As Physics Unit 2

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AS Physics Unit 2
Waves and Quantum Behaviour
Superposition and Coherence
Waves are not physical objects therefore they do not bounce off each other when they
Instead, they combine temporarily and then continue to travel as before. This combination is
called superposition.
The interference caused by superposition can be constructive, where two peaks meet and
make a bigger peak, or destructive, where a peak meets a trough and they cancel out to
some extent.
The phase of a wave, how far it is displaced from 0 at that point, can be represented by a
The resultant amplitude when two waves meet can be found by joining their phasors tip to
If the waves are at the same point in their cycle, they are said to be in phase, with a phase
difference of a multiple of 2 radians.
Waves in phase at a point in space interfere constructively. Their phasors point in the same
direction, giving the maximum possible resultant.
Waves at points in their cycle as far apart as possible are said to be in antiphase, with a
phase difference of an odd number times radians.
Waves in antiphase at a point in space interfere destructively. Their phasors point in
opposite directions, giving a minimum resultant.
Waves at different points interfere with each other to a lesser degree, whether this is
destructive or constructive can be worked out by drawing phasors.
Interference patterns can be established when waves line up so that the phase difference is
always the same this requires waves to be coherent. Sources are coherent when they have
the same wavelength and frequency and a fixed phase difference.
The phase difference of two waves depends on the difference in the lengths of the paths
between the sources and the detector, known as the path difference. If the path difference is
a whole number of wavelengths the phase difference is 0, if the path difference is a half
number of wavelengths the phase difference is /2.

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Standing Waves
Standing waves are formed when waves are confined, such as on a length of string, and a
whole number of waves is produced in the time it takes one wave to go to the end and
The effect of the moving pulses is to create a series of apparently stationary pulses that move
up and down instead of forwards and backwards.
The frequencies at which this occurs are called resonant frequencies, the relationships
between them is the basis of musical scales.…read more

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Two Source Interference
If waves from two sources are coherent then an interference pattern can be formed.
With Young's slits this interference pattern is a series of alternating bright and dark fringes
on a screen.
Points where the path difference from the two slits is a whole number of wavelengths are
maxima, points where it is some and a half wavelengths are minima.…read more

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When light is shone on a metal it may or not emit electrons. Experiment showed that:
For a given metal, no photoelectrons are emitted when exposed to radiation below
its own threshold frequency.
The kinetic energy of the electrons ranged from 0 to a maximum, this maximum
increased with the frequency of radiation but was not affected by intensity.
The number of photoelectrons emitted per second was proportional to the intensity
of the radiation.…read more

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Hot things emit continuous spectra of visible and infrared radiation. If this is shone through a
cool gas the electrons absorb photons of the right energy to move up a level and so when
the light is split dark lines appear matching the bright lines of the emission spectrum.
Each element has a particular set of lines which can be used to identify it, this method was
used to discover helium in the Sun before it was found on Earth.…read more

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If the speed of the electrons is increased the rings of the diffraction patter become smaller,
which is predicted by the smaller wavelength given by the de Broglie equation.
Electrons also behave like waves in other situations, such as in Young's slits.
It has since been proven that all particles, including molecules and even large objects, act
like waves.…read more

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If objects are projected horizontally you may have to work out how long they take to fall
with suvat and then how far they go in that time.
If objects are projected at an angle resolve the original vector into horizontal and vertical
vectors, use the vertical as the initial velocity upwards and then suvat with a=g, then work
out the horizontal distance in that time.
If an object is under thrust at an angle it sucks to be you.…read more

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Work=force x displacement. The work is only the energy transferred, other types of energy
may be present in the system.
If the force is at an angle to the direction of motion only the component in the direction of
travel contributes to the work done.
Power = work done / time = force x velocity.
Conservation of energy means that a system can never lose energy it is always transferred
to another part of the system in a different form.…read more


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