quantum physics

Question 1

define photon

Answer 1

discrete quantum of electromagnetic energy

Question 2

nature of electromagnetic radiation

Answer 2

1. behave like wave:
   - diffraction
   -refraction
2. behave like particle
   - photoelectric effect
   - atomic line spectra

Question 3

equation energy of photon (Einstein’s relation)

Answer 3

E(eV) = h * f = (h * c)/ λ
E=energy(J)
h=Planck’s constant(Js)
f=frequency(Hz)

Question 4

what is photoelectric effect

Answer 4

an interaction btw a photon and a electron in a metal, in which the electron is removed from the surface of a metal

Question 5

equation momentum of photon in terms of E

Answer 5

P = E/ c
E=energy of photon
c=speed of light

Question 6

what is electronvolt (eV)

Answer 6

The energy gained by an electron travelling through a potential difference of one volt

Question 7

1 eV to joule

Answer 7

1 eV = 1.6 * 10^-19 J (elementary charge of electron)

Question 8

kinetic energy of electron accelerated by potential difference from rest

Answer 8

eV = 1/2 * m * v^2
note: this does NOT apply when v approaches c (3*10^8)

v = √(2 * eV / m)

Question 9

what is threshold frequency

Answer 9

the minimum frequency of an incident electromagnetic radiation required to remove a photoelectron form the surface of a metal

Question 10

what is threshold wavelength

Answer 10

the longest wavelength of an incident electromagnetic radiation required to remove a phoelectron from the surface of a metal

note: related to threshold frequency by c = f * λ

Question 11

define work function (threshold energy) Φ

Answer 11

the minimum energy required to release a photoelectron from the surface of a material

note: electron will be only emitted when absorbed a photon has energy more than Φ

Question 12

how does work function varies with metals

Answer 12

1. more tightly packed electron => more work function
2. metallic bonding stronger

Question 13

what is stopping potential

Answer 13

potential difference required to stop photoelectrons emission from occurring

Question 14

equation max Ek of phototelectron in terms of stopping voltage

Answer 14

Ek max = e * Vs
e=elementary charge of electron
Vs=stopping voltage

Question 14

how is max Ek determined

Answer 14

1. set up circuit with emitter plate, collector plate and variable emf
2. as photon reaches emitter plate, photoelectron is emitted and collected by the collector plate, creating a current, thus pd
3. the variable emf is set to be opposite to the photo cell, only electron possess more Ek than the emf can reach the collector plate
4. as the opposition emf increases, eventually no electron can reach the collector plate, thus max Ek is determined by this stopping potential

note: V = W/Q = E/Q = Ek/e

Question 15

factors that effect the maximum Ek of photon

Answer 15

1. frequency (or wavelength) of incident wave
2. work function of the metal

which means that the stopping potential will be the same

Question 16

what does increasing intensity effect

Answer 16

1. increases the number of photon incident to the metal surface (if frequency kept the same)
2. increase the photoelectric current (the number of photoelectron emitted)
3. increase the energy transferred per unit area in a given time
4. will not effect Ek

note: intensity is power per unit area (rate of energy dissipated per unit area)

Question 17

what happens to photoelectric current when frequency increases but intensity remain the same

Answer 17

1. f incr => E of each photon incre (E = hf)
2. intensity constant => energy transferred per unit area in a given time is constant
3. therefore, the number of photo per unit area decreases
4. one electron absorb one photon => photoelectron emitted decre

Question 18

photoelectric equation

Answer 18

E = h*f = Φ + Ek = Φ + 1/2 * m * v^2(max)

E=energy of photon
Φ=work function(energy required for photoelectron to be emitted)
m=mass of photoelectron
vmax= max velocity of photoelectron
Ek=kinetic energy gained by photoelectron

Question 19

relation Ek against f

Answer 19

Ek = h*f - Φ

Question 20

what is photoelectric current

Answer 20

1. number of photoelectron emitted per unit time
2. value of photoelectric current = number of electron emitted * charge of one electron

Question 21

define de Broglie wavelength

Answer 21

wavelength associated with a moving particle

Question 22

equation of de Broglie wavelength

Answer 22

λ = h / p. = h / √(2 * m * Ek)

λ=de Broglie wavelength
h=plank’s constant
p=momentum of particle

m=mass of particle
Ek=kinetic energy of particle

Question 23

equation for kinetic energy in terms of moementum

Answer 23

Ek = p^2 / 2m
p=momentum
m=mass

Question 24

what is electron energy level

Answer 24

discrete electron energy level in isolated atoms electrons are usually at ground state (lowest energy level)

Question 25

what happens when electron moves btw energy level

Answer 25

1. when electron absorbs enough energy to move up energy level, it is excited 2. when gains enough energy to be removed from the atom, it is ionised 3. when drops energy level, it releases energy in form of photon

Question 26

how to find the ionisation energy an electron hydrogen atom

Answer 26

the difference btw Einfinity (which is 0) and E1, therefore same as ground level energy

Question 27

how to find threshold frequency

Answer 27

Φ = h*f

Question 28

what is line spectra

Answer 28

1. when excited atoms emit light of specific wavelength, which creates a unique pattern of coloured lines 2. there are discrete energy levels => discrete wavelength will be emitted 3. useful to determine the material 3. Emission Spectra: when excited electron move from higher to lower energy level, light of specific wavelength is emitted 3. absorption Spectra: when shine a light through a gas of substance, light of specific wavelength will be absorbed to excite the electrons, so certain wavelength will be missing

Question 29

why only certain wavelength is absorbed

Answer 29

1. photon is indivisible 2. only those possess energy exactly the same as the difference btw energy levels will be absorbed to excite electrons (other ones will be ignored) 3. this only applies to photon

Question 30

calculate discrete energy changes

Answer 30

hf = E2 - E1 note: dont forget the negative sign