Quantum Physics Part 1 Flashcards
Describe photoelectric effect
Phenomenon where e- r emitted fr metal surface when e-magnetic rad n of sufficient high f is incident on surface
Define electromagnetic radiation
exists as discrete bundles (quanta) of energy aka photons
Define photon
A discrete bundle/quantum of e-magnetic energy
Give formula for energy of photon
energy, E = hf = hc/λ (c=fλ)
where
h is Planck constant (6.63e-34 J s),
f is frequency,
c is speed of light in vacuum (3.00e8 m s-1),
λ is wavelength
Describe photon’s characteristics
- only dependent on f
- for given f, beam of EM rad n of greater intensity simply hv more photons per unit time
- changing intensity and photon per unit time no affect whether e- can be emitted
Give formula for intensity of a beam of EM radiation
Intensity I = P/A = Etotal/(tA) = NE/(tA )= Nhf/(tA) = nhf
Units: W m-2 or J s-1 m-2
where
E is energy of 1 photon,
N is no of photon,
N/t is rate of incident photon,
n is no of photon passing unit area per unit time
What to take note about photons bombarding metal surface?
- 1 e- can oni absorb 1 photon (1:1 ratio)
- not every incident photon cause e- be emitted (due to various factors)
- transfer of energy is immediate (no time delay)
Give Einstein’s photoelectric equation
hf = Φ + Ek,max
where
hf is photon energy,
Φ is work funct n energy (of material),
Ek max is max kinetic energy of emitted e-
Define work function energy Φ of a material. What to take note?
Φ is min amt energy needed to remove e- fr surface of material
*Φ is a material property (ie diff metal hv diff value)
Define threshold frequency, f0 and give formula
f0 is min frequency of incident rad n for e- to escape
f0 = Φ/h
Discuss how the photoelectric effect provides evidence for the particulate nature of electromagnetic radiation
- Existence of threshold frequency below which no photoe- r emittef prove EM rad n consist og discrete quanta of energy given by hf
- Instantaneous emis n of photoe- when all photon energy is transferred immediately to e- in single collis n give evidence to particulate nature of EM
- Max Ek of photoe- being dependent oni on f of rad n, relating to discrete energy of photon, & independent on intensity of rad n oso give evidence for particulate nature of EM
What is stopping potential?
Min retarding potential (diff) to stop all emitted e- fr reach collector plate
Using stopping potential, what formula can be made?
Ek,max = 0.5m(v max)² = e(Vs)
where
Ek max is Ek of e- converted to electric PE,
e is charge of electron,
Vs is stopping potential
How to get stopping potential against frequency graph?
hf = Φ + Ek, max
hf = Φ + e(Vs)
e(Vs) = hf - Φ
Thus, Vs = (h/e)f - Φ/e
Give photoelectric equations
f = c/λ
hf = Φ + Ek,max
hf0 = Φ
Ek,max = 0.5m(v max)² = e(Vs)
Explain photoelectric graphs
- Vs against f of EM rad for three diff metal (same grad, diff f intercept)
- recall Vs = (h/e)f - Φ/e
- threshold f diff bcos diff work funct n of metals
- slope const as given by h/e (const) - current I against intensity (origin straight positive grad line)
- photocurrent I (rate emis n photoe-) proportional to intensity of EM rad n (rate photon incident); straight line graph obtained (assuming const f and f > threshold f) - Photocurrent I against potential of collector plate V (start fr same pt in -ve V axis but Q rise until twice high as P just after passing V=0)
- consider graph due to 2 light beam of same wavelength, but diff intensity. Q high intensity, P low intensity
- stopping potential Vs is same due to light beam low intensity P & high intensity Q, since photon energy (& thus Ek,max) is same
- beam of high intensity Q produce more e- than low intensity P (if Q double P in intensity, current I oso double)
* assume probability of photoe- emis n const
How to convert between Joules and Electron-volts?
1eV = 1.6E-19 J
Give de Broglie formula
λ = h/p = h/mv
where
λ is wavelength,
h is Planck const,
p (=mv) is momentum of particle
Give two formulae for kinetic energy
Ek = 0.5mv² = p²/(2m)
Name evidence showing wave-particle duality of light and electrons
- light as wave: interference/diffraction
- light as particle: photoelectric effect
- electron as particle: e- undergo collis n, hv mass & charge
- electron: e- diffract n
Give formula for magnitude of energy change during transition of electron from one energy level to another (emission or absorption of photons)
ΔE = E higher - E lower = hf
where
E higher is higher energy level,
E lower is lower energy level,
hf is energy of emiited/absorbed photon
Describe all key features of energy-level diagram
- n represent quantum no. (oni take integer, discrete value fr 1 to infinity)
- Ground state refer to lowest energy lvl in which atom is most stable. e- normally occupy this lvl unless given suff energy move up higher lvl
- highest energy lvl n=infinity correspond to energy state whereby e- no longer bound to atom (ie escaped atom, recall gravitation). By convent n, assigned value 0 eV
- lower energy lvl, more -ve value associated w that lvl. So, lower energy states correspond to more stable state
- energy diff btw any 2 adj lvl get smaller as n increase (so higher energy lvl become practically continuous at as n approach infinity)
- ionisat n energy of atom is energy needed remove orbital e- completely fr atom (ie transit n fr ground state to n = infinity)
- transit n btw diff energy lvl represented by arrow (relative length of arrow relate to amt energy absorbed/emitted when e- transits btw energy lvl)
What must happen during transition between energy levels (or allowed orbits)?
atom can oni absorb (more to higher energy lvl) or emit (drop to lower energy lvl) energy in fixed amt equal to diff btw energy lvl
Describe excitation
- an atom said to b excited state when e- found in higher energy lvl (transit n to higher energy lvl)
- there are 2 mechanisms:
- Absorp n proton
- energy of photon MUST b exactly equal energy diff btw 2 energy lvl b4 absorbed
ie. ΔE = E higher - E lower = hf
Otherwise, not absorbed by e- at ALL - high speed collis n by another particle
- Ek of colliding particle must b >= energy diff btw 2 energy lvl (no need match)
- colliding particle transfer part of its energy to e- for excitat n (equal to diff btw 2 energy lvl), while keeping rest
Describe de-excitation
- transit n of e- to lower energy lvl
- atom can de-excite oni thru 1 mechanism: e- of atom lose exact amt energy by emitting photon of exact f OR λ
ie. ΔE = E higher - E lower = hf = hc/λ
Describe emission line spectrum
- consists of discrete bright coloured lines in dark background
Explain how an emission spectrum is produced
- gases eg H, Ne, can b places in discharge tube low Pa. High voltage applied btw metal electrodes in tube large enough to produce current in gas. Gas atoms bcome excited by collis n w e- passing thru tube, fr cathode to anode of discharge tube
- excited gas atom r unstable, transits to lower energy lvl by emit photon
- since atom hv discrete energy lvl, oni specific high-to-low energy lvl transit n r possible, so photon emitted of specific energy, f
- so, coloured lines in emiss n spectrum correspond to emitted photon of specific f
Describe absorption line spectrum
consist of dark lines against continuous spectrum of white light
Explain how absorption spectrum is produced
- produced when white light pass thru cold gas
- gas atoms r excited to higher energy lvl when absorb photon fr white light
- since atom hv discrete energy lvl, oni specific low-to-high energy lvl transit n possible, so photon absorbed of specific energy, f
- when atom transit back to ground state, photon of same f re-radiated but in all direct n
- so, parts of absorpt n spectrum corresponding to f appear as dark lines as compared to other non-absorbed f
Describe how line spectra can be explained using the idea of discrete energy levels in isolated atoms
- each line correspond to specific photon energy
- photon emitted when e- make transit n fr higher to lower energy lvl
- since photon energy is of specific value, implies that energy change btw energy lvl is of discrete amt
- discrete energy change thus implies energy lvl discrete
Explain why energy levels of hydrogen atom are labelled negative
- e- & nucleous form bound system, where force btw e- & nucleus is attractive
- since potential at infinity defined to b zero, energy lvl closer to nucleus is lower, so more -ve
