Quantum Flashcards
Charge on electron
1.602 x10^-19
Planck’s constant
E=hf
h=6.63 x10^-34
explain threshold frequency
The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal
The photoelectric effect
The photoelectric effect is the phenomena in which electrons are emitted from the surface of a metal upon the absorption of electromagnetic radiation
Electrons removed from a metal in this manner are known as photoelectrons
The photoelectric effect provides important evidence that light is quantised, or carried in discrete packets
This is shown by the fact each electron can absorb only a single photon
This means only the frequencies of light above a threshold frequency will emit a photoelectron
Work function Φ
The minimum energy required to release a photoelectron from the surface of a metal
The Photon Model
Light waves can behave like particles(i.e. photons) and waves.
Light interacts with matter, such as electrons, as a particle (photoelectric effect).
Light propagates through space as a wave (diffraction and interference of light in Yung’s double slit experiment).
Light as a particle
- electromagnetic waves carry energy in discrete packets called photons
- the energy of the photons are quantised as E=hf
- In the photoelectric effect, each electron can only absorb a single photon.
Why does the wave model fail to explain the photoelectric effect?
The wave theory suggests any frequency of light can cause photoelectric emission if exposure is long enough - electrons are released immediately if above threshold frequency.
The wave model implies energy absorbed by each electron will increase over time with each wave - energy is absorbed instantaneously; electrons are either emitted or not emitted.
The kinetic energy of the emitted electrons should increase with radiation intensity - the intensity of the light only affects the rate of the photoelectric effect.
Define the Photon
A massless [“packet” or a “quantum”] of electromagnetic energy
discuss Electonvolt (eV)
used to express very small energies
used for quantum energies, many orders of magnitude below what joules would describe
Is the energy gained by an electron travelling through a potential difference of 1V
Therefore The energy will be the charge of the electron*1 = 1.602 x10^-19 J
Electron diffraction tubes
used to investigate properties of electrons
The electrons are accelerated in an electron gun to a high potential, such as 5kV, and are then directed through a thin film of graphite.
The electrons diffract from the gaps between carbon atoms and produce a circular pattern on a fluorescent screen made from phosphor
Increasing the voltage increases the energy and hence speed of the electrons(proportional relationship)
Electron diffraction (in electron diffraction tubes)
de Broglie discovered that matter, such as electrons, can behave as a wave
In order to observe the diffraction of electrons, they must be focused through a gap similar to their size, such as an atomic lattice
Graphite film is ideal for this purpose because of its crystalline structure
The gaps between neighbouring planes of the atoms in the crystals act as slits, allowing the electron waves to spread out and create a diffraction pattern
The diffraction pattern is observed on the screen as a series of concentric rings
This phenomenon is similar to the diffraction pattern produced when light passes through a diffraction grating
If the electrons acted as particles, a pattern would not be observed, instead, the particles would be distributed uniformly across the screen
It is observed that a larger accelerating voltage reduces the diameter of a given ring, while a lower accelerating voltage increases the diameter of the rings
de Broglie equation
relates particle’s momentum to its wavelength
λ=h/p
The de Broglie question, therefore, links a particle-like property (momentum) to a wave-like property (wavelength) demonstrating wave-particle duality for all particles
Since momentum p = mv, the de Broglie wavelength can be related to the speed of a moving particle (v) by the equation
