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Physics Term 2 Week 3

From radio to photocells

Beverley Sampford

on 31 May 2010

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Transcript of Physics Term 2 Week 3

In 1873, Maxwell developed mathematical relationships connecting light, electricity and magnetism and predicted the existence of a complete spectrum of electromagnetic radiation - of which only light and infra-red were known at the time.
Hertz, a German physicist, was the first to discover one ot these radiations. In 1887 he set out to see if there was emr associated with electrical discharges. Hertz observed that discharges in one coil produced discharges in a second coil about 1.5m away. He propsed that an invisible radiation was produced by discharges in the first coil adn travelled through the air to the second coil, producing discharges as a result. Hertz calculted the wavelength of his radiation by studying the interference patterns produced when one ray, reflected off a metal plate to a detector was superimposed on a ray that travelled directly. Knowing the frequency of the oscillator producing the sparks, Hertz then used the wave equation to calculate the speed of the rays, proving them to be part of Maxwell's electromagnetic spectrum Hertz also observed that if he shone UV light on the second coil the spark jumped the gap more easily. He realised from this that light and electricity must be connected some way and called this the photoelectric effect. Unfortunately he did not get a chance to follow up on this as he died a short time later at the age of 36. Hertz found that
electromagnetic waves could be generated, transmitted and received
they travelled in straight lines
they could be reflected
they travelled through non-conducting materials but were absorbed by conducting materials
they travelled at the speed of light
they were polarised (gaps had to be parallel) so electromagnetic transverse waves.

Oscillating charges in antennae radiate em waves.
Using AC electrons are accelerated back and forth.
Moving charges produce emr with the same frequency as the AC.
The current travels the entire length of the aerial so the greater the length the longer the time for each cycle --> the lower the frequency.
Long antennae generate radio waves. Short antennae produce short wavelength radio waves (microwaves) Around 1860 many scientists started observing the light given out by hot, incandescent metals such as tungsten.
They observed that all hot materials emitted radiations of varying wavelengths which depended not on the substance but on the temperature of the substance. 2 problems
Why was the curve the same general shape for all substances?
What was the range of emitted radiation both limited and peaking at similar wavelengths? To explain this they generalised their ideas to a black body radiator or cavity radiator.
A perfect black body will absorb all radiation falling on it, increasing its temperature.
It will radiate this energy out again as its temperature falls to room temperature.
The peak radiation it emits reflects teh temperature it reaches as it absorbs energy. Max Planck came up with a model to explain black body radiation in 1900.
He had atoms inside the cavity oscillating back and forth and therefore emitting radiation in a similar way to radio antennae.
Like antennas, the oscillating atoms could also receive emr (in this case heat) from their surroundings. Electromagnetic energy associated with atom oscillation was quantised and could only have energy values consistent with E = hf where h = Planck's constant and f = the frequency of the radiation.
An atom could only absorb or release whole numbers of quanta of energy.
Quanta of energy were absorbed or emitted only when an atom changed from one quantised energy level to a different quantised level. Planck's ideas were not readily accepted - they were too different from the 'classical physics' of the time.
Einstein provided a practical application using quantum theory to explain the photoelectric effect.
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