Resonance application applet

1. A Resonance Applet
http://www.walter-
fendt.de/ph11e/resonance.htm

2. A quick review
Simple oscillations
period, frequency, amplitude
Many simple oscillations -> waves
Wavelength,
Speed = wavelength X frequency
Electromagnetic waves are produced by
oscillations of charges!

3. Electric Charges and Fields
There are two kinds of electric charges:
Positive and negative. The same
charges repel and the different ones
attract.
There are electric fields (lines) around
charges. Charges interact with the fields
+ --

4. When the charges oscillate, so do the
the electric field lines which send out
ripples. The ripples can be created in
the directions orthogonal to the
direction of oscillation (transverse
wave)
When a positive charge oscillates
against a negative one, the ripples are
loops of electric fields which propagate
away from the charges.
Electromagnetic waves

5. Oscillating charges also generate a
oscillating current, which in turn
produces oscillating magnetic field
ripples propagating away.
The propagating electric and magnetic
ripples interact with and support each
other. They can go forever until
absorbed by some charges along the
way.
They are electromagnetic waves!

6. Summary of Concepts
Electric and magnetic fields of Faraday can
go off the wiggling charges, and propagate
in space by themselves!
(Maxwell, 1861-1862)
Hertz confirmed this by experiments in 1888.
Electromagnetic wave is a transverse wave.
The E&M wave waves without a medium!

7. E&M Radiation:
Frequency and wavelength
C=ln (Speed is a constant in vacuum)
Radio wave:
l = a few meters, n ~ 1 –10 MHz
1 M (mega) = 106 (oscillation freq. in a
circuit)
Microwave:
l = centimeters, n ~ 1 –10 GHz
1 G (giga) = 109

8. Light
l = 300-1500 n(ano)m,
n ~ 1014–1015 Hz
(Resonance frequency of the receptors in eyes)
1 nanometer = 10-9 meter
1 mm = 10-6 meter
X-ray
l ~ Ångstrom=0.1nm, n ~ 1018–1019 Hz
1 Å = 10-10 meter (size of an atom)

11. Light in a Medium
When light enters a medium, it wiggles
the charges in it, which in turn radiates
more light. All add up to a single light
wave traveling in the media
(transparent). The oscillation frequency
does not change.
The speed of light is slower in a medium
v=c/n (n: index of refraction, 1.33 inWater)
thus the wavelength becomes shorter!

12. Making E&M Radiation
Find devices that wiggle charges!
High frequencies: atoms and nuclei
Intermediate frequencies:
heating (motion of atoms),
Temperature ~ frequency
Your face: 10,000 nm
Heating and cooking: 1000 nm
Sun: 500 nm
Low frequencies: electric circuits