2.1 The Photoelectric Effect

Question 1

what is the photoelectric effect?

Answer 1

the emission of electrons from a metal surface when the surface is illuminated by light of a frequency greater than a minimum value known as the threshold frequency

Question 2

what are photoelectrons?

Answer 2

electrons emitted in the photoelectric effect

Question 3

what is the work function?

Answer 3

the minimum energy needed to release a photoelectron from the surface of a metal

Question 4

what is the threshold frequency?

Answer 4

the minimum frequency of light required to cause the photoelectric effect

Question 5

what causes the value of the maximum kinetic energy of the photoelectron to increase?

Answer 5

photoelectrons are emitted with a variety of kinetic energies from 0 to a maximum value
if the frequency of radiation increases the maximum value increases

Question 6

what is the intensity of radiation in the photoelectric effect?

Answer 6

the amount of energy per second hitting an area of the metal

Question 7

does the intensity of radiation affect the maximum kinetic energy of the photoelectrons?

Answer 7

it doesnt affect maximum kinetic energy

Question 8

what is the relationship between number of photoelectrons emitted per second and the intensity of the radiation?

Answer 8

directly proportional

Question 9

why doesnt wave theory explain the threshold frequency?

Answer 9

• if an EM wave were shone onto a metal, each free electron on the surface of the metal would gain a bit of energy from each incoming wavefront.
• gradually, each electron would gain enough energy to leave the metal.
• even if the EM wave had a lower frequency than the threshold frequency, the electrons would eventually gain enough energy and be emitted.
• however that does not occur

Question 10

why doesnt wave theory explain the fact that kinetic energy depends only on the frequency?

Answer 10

• the higher the intensity of the wave, the more energy it should transfer to each electron
• so the kinetic energy of the electrons should increase with intensity

Question 11

what is the relationship between energy of a photon, Planck’s constant and frequency?

Answer 11

E = hf

Question 12

what is the relationship between energy of a photon, Planck’s constant, speed of light and the photons wavelength?

Answer 12

E = hc/lambda

Question 13

which model can explain the photoelectric effect?

Answer 13

the photon model

Question 14

what is the photon model?

Answer 14

• EM waves can only exist in discrete packets known as photons
• photons have a one-on-one, particle-like interaction with an electron in a metal surface
• each photon would transfer all its energy to one specific electron

Question 15

how do you experimentally demonstrate the photoelectric effect?

Answer 15

• attach a zinc plate to the top of an electroscope (a box containing a piece of metal with a strip of gold leaf attached)
• the zinc plate is negatively charged and so is the metal in the box
• the negatively charged metal repels the gold leaf, causing it to rise up
• UV light is then shone onto the zinc plate, and electrons are lost due to the photoelectric effect
• as the zinc plate and metal loose their negative charge, the gold leaf falls back down as its no longer repelled

Question 16

what does the work function value depend on?

Answer 16

the metal

Question 17

what happens if the energy gained from the photon is greater than the work function?

Answer 17

the electron is emitted

Question 18

what happens if the energy gained from the photon is lower than the work function?

Answer 18

the electron will not be emitted but instead will release the energy as another photon

Question 19

what is the relationship between threshold frequency, work function and Planck’s constant?

Answer 19

f0 = phi/h

Question 20

what is the energy transferred from EM radiation to an electron?

Answer 20

the energy of the photon absorbed (hf)

Question 21

what is the kinetic energy the electron will be carrying when it leaves the metal equivalent to?

Answer 21

hf minus any other energy losses

Question 22

why are electrons emitted from the metal with a range of kinetic energies?

Answer 22

the kinetic energy of the electron is equivalent to hf minus any other energy losses, which can vary

Question 23

what is the relationship between Planck’s constant, frequency, the work function and the maximum kinetic energy of the electron?

Answer 23

hf = phi + Ek(max)

Question 24

what is the relationship between the maximum kinetic energy of the electron, mass and velocity?

Answer 24

Ek(max) = 1/2m(vmax)^2

Question 25

what is the relationship between the kinetic energy of the electrons and intensity of UV radiation?

Answer 25

they are independent of each other, as electrons can only absorb one photon at a time, no matter how many are around

Question 26

what is stopping potential?

Answer 26

the potential difference needed to stop the fastest moving electrons travelling with kinetic energy Ek(max)

Question 27

what is the relationship between charge on the electron, the stopping potential (in V) and the maximum kinetic energy (in J)?

Answer 27

eVs = Ek(max)