quantum physics

Question 1

describe experiment which can be used to estimate the value of the Planck constant

Answer 1

• LED will only allow current to pass after a minimum voltage has been put across it
• at this voltage all the electrons will have the same energy as a photon emitted by the LES which you know the frequency of
• finding the threshold voltage by seeing when current flows in the circuit can then be used to find h = e/f

Question 2

describe how to improve the accuracy of the estimate of this experiment

Answer 2

• to improve accuracy of experiment the experiment can be repeated with a variety of different coloured LEDs which each emit different wavelength of light
• the values of wavelength and threshold pd for each can be recorded and a graph drawn of v against 1/lambda
• the gradient of this graph will be equal to hc/e
• as the speed of light and the electron charge are known constants can calc h from this

Question 3

photoelectric effect

Answer 3

phenomenon where shining light with enough energy onto a metal releases electrons and can cause current to flow. the electrons emitted are called photoelectrons

Question 4

which features of the photoelectric effect can’t be explained if light is a wave

Answer 4

if light was a wave then the energy of the electrons released would increase with increasing intensity if the light - but this isn’t the case. instead the energy of electrons depends on frequency and no electrons are released below a certain threshold value no matter how intense the light is

Question 5

how does the photon model of light explain the threshold frequency seen in the photoelectric effect

Answer 5

each electron absorbs a single photon. this single photon must have enough energy for the electron to be released, it it doesn’t the energy is re-emitted. the electron can’t build up energy as It could if light was a wave

Question 6

true or false: the rate of emission of photoelectrons is proportional to intensity

Answer 6

true

higher intensity means more photons, this means more electrons can absorb energy and be released

Question 7

does the maximum ke of a released electron depend on the intensity of light hitting the surface

Answer 7

no, energy transferred is due to a one-to-one interaction and so depends on frequency not intensity

Question 8

what experimental evidence appears to show particles behaving as waves

Answer 8

• electron diffraction
• electrons will diffract is passed through the spaces between atoms in graphite
• this wouldn’t happen if electrons were behaving as particles only