electromagnetic radiation and quantum

Question 1

what is the electron volt?

Answer 1

the kinetic energy carried by an electron after its been accelerated from rest through a pd of 1 volt

Question 2

what is one eV in Joules?

Answer 2

1.6x10-19 J

Question 3

what’s the lowest energy level an electron can be in?

Answer 3

the ground state

Question 4

when an electron is excited, what is its energy level?

Answer 4

higher than the ground state

Question 5

how do we represent the energy level an electron is in?

Answer 5

n=1 (ground state), n=2, n=3…

Question 6

can electrons move energy levels?

Answer 6

yes, they can move down energy systems by emitting a photon
and can move up by absorbing a photon with the exact same energy difference as the two levels

Question 7

what is excitation?

Answer 7

the movement of an electron to a higher energy level.

Question 8

what is the ionisation energy of an atom?

Answer 8

the amount of energy needed to remove and electron from the ground state atom

Question 9

how is ionisation energy and energy to change energy levels linked?

Answer 9

they are both the exact same

Question 10

fluorescent tubes use excitation of electrons and photo emission to produce visible light, how does this occur

Answer 10

they contain mercury vapour, high pd is applied across this
this high pd accelerates fast-moving free electrons that ionise some of the mercury atoms, producing more free electrons
when this flow of free electrons collides with the electrons in mercury atoms, the atomic mercury electrons are excited to the highest energy level
when these excited electrons return to their ground state, they lose energy by emitting high-energy photons in the UV range
the photons emitted have a range of energies and wavelengths that correspond to different transitions of the electrons
a phosphor coating on the inside of the tube absorbs these photons, exciting the electrons to much higher energy levels
electrons then cascade down energy levels and lose energy by emitting many lower energy photons of visible light

Question 11

what do fluorescent use in order to produce visible light?

Answer 11

excitation of electrons and photon emission

Question 12

if you split light from a fluorescent tube with a prism or a diffraction grating what will you get?

Answer 12

a line spectrum

Question 13

what is a line emission spectrum?

Answer 13

a series of bright lines against a black background, each line corresponds to a particular wavelength of light emitted by a source

Question 14

what does the line emission spectrum provide evidence for?

Answer 14

that electrons in atoms exist in discrete energy levels

Question 15

why do you only see certain wavelengths in the line emission spectrum?

Answer 15

because only certain photon energies are allowed so you can only see the corresponding wavelengths as atoms can only emit photons with energies the same as the difference between two energy level

Question 16

what causes a continuous spectra with a line absorption spectra?

Answer 16

white light, if you split the light with a prism, the colours all merge into each other
hot things cause a continuous spectrum in the visible and infrared

Question 17

why are all wavelengths allowed in the continuous spectra?

Answer 17

because electrons are not confined to energy levels in the object producing the continuous spectrum. the electrons are not bound to atoms and are free

Question 18

when will you get a line absorption spectra?

Answer 18

when light with a continuous spectrum of energy (white light) passes through a cool gas

Question 19

what does a line absorption spectra look like and how does this happen?

Answer 19

multicoloured with black lines corresponding to absorbed wavelengths
at low temperatures, most electrons are at ground state
photons of correct wavelength are absorbed by the electrons to excite them to higher energy levels
these wavelengths then become missing from the continuous spectrum when it comes out the other side of the gas

Question 20

whats an example of light acting as a wave?

Answer 20

the photoelectric effect

Question 21

what is the photoelectric effect?

Answer 21

if an electron absorbs enough energy from UV radiation then the bonds holding it to the metal can break and the electron can get released (these are called photoelectrons)

Question 22

what conclusions do you get from the experiment of the photoelectric effect?

Answer 22

1. for a certain metal, no photoelectrons are released if the radiation has a frequency below a certain value (threshold frequency)
2. the photoelectrons are emitted with a range of kinetic energies. max value increases with frequency
3. the intensity of radiation is the energy per second hitting an area of the metal
4. the number of photoelectrons emitted per second is proportional to the intensity of the radiation

Question 23

what model can be used to to explain the photoelectric effect?

Answer 23

photon model- seen that photons of light had one-on-one particle interactions with an electron in a metal surface. each photon would transfer all its energy to one specific electron

Question 24

experiment to demonstrate the photoelectric effect

Answer 24

attach a zinc plate to the top of an electroscope. the zinc plate is negatively charged so the metal in the box is also negatively charged. metal then repels the gold leaf causing it to move up.
UV light is then shone onto the zinc plate
the energy from the light then causes electrons to be lost from the zinc plate. the zinc plate and metal then lose their negative charge so gold leaf is no longer repelled and goes back down

Question 25

what is the work function?

Answer 25

the amount of energy needed to break the bond between an electron and surface of a metal

Question 26

what will happen if the energy is less than the work function?

Answer 26

the electron will shake about a bit then release the energy as another photon. the metal will heat up a bit but no electrons will be emitted

Question 27

how can you work out threshold frequency?

Answer 27

work function/ Planck constant
ø/h

Question 28

whats the reason electrons emitted from a metal have a range of kinetic energies?

Answer 28

energy losses

Question 29

what is stopping potential?

Answer 29

the potential difference needed to stop fast moving electrons travelling with maximum KE

Question 30

how do you calculate stopping potential?

Answer 30

max KE/ charge of an electron

Question 31

how has light been shown to act like a wave?

Answer 31

diffraction- when a light beam passes through a narrow gap, it spreads out

Question 32

how has light been shown to act like a particle?

Answer 32

the results of photoelectric effect experiments can only be explained by thinking light as a series of particle-like photons

Question 33

how can we provide evidence that electrons show wave-like qualities?

Answer 33

electron diffraction
-diffraction patterns can be shown using a electron diffraction tube
-electrons are accelerated to high velocities in a vacuum and then passed through a graphite crystal
-as they pass through spaces between the atoms of the crystal, they diffract like waves passing through a slit

Question 34

if wavelength of a wave is greater, what happens to the spread of lines in the diffraction pattern?

Answer 34

the spread increases

Question 35

in general, what is the wavelength for electrons accelerated in a vacuum tube about the same size as?

Answer 35

electromagnetic waves in the X-ray part of the spectrum

Question 36

when will a electron beam diffract?

Answer 36

when the de Broglie wavelength is around the same size as the gap between the atoms

Question 37

what happens to the diffraction pattern if you use particles with a larger mass?

Answer 37

more tightly packed

Question 38

if you are using an electron microscope and want to resolve tiny data on an image, what wavelength do you need?

Answer 38

shorter

Question 39

are electron microscopes or light microscopes in more detail?

Answer 39

electron microscopes