physics for scientists and engineers ii, summer semester 2009 1 lecture 20: july 10 th 2009 physics...

Physics for Scientists and Engineers II , Summer Semester 2009

1

Lecture 20: July 10th 2009

Physics for Scientists and Engineers II


2

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7

The 500000 Volt Tesla Coil – featuring Adam Beehler succesfully trying to not electrocute himself


8

Chapter 34: Electromagnetic Waves

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General Form of Ampere’s Law

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10

Calculating displacement current: Example

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11

Major Point

B

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Magnetic fields are produced both by conduction currents andby time-varying electric fields.


12

Example: Chapter 34, Problem 1

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13

A Complete Description of All Classical Electromagnetic Interactions in a Vacuum

law Maxwell-Ampere B

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law sGauss'AdE

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14

Maxwell’s Prediction

law Maxwell-Ampere B

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law sGauss'0AdE

0I and 0q when space freein Equations sMaxwell'

00 dt

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Can be combined Result: Wave equations for electric and magnetic fields

Solution of wave equations shows that electric and magnetic field waves travel(“electromagnetic radiation”) with the speed of light, leading Maxwell to predict that light is a form of electromagnetic radiation.


15

Hertz’s Experiment

Hertz showed with this apparatus that electro-magnetic waves exist.


16


Details of the apparatus:

How it works: Battery charges the capacitor in the primary circuit on the left. The interrupter periodically creates a short circuit across that capacitor and alternating charging and discharging of capacitor occurs. A large periodic change of flux occurs in the primary winding. The secondary winding picks up this large flux change and transforms the primary voltage fluctuation into a much higher voltage fluctuation in the secondary circuit. The secondary circuit is tuned (LC) creating a very large voltage across the spark balls. High electric fieldsoccur between the spark balls, causing a sudden charge transfer (spark). The electric fieldbreaks down but is then built up again. This fluctuating electric field induces a fluctuatingmagnetic field (Ampere-Maxwell law), which in turn creates a fluctuating electric field (Faraday’s law)….. electromagnetic radiation, which is then picked up by the detection ringcausing it to spark across it’s spark gaps (detection ring is also tuned to the same frequency).


17


Other experiments by Hertz:

em wave generator

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18

Plane versus Spherical Waves

Let’s first look at mechanical waves: Imagine looking down on a pond. Someone throws rocks into the middle ofThe pond, creating spherical waves in two dimensions.

Direction of wave propagation

If instead, the person throws very long metal bars into the pond,plane waves are created.

Metalbar

All parts of the wave front have the samephase.


19

Plane Electromagnetic Waves

Plane linearly polarized electromagnetic waves with propagation in x-direction.

y

zx

Properties:1)Wherever in the yz plane em wave comes from, it propagates in the x-direction. (all “rays” of this wave are parallel).2) All em waves coming from different parts of the yz plane are all in phase.3) Electric and magnetic waves oscillate perpendicular to each other (take this by faith for now).4) Electric field is in the same direction (here in y-direction) “linearly polarized”.5) Magnetic field is in the same direction (here in z-direction) “linearly polarized”.6) And B are only function of x and t and do not depend on y and z E(x,t), B(x,t).

E

B

cc

E

B

physics for scientists and engineers ii, summer semester 2009 1 lecture 20: july 10 th 2009 physics...

Documents

summer semester

vacuum slide

magnetic field waves

high electric fields

primary circuit

secondary circuit

electro magnetic waves

large voltage