 Tuesday May 7 at 9 pm in boxes outside 32-082 or 26-152

Next Reading Assignment W13D3 Course Notes: Sections 13.9, 13.11, 13.12

Outline

Maxwell’s Equations and the Wave Equation

Understanding Traveling Waves

Electromagnetic Waves

Plane Waves

Energy Flow and the Poynting Vector

Wave Equations: Summary

Electric & magnetic fields travel like waves satisfying:

with speed

But there are strict relations between them:

Example: Traveling Wave

Consider

The variables x and t appear together as x - vt

At t = 0:

At vt = 2 m:

At vt = 4 m:

is traveling in the positive x-direction

Direction of Traveling Waves

Consider

The variables x and t appear together as x + vt

At t = 0:

At vt = 2 m:

At vt = 4 m:

is traveling in the negative x-direction

General Sol. to One-Dim’l Wave Eq.

Consider any function of a single variable, for example

Change variables. Let then

and

Now take partial derivatives using the chain rule

Similarly

Therefore

y(x,t)satisfies the wave equation!

Generalization

Take any function of a single variable , where Then or (or a linear combination) is a solution of the one-dimensional wave equation

corresponds to a wave traveling in the positive x-direction with speed v and

corresponds to a wave traveling in the negative x-direction with speed v

Traveling Sinusoidal Wave: Summary

Two periodicities:

Traveling Sinusoidal Wave

Alternative form:

Plane Electromagnetic Waves

http://youtu.be/3IvZF_LXzcc

Electromagnetic Waves: Plane Sinusoidal Waves

Watch 2 Ways:

1) Sine wave traveling to right (+x)

2) Collection of out of phase oscillators (watch one position)

Don’t confuse vectors with heights – they are magnitudes of electric field (gold) and magnetic field (blue)

http://youtu.be/3IvZF_LXzcc

are special solutions to the 1-dim wave equations

where

Show that in order for the fields

to satisfy either condition below

then

Group Problem: Plane Waves

1) Plot E, B at each of the ten points pictured for t = 0

2) Why is this a “plane wave?”

Electromagnetic Radiation: Plane Waves

Magnetic field vector uniform on infinite plane.

http://youtu.be/3IvZF_LXzcc

Direction of Propagation

Special case generalizes

Concept Question: Direction of Propagation

The figure shows the E (yellow) and B (blue) fields of a plane wave. This wave is propagating in the

- +x direction
- –x direction
- +z direction
- –z direction

Concept Question Answer: Propagation

Answer: 4. The wave is moving in the –z direction

The propagation direction is given by the

(Yellow x Blue)

Properties of 1 Dim’l EM Waves

1. Travel (through vacuum) with speed of light

2. At every point in the wave and any instant of time, electric and magnetic fields are in phase with one another, amplitudes obey

3. Electric and magnetic fields are perpendicular to one another, and to the direction of propagation (they are transverse):

Concept Question: Traveling Wave

The B field of a plane EM wave is

The electric field of this wave is given by

Concept Q. Ans.: Traveling Wave

Answer: 4.

From the argument of the , we know the wave propagates in the positive y-direction.

Concept Question EM Wave

The electric field of a plane wave is:

The magnetic field of this wave is given by:

Concept Q. Ans.: EM Wave

Answer: 1.

From the argument of the , we know the wave propagates in the negative z-direction.

Poynting Vector and Intensity

Direction of energy flow = direction of wave propagation

units: Joules per square meter per sec

Intensity I:

An electric field of a plane wave is given by the expression

Find the Poynting vector associated with this plane wave.

Standing Waves

What happens if two waves headed in opposite directions are allowed to interfere?

Momentum & Radiation Pressure

EM waves transport energy:

They also transport momentum:

And exert a pressure:

This is only for hitting an absorbing surface. For hitting a perfectly reflecting surface the values are doubled, as follows:

Problem: Catchin’ Rays

As you lie on a beach in the bright midday sun, approximately what force does the light exert on you?

The sun:

Total power output ~ 4 x 1026 Watts Distance from Earth 1 AU ~ 150 x 106 km

Speed of light c = 3 x 108 m/s

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