Quantum Physics Flashcards
What is the basic principle of Quantum Physics
What phenomenon brought up the field of Modern Physics as it disputed Classical Physics theories?
The Photoelectric Effect
What phenomenon provides evidence that particles can have wave-like properties?
Electron Diffraction
Electrons travelling through polycrystalline graphite will be diffracted by the atoms and the spacing between the atoms.
What are the apparatus needed in an electron diffraction experiment?
A beam of electrons, a detection screen, and a graphite film
The layered-like structure of graphite allows it behave like a diffraction grating
What can electron diffraction be used for?
To determine the arrangement of atoms in matter and the size of nuclei.
How does
the Photoelectric Effect dispute Classical wave theory?
It proves that the emissionof electrons is dependent on the frequencyof the wave and the no. of e- is dependent on the energy of the wave
Classical wave theory would have suggested that the reverse is the case
What then is proportional to
the wave intensity in the Photoelectric Effect?
The Photoelectric current
This is the number of photoelectrons emitted per second
What is
the Photoelectric Effect?
The emission of e- from the surface of a metal when radiation is incident on it
What is the prerequisite
for incident radiation to liberate photoelectrons from metals?
The frequency of the incident radiation must be ≥ the threshold frequency of the metal
What is
Threshold frequency?
Theminimum frequency of the incident radiation on a metal’s surface needed to liberate photoelectrons from it
What is
Threshold wavelength?
The maximum wavelength of the incident radiation on a metal’s surface needed to liberate photoelectrons from it
Do all the emitted photoelectrons have the same KE when liberated?
No
Which photoelectrons typically have the highest (maximum) KE?
The valence electrons of surface atoms
What happens to the leaf of a gold-leaf electroscope when -vely charged metal plate is irradiated by light of a sufficient frequency to release photoelectrons?
The leaf falls
When photoelectrons are emitted, the -ve plate loses its -ve charge
Will any change to the leaf of a gold-leaf electroscope be observed when the metal plate is given a +ve charge?
No
As photoelectrons are emitted, the +ve charge of the plate creates an attractive force between the electron and the metal plate, preventing them from being liberated
What is Einstein’s photoelectric equation?
(Total) Σ = hf0 + KEmax.
The total energy of the incident radiation is equal to the energy needed to liberate the electron from the metal and the maximum KE it can get on liberation
What is a photon
A packet (or quantum) of energy possessed by an EM wave
What is the implication of the Photon model of a wave
It implies that waves can have particle-like properties
What are the particle-like properties of waves?
- Elements of it (photons) collide elastically with e- to liberate photoelectrons
- Elements of it (photons) possess momentum
What is the relationship between a photon and an electron
in the Photoelectric Effect under the Photon model of a wave?
It is a one-to-one relationship
One electron absorbs the energy of one photon and it is liberated if this Σ value is greater than hf0
What happens to the photon when it collides with an electron?
The electron absorbs its energy and it ceases to exist
What happens when the Σ supplied by the photon is less than hf0?
The electron absorbs its energy and it as KE
which it can lose upon collision with metal ions
What is the work function (Φ) of a metal?
This is the energy required to liberate e- from the surface of a metal
Each metal has their own w0 (another way of representing work function) value
Why is the value of the KE of photoelectrons up to a maximum?
For e- that are not as loosely bounded, work is done against the attractive forces of the nucleus to bring the electron to the surface, hence its emission energy will be reduced
What unit is used to better express photon energies?
The electron-volt
What is electron-volt?
The work done in accelerating an electron through a PD of 1 volt
What is the value of one electron-volt in Joules?
1.6 x 10-19 J
How does KEmax. relate to stopping potential?
The value of Voltage that gives the maximum KE is the PD required to stop emitted photoelectrons from leaving the surface of the metal
What are
Energy Levels?
Specified regions of space around an atom where its e-can be found
they are quantised energy states within the atom
What is
the ground state of an element?
The state at which the atom possesses the least amount of energy with all the e- in their lowest energy state
What are the 2 main characteristics of
Energy Levels?
They are fixed and are peculiar to each element
What is required for
an e- to transit between 2 energy levels?
It has to absorb the exact amount of energy needed for a specific transition
E.g. if an electron wanted to move btw Σ levels 2 and 3, it has to absorb Σ equivalent to the energy difference between the 2 levels
What happens if
an e- absorbs less or more Σ than what is required?
No transition will occur
Why do
have energy levels -ve values?
Because external energy is supplied to the atom in order to make the transition
What is the value in eV
of the ionisation level?
0 eV
At ionisation the e- has escaped from the attractive pull of the atom’s nucleus
What are all other energy levels apart from the ground state called?
Excited states
What happens when an electron ‘falls’ from a higher energy level to a lower one?
A photon is emitted ; its frequency is equal to the frequency of the energy difference between the 2 levels
What equation helps find the energy difference btw 2 energy levels?
ΔΣ = hfhigher - hflower
How many types of emission spectra are there?
Two : Line emission spectrum and Continuous emission spectrum
What is
the Line emission spectrum?
It is a spectrum that consists of bold, distinct coloured lines, that show the specific wavelengths absorbed (and thus emitted) by electrons in gaseous atoms
What is the nature
of the gases that are used to produce the Line emission spectrum?
They must be hot
Hot gases are raised past their ground states. e- can absorb energy from the heat to make transitions btw Σ levels
What is
the Continuous emission spectrum?
It is a spectrum that consists of a wide, continuous range of wavelengths with no clear-cut distinction between the colours.
What type of light
produces the Continuous emission spectrum?
White Light
What is
an absorption spectrum?
It is a spectrum that consists of dark lines against the background of a continuous spectrum to show the absence of light of particular wavelengths.
How is
an absorption spectrum produced?
When white light passes through a substance (such as a cool gas) before being diffracted
What do the dark lines
in an absorption spectrum show?
The wavelengths absorbed by the electrons in the atoms of a substance when white light is incident on it
Why are cool gases at low pressure used when trying to produce an absorption spectrum?
When gases are cool, its atoms are more likely to be in their ground state. When gases are at low pressure, its atoms are duly separated from each other
This allows photons interact with electrons on an atom-by-atom basis
What does the line emission spectrum of an atom tell us about it?
Since there are sharp distinct, coloured lines, electrons in the atom can only exist in specific energy levels.
Why do the line emission spectra of different atoms differ?
Because they have different spacings between their energy levels
Is a photon a particle?
No, it is an element of an electromagnetic wave
Do photons have mass?
No
Who suggested that matter can have wave-like properties?
Louis de Broglie
Who suggested that waves can have matter-like properties?
Albert Einstein
What is the equation for the de Broglie wavelength?
λ = h/p
h = Planck’s constant
p = momentum of the photon
where λ = wavelength
Make sure “de Broglie wavelength” is specified before you use this formula - Mr Onanuga
