Quantum phenomena

Question 1

what is the photoelectric effect

Answer 1

where electrons are emitted from the surface of a metal when photons are incident on it

Question 2

how do you achieve the photoelectric effect

Answer 2

you could use a gold leaf electroscope, a metal plate attached to a metal pole and a flexible gold leaf

Question 3

how does a gold leaf electroscope work

Answer 3

the metal in the electroscope is set up to be negatively charged causing the leaf to repel away. when light of a certain frequency was incident, electrons were released causing less repulsion.

Question 4

when would an electroscope not work

Answer 4

the electroscope is positively charged, (protons are fixed in the metal so they cant be released), harder for the electrons the escape (attracted back). gold leaf will repel slightly but won’t fall back when photons are incident.

Question 5

what did wave theory predicted would happen when photoelectric effect was replicated

Answer 5

• any frequency should cause the photoelectric effect
  -increasing intensity (brightness) increases energy
• it should take longer for electrons to be emitted with lower intensity light

Question 6

what happened in the photoelectric effect (wave vs actually)

Answer 6

-only above a certain threshold frequency were electrons were emitted
- as long as frequency was above threshold the electrons had a consistent energy (more got released)
- electrons were emitted instantly regardless of intensity

Question 7

conclusions of the photoelectric effect

Answer 7

• energy is delivered in packets, and is proportional to the frequency of the photon (electrons cant store energy)
  -one to one interaction between photon and electron
  -energy is delivered in one packet in one go

Question 8

what is the work function

Answer 8

the minimum energy required to release an electron from the metals surface

Question 9

how to calculate threshold frequency

Answer 9

f0=work function/h

Question 10

why are electrons released with different speeds

Answer 10

not all electrons are at the metals surface so more energy is needed to release it. that’s why when electrons are emitted with different kinetic energies.

Question 11

what is the photoelectric graph

Answer 11

x-axis kinetic energy
y-axis frequency
gradient plancks constant
y-intercept work function

Question 12

what is intensity

Answer 12

the amount of energy arriving per second per unit area

Question 13

what is a vacuum photocell

Answer 13

measures the amount of energy to stop an electron
photons release photoelectrons (from the smile)
they travel to the ‘eye’
completing the circuit
(happy cyclops)

Question 14

how does a vacuum photocell work

Answer 14

as voltage increases the ‘eye’ becomes more negative, making it harder for electrons escape
eventually the voltage would be high enough that none cross the gap so the circuit is no longer complete

Question 15

stopping potential equation

Answer 15

eVs=Ek(max)

Question 16

what happens when you excite a gas

Answer 16

if you excite a gas they glow
different elements glow with different colours
this could be done by passing a high current through them

Question 17

what happens with emission of photons from a excited gas

Answer 17

even though we see one colour, its made up of multiple photons which can be split up with a prism

Question 18

what is an emission spectra

Answer 18

the visible light colour that can be seen (on a black strip)
a continuous spectra is all the visible light (white light)

Question 19

how to create and absorption spectra

Answer 19

if you take the same gas (de-excited) and shine white light through it then it will create an absorption spectra (opposite of emission spectra)
it blocks the photons that are emitted by the gas

Question 20

what are energy levels

Answer 20

around atoms, electrons only exist in energy levels (not the same as shells)
they can absorb and emit photons to move up and down energy levels

Question 21

what is ground state

Answer 21

where electrons have the least amount of energy/ need to receive the most amount to be ionised (leave the atom). they are most stable in ground state

Question 22

how can electrons gain energy

Answer 22

they can absorb a photon (one to one) of he exact same energy needed to move up a level.
hit with an electron with kinetic energy- only absorbs what it needs

Question 23

what is ionisation energy

Answer 23

the amount of energy needed to get an electron released from the tom turning it into an ion

Question 24

what’s the energy levels required in an atom

Answer 24

there are discrete energy levels that are different for each element, the lower down the atom is, the more energy needed to escape

Question 25

what happens when an electron in ground state is hit with a slow electron

Answer 25

nothing happens as there isn't enough energy to move up energy levels

Question 26

what happens when an electron in ground state is hit with fast electron

Answer 26

the exact energy needed to move up a level is transferred to the ground state electron and the incoming electron moves away with the left over energy

Question 27

what happens when an electron in ground state is hit with a low energy photon

Answer 27

nothing happens, not enough energy to move up and you cant absorb part of a photon

Question 28

what happens when an electron in ground state is hit with a high energy photon

Answer 28

if its the exact right energy of the energy level it will move up to that level

Question 29

why do electrons jump down energy levels

Answer 29

they are most stable in ground state

Question 30

how do electrons jump down energy levels

Answer 30

electrons will emit photons of the exact energy of the energy jump in photons to move down. they can cascade down levels or move straight to ground state in one jump

Question 31

why are only certain photons are absorbed in energy levels

Answer 31

absorption spectra are dependant of the photons emitted by the electrons cascading down of certain energies. these being different for different elements

Question 32

how do fluorescent tubes use energy levels

Answer 32

-Fluorescent tubes are filled with mercury vapour -a high pd is put through the tube causing fast electrons to pass through. -free electrons collide with ground state electrons in the mercury atoms, moving them up -excited electrons cascade down the energy levels releasing UV photons -tube has fluorescent coating turning UV photons into visible light, using ground state electrons in the coating. -cascading electrons back down release lower energy photons (visible light) of all colours (white light)

Question 33

when does light behave like waves

Answer 33

in diffraction- light bends around objects

Question 34

when does light behave like particles

Answer 34

in the photoelectric effect- packets of light arrive (photons) one to one interaction

Question 35

what is wave particle duality

Answer 35

light can show wave like behaviour and particle like behaviour depending on the conditions and circumstances

Question 36

what is the de broglie theory

Answer 36

If waves can behave like particles then particles should be able to behave like waves

Question 37

how can you prove de Broglie theory

Answer 37

particle diffraction- the greatest diffraction is when the wavelength is similar in size to the gap its being diffracted through

Question 38

what is the electron diffraction experiment

Answer 38

accelerated electrons are fired past a thin graphite screen causing them to diffract and hit a fluorescent screen. if they behaved like particles there would be a single spot on the screen however there are rings that come out from the centre spots (constructive/destructive interference)

Question 39

why is graphite used to diffract the electrons in electron diffraction

Answer 39

the graphite lattice are in similar size to the de broglie wavelength of the electrons

Question 40

why does electron speed effect the diffraction rings

Answer 40

slow electrons- bigger wavelength- more diffraction- more spaced out rings