Quantum Physics Flashcards
What is a photon?
A discreet packet of energy of an electromagnetic radiation with energy hf, where h = Planck’s constant and f is the frequency of EM radiation
What is photoelectric effect?
Release of electron from a surface due to incident photon with frequency at least equal or greater than threshold frequency
What is photocurrent?
rate of electron arriving at collector
What is work function, Φ?
minimum photon energy required to remove an electron from a surface
What is threshold frequency?
minimum frequency of photon for electron to be emitted from a surface
What is threshold wavelength, λₒ?
maximum wavelength of EM radiation incident on a surface for photoelectric effect to take place
just think opposite of fₒ
When is saturated current achieved?
When all emitted electrons are collected
electrons are scattered randomly in all directions
What happens to the electrons emitted at low pd and high pd?
- at low pd: some electrons may not be attracted by collector
- at high pd: path of electrons will be altered so that more electrons could be attracted
What happens to the electrons emitted when there’s negative pd?
- liberated electrons experiences opposing electric force
- energetic electrons could still reach the collector while slower ones may not
When does stopping potential, Vs occur?
when no electrons could reach the collector
Why is there a slope for current-pd graph?
photoelectrons liberated possess varying kinetic energy
Why do photoelectrons have a range of kinetic energy in photoelectric effect?
- energy of each photon is fixed
- KE is varying because varying energy is required by inner electrons to reach its surface
- max KE equals to photon energy minus work function energy, only surface electrons emerge with max KE
What is the effect on photocurrent when intensity of radiation changes?
- higher intensity of radiation means more photons incident on the emitter per unit area per unit time
- more photoelectrons will be emitted per unit area per unit time, which gives rise to larger photocurrent
What is the effect on saturation current when intensity of radiation changes?
- When the intensity of radiation increases, more photons is incident on the emitter per unit time
- more photoelectrons will be emitted per unit time, resulting in larger photocurrent
What is the effect of changing the intensity of radiation on KEmax and Vs?
- the energy of individual photon remains the same since frequency is unchanged
- no effect on KEmax and Vs as the same amount of work has to be done on photoelectrons to stop them from reaching the collector
What is the effect on Vs and KEmax when frequency of radiation changes?
- When frequency of radiation increases, the energy of individual photons increases
- emitted photoelectrons have greater KE, hence a larger pd is required to stop then from reacher the collector
How does photoelectric effect show the particulate nature of EM radiation?
- only radiation above certain minimum threshold frequency
- rate of emission of electrons is proportional to intensity
- max KE of electrons emitted is dependent only on frequency and independent of intensity
- no observable time lag
Why does the wave theory not explain photoelectric effect?
- By wave theory, any frequency would give rise to emission of electrons, if intensity is sufficiently high but in reality electrons will only be liberated if light above of a certain threshold frequency is used, regardless of intensity
- electrons will take time to absorb light energy before it is removed from a surface, in reality, instantaneous emissions were observed
- if intensity increases indefinitely, KE of emitted electrons increases indefinitely too since intensity is proportional to energy but in reality, there is a limiting maximum KE of electrons emitted
What happens to photoelectrons if intensity of EM radiation is constant while frequency is increased?
- each photon has larger energy
- since intensity is constant, there will be fewer photons incident on the plate per unit time (n)
- since current is proportional to rate of incident photons, rate of electrons emitted decreases too
use equation to see
What is de Broglie’s wavelength?
Wavelength associated with a particle that is moving
What happens when white light passes through a prism or diffraction grating?
A continuous spectrum is formed
What is a continuous spectrum?
- A distribution of emitted wavelengths of EM radiation of a body, with no breaks between its extreme values
- it is produced by hot bodies at high spectrum like white light
What is emission spectrum?
Separate coloured lines on dark background
What is absorption spectrum?
Separate dark lines on coloured background
What is the signification of equation: hf = Φ + 1/2mv²
- conservation of energy
- When an electron absrobs energy hf from an incident photon, part of it is used to overcome the work function energy Φ of the metal
- remaining energy becomes the maximum KE of liberated electrons
How can Planck’s constant h be determined using the photoelectric effect?
- reverse the polarity of the battery to obtain stopping potential Vs when a metal is irradiated with EM radiation of fixed frequency, f
- Vary f to obtain different values of Vs
- Plot Vs against f graph
- h equals to the product of gradient and elementary charge e
E = hfₒ + eVₛ
Vₛ = (h/e)f - (h/e)fₒ
What does the graph of stopping potential V against frequency not extend below the x-axis?
- the value of x-intercept is the threshold frequency of the metal
- no electrons will be liberated using any frequency lower than that of threshold frequency
What are the three observations for photoelectric effect that is valid for quantum theory but not for wave theory?
- KEmax depends on frequency, independent on intensity
- minimmum frequency of EM radiation is required to liberate electrons from a surface
- no observable lag time in photoelectron emissions
How does wave theory and quantum theory differ in explaining how KEmax depends on frequency, independent of intensity?
- Wave theory: higher intensity, high energy, supposedly KE increases, however KE remains constant
- Quantum theory: higher intensity, higher rate of incident photon per unit area, however does not affect energy of each photon
- With same energy, each photon gives same amount of energy to each electron so KE same
How does wave theory and quantum theory differ in explaining how minimum frequency of EM radiation is required to liberate electrons from a surface?
- Wave theory: higher intensity, higher energy so electrons should be liberated regardless of frequency, however in reality, low frequency does not produce photoelectrons regardless of intensity
- Quantum theory: energy of each photon is absorbed by each electron, as long as the incident frequency is greater than threshold frequency of the metal, photoelectrons will be produced
How does wave and quantum theory differ in explaining no observable lag time in photoelectron?
- Wave theory: energy of EM radiation will be absorbed by electrons and should take some time before photoelectrons can be produced, however in reality electrons are liberated almost instantaneously
- Quantum theory: photon has discreet energy, photon is emitted/absorbed almost instantaneously
How does quantum theory support how current is proportional to intensity?
higher intensity, higher rate of incident photon per unit area, higher number of one to one interaction, higher number of electrons liberated
Since all matters contain wave characters, why does a person running through a door fail to show diffraction effect?
- the mass is too large as compared to Planck’s constant
- de Broglie wavelength is extremely small
- When the wavelength is too small compared to the door’s opening, diffraction effect is not observable
What is the use of emission spectrum?
Each element has its own unique line spectrum, this allows chemist to identify based on their line spectrum
What is ionization?
Ionization is the process of creating charged particles, a positive ion is produced when electrons are removed from a neutral atom
What is excitation?
Process whereby atoms absorb energy without ionization
What could cause electrons to be ionized or excited to a higher energy state?
- high speed electron colliding with atom (energy equal or more)
- high speed particle collision (energy equal or more)
- photons (need absorb exact energy)
How does absorption spectrum form?
- white light, consisting of photons of a range of frequencies is passed through a cool gas in the discharge tube
- atoms of cool gas absorb photons of certain frequencies to jump from a lower energy level to a higher one
- excited atom will eventually return to the lower energy state by emitting the same photons or less energetic ones, however, these emissions occur in all directions and therefore have very weak intensities
- thus image is separate dark lines on coloured background
How does emission spectrum form?
- atoms of the gas is excited to higher energy levels when a high potential difference is applied across a discharge tube, atoms are made to undergo inelastic collisions with accelerating electrons
- when they return to lower energy levels, photons are emitted with energy corresponding to the energy difference between two energy levels
- because energy levels are discrete, energy difference between energy levels are also discrete, resulting emission spectrum also consists of discrete lines
- discrete bright lines of different colours on a dark background
How does emission spectrum lead to understanding that energy levels in the atoms are discrete?
- discrete wavelengths emitted imply that photons have specific energies hf
- energy of emitted photons is equal to energy difference between two energy levels when orbital electrons transit from higher to lower energy level
How does absroption spectrum lead to an understanding that energy levels in atoms are discreet?
- discrete wavelength absorbed imply photons have specific energy hf
- energy of absorbed photons is equal to the energy difference between two energy levels when orbital electrons transit from lower to higher energy levels
Why do street lamps give off distinct orange yellow colour?
Light is being emitted when energised electrons transit from higher to lower energy levels, energy gap of each element is distinct
How is x-ray produced?
X-ray is produced when high speed electrons are accelerated through a large potential difference of a few kV
What does X-ray intensity against wavelength graph look like?
sharp peaks or lines superimposed on a broad continuous spectrum
What are sharp peaks on X-ray intensity against x-ray graph called and what are they dependent on?
- characteristic peaks
- dependent on type of metal used
How is breaking radiation formed?
electrons lose various energies due to a range of decelerations when they collide with target metal, thereby giving off X-ray photons of different energies, forming the continuous spectrum
How is characteristic radiation formed?
- orbitals from inner shells are knocked out by energetic incident photons, creating a hole
- Electrons from higher energy shell falls to occupy the hole and emits a characteristic X-ray photon
