TOPIC 19: QUANTUM PHYSICS (COMPLETE) Flashcards
[Definition + Formula] Photon
A photon is discrete packet of energy of electromagnetic radiation
E = hf = hc/λ
E: Energy of 1 photon
f: frequency of the EM radiation
[Definition] Photoelectric Effect
Photoelectric effect is the emission of electrons when electromagnetic radiation of high-enough frequency is incident on a metal surface
[Formula] Photoelectric Equation
hf = Φ + KEmax
[Definition] Work function energy
It is the minimum energy needed to cause an emission of electron from the surface of a metal
[Definition] Threshold frequency
It is the minimum frequency of electromagnetic radiation to cause emission of electron from the surface of a metal
[Definition] Threshold wavelength
It is the maximum wavelength of electromagnetic radiation to cause emission of electron from the surface of a metal
[Explanation] Why for any wavelength of electromagnetic radiation, most of the electrons are emitted with KE less than the max KE?
The photoelectrons only gain max KE when they are at the surface of the metal, losing the minimum amount of energy before emission.
For electrons that are below the surface of the metal, additional energy is required to bring the electron up to the surface before emission.
[Definition] Stopping potential, Vs
Stopping potential is the minimum potential difference between the emitting metal and the collector that prevents the most energetic photoelectrons from reaching the collector plate, resulting in zero photoelectric current
[Formula] Stopping potential, Vs
KEmax = eVs
[Graph] Photoelectric current, I - V
Positive V: horizontal line (saturation current)
Negative V: Curve (from increasing gradient to decreasing gradient)
Key concept of how the photoelectric current - V graph would change when the variables are changed
Intensity = Power / Area
(aka Energy per unit Time per unit Area)
Energy = no. of photons x Energy of 1 photon
Frequency –> Energy of each photon
Intensity –> Number of photons
Power –> Both Frequency & Intensity
So if constant power, either lots of electrons, but each of low energy OR less electrons, but each with high energy
[Explanation] Increasing intensity on photoelectric current - V graph
If frequency remain constant, energy per photon constant, so max KE of photoelectrons constant, stopping potential same
Since energy per photon constant, with increasing intensity, more photons reaching emitter. So more photoelectrons emitted per second, saturation current increases
[Explanation] Increasing frequency on photoelectric current - V graph, with same number of photons.
Increasing f, energy per photon increases, max KE increases, stopping potential increases
No. of photons same, so no. of photoelectrons emitted same, photoelectric current same
[Explanation] Increasing frequency on photoelectric current - V graph, with same power output
Increasing f, energy per photon increases, max KE increases, stopping potential increases
Since P same, with increasing f, the number of photons decrease, so no. of photoelectrons decrease, saturation current decreases
[Definition] Absorption line spectra
Continuous spectrum crossed by dark lines
[Definition] Emission line spectra
Discrete bright lines of different colours on a dark background
[Understanding] How does the absorption and emission lines differ on the energy level diagram?
Absorption lines will be upwards, because when the electrons absorb the photons, it causes the photons of that energy level to be ‘gone’. so it will be continuous spectra crossed by dark lines
Emission spectra will be downwards, because when the electrons de-excite back to ground state, it emits photons. so it will be different coloured lines on a dark background
[Understanding] What causes the emission and absorption line spectra to occur in v simple terms?
Emission: electrons de-excite. simple
Absorption: by absorbing photon, OR through high speed collisions with other particles
[Explanation] How is absorption spectrum formed?
- Atoms in the cool hydrogen absorbs photons of specific energy levels that are equal to the difference between the discrete energy levels
- The specific energy of the absorbed photons shows up as dark lines of specific frequencies against the white / continuous spectrum
- As the electrons de-excite, the absorbed photons are re-emitted in all directions. So only a small negligible fraction is observed on the spectrum, so its still dark lines
[Explanation] How is emission spectrum formed?
- When the cool hydrogen atoms absorb photons, they are excited and unstable
- Thus, they will shortly de-excite and re-emit photons in all directions.
- These photons have specific energy values that are equals to the difference in discrete energy levels, hence showing up as coloured lines of specific frequencies on a dark background
[Understanding] What can the x-ray production process be compared to?
Opposite of photoelectric effect!
Photoelectric effect –> Photons of high frequency results in emission of electrons
X-ray production –> Electrons strike metal, resulting in emission of photons when electrons lose their energy
[Explanation] How is the continuous spectrum of the X-ray spectrum formed?
When electrons with very high kinetic energies bombard with the nuclei in the metal, they lose energy and emit photons in the X-ray energy range aka X-ray photons
[Explanation] Why is there a continuous distribution of wavelengths for the x-ray continuous spectrum?
X-ray photons are produced when the bombarding electron loses energy when it interacts with a nucleus
Larger acceleration of electron results in larger photon energy
Since the electrons hitting the metal target have a distribution of accelerations, there will be a distribution of wavelengths.
[Formula] Main formula for the formation of x-ray photons
Electrons accelerated through potential difference to gain KE. KE is then lost to produce photons
qV = 1/2mv^2 = E(photon) = hf
[Explanation] How are characteristic peaks of the X-ray spectrum formed?
- Inner shell electron is being knocked out by the bombarding electrons, leaving a vacancy
- Electrons from the nearby shells with higher energy de-excites and fills up the vacancy, producing an x-ray photon
[Understanding] What is the similarity / difference between emission line spectrum and x-ray characteristic peaks?
Both processes involves the electrons transiting from a higher energy level to a lower energy level
However, the electrons involved in the emission line spectrum are VALENCE ELECTRONS, while electrons involved in x-ray production are those CLOSEST TO NUCLEUS
because the difference in discrete energy levels closer to the nucleus are larger, so more energy released, resulting in high-energy x-ray photons produced
[Explanation] What is the difference between Kα and Kβ of the characteristic peaks in the X-ray spectrum?
Inner most shell: K, L, M
Kβ: M –> K ; Kα: L –> K
Energy difference between M and K LARGER than energy difference between L and K, so X-ray photons corresponding to Kβ is of higher energy (shorter wavelength) than Kα
L shell closer to K, so higher probability of electron de-exciting to fill up vacancy in K shell, so Kα peak is higher
[Formula] de Broglie wavelength
λ = h/p
[Formula] Heisenberg Uncertainty Principle
∆p∆x ≥ h
[Understanding] What is the key thing to take note of regarding the heisenberg Uncertainty Principle
p and x, meaning momentum and position MUST both be along the same direction
