Transcript IB Phys..

IB Physics 12
Mr. Jean
February 25th, 2014
The plan:
• Video clip of the day
• Wave Duality
Wave Duality:
• Small Wave Length:
– Behaves like a particle.
• Large Wave Length:
– Behaves like a wave.
Compton scattering:
• This was the second big experiment which
proved that photons existed.
• It also had some interesting results.
With collisions we must consider:
• Conservation of Energy:
• Conservation of momentum:
Compton scattering:
• Major discovery supporting Quantum Theory
Compton Scattering:
• Compton scattering depends on how big
the wave is as it interacts with the
material’s surface in comparison to the
Compton wave length.
Compton Scattering
• http://www.youtube.com/watch?v=fI2C4VlR1OM
• http://www.youtube.com/watch?v=fI2C4VlR1OM
&list=PLFIIOUbmbUQI6vJD0kvxqcNoRAGXZ0dx
– This is a list of different videos explaining (some with
examples) of the Compton scattering model.
• The Problem:
– Electrons moving around a nucleus in roughly
circular or elliptical paths would feel a
centripetal force and would therefore have
centripetal acceleration.
– But accelerating charges produce electric
magnetic radiation, therefore these electrons
should continuously emit energy, slow down,
and eventually collapse into the nucleus.
– Obviously, this does not happen.
Limitation of the nuclear model
of the atom:
• According to classical physics, an orbiting
electron is accelerating, and accelerating bodies
radiate energy. This would mean that electrons
would radiate energy as they orbit the nucleus.
This contradicts observations for two reasons:
– 1. Electrons would lose energy and spiral into the
nucleus. This would destroy all matter.
– 2. Electrons would radiate energy as light in a
continuous spectrum of colors. This contradicts
experimental observation since the emission spectra
of atoms are observed to consist of only well-defined
discrete wavelengths.
Bohr's Explanation:
• Laws of electromagnetism do not apply at
atomic level.
• Within an atom energy is only absorbed or
emitted when electrons change energy levels
and this energy is QUANTIZED.
• Electrons are either in lowest energy state
(ground state) or certain allowed levels (excited
state).
Conclusion:
• Observations of atomic emission and absorption
spectra indicate that:
– electrons do not radiate energy when in stable orbits.
Stable orbits only occur at certain radial distance from
the nucleus. Thus, electrons in these orbits have a
well-defined discrete amount of energy.
– electrons only radiate or absorb energy only when
they move (transition) between stable orbits. This
energy is quantized and fixed by the energy
differences between the allowed orbital levels.
Emission Spectrum
– 1. Low pressure gas is energized by applying a
potential difference across it causing it to heat up.
– 2. The hot gas emits light energy only at certain
well-defined frequencies, as seen through a
diffraction grating (spectroscope) or prism.
Absorption Spectra
– 1. Light is shone through a cool low pressure
gas.
– 2. A diffraction grating or prism is used to
determine what frequencies pass through the
gas and which are absorbed.
• To move from one level to another,
electrons must emit or absorb a photon
with a certain amount of energy (E = hf).
• This absorbed or emitted photon had an
energy value equal to the difference
between energy levels.
Energy Levels:
• Bohr found that for the hydrogen atom, the
energy associated with a particular level was
given by:
• En = Energy from change (J)
• n = Quantum level
• Where n is principle quantum number (energy
level) and energy is negative because energy is
being added to pull the electron away from the
nucleus.
Changing Energy Levels
to Electrons:
• This is a great website highlighting states
of energy in matter.
• http://www.kcvs.ca/martin/astro/au/unit2/6
1/chp6_1.html
For the remainder of class:
• Work on Quest work