the photoelectric effect

Question 1

what does shining light on a metal do

Answer 1

release electrons

Question 2

how do you get a metal to to emit electrons

Answer 2

if you shine a light of a high enough frequency onto the surface of the metal

Question 3

what is the high enough frequency for most metals

Answer 3

in the UV range

Question 4

what do free electrons on the surface of the metal do

Answer 4

absorb energy from light

Question 5

what happens if an electron absorbs enough energy

Answer 5

the bonds holding the electrons to the metal break and the electron is released

Question 6

what is this called

Answer 6

the photoelectric effect

Question 7

what are the electrons emitted called

Answer 7

photoelectrons

Question 8

what is the first conclusion

Answer 8

for a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value - called the threshold frequency

Question 9

what is the second conclusion

Answer 9

the photoelectrons are emitted with a variety of kinetic energies

Question 10

what do the kinetic energies range from

Answer 10

0 to some maximum value

Question 11

how does the maximum kinetic energy increase

Answer 11

it increases with the frequency of the radiation and is unaffected by the intensity of the radiation

Question 12

what couldnt explain the photoelectric effect

Answer 12

wave theory

Question 13

according to wave theory, what is the energy carried proportional to

Answer 13

to the intensity of the beam

Question 14

according to wave theory, what would happen to the energy carried by the light

Answer 14

spread evenly over the wave front

Question 15

according to wave theory, what would each free electron on the surface of the metal gain

Answer 15

a bit of energy from each incoming wave

Question 16

according to wave theory, what would gradually happen

Answer 16

each electron would gain enough energy to leave the wave

Question 17

what does higher intensity of the wave equal

Answer 17

the more energy it should transfer to each electron

Question 18

therefore what else should increase with intensity

Answer 18

kinetic energy

Question 19

what is the issue with this

Answer 19

there is no explanation for the kinetic energy depending only on the frequency

Question 20

what is there also no explanation of

Answer 20

the threshold frequency. according to wave theory the electrons should be emitted eventually no matter what the frequency is

Question 21

what can this whole problem be explained by

Answer 21

einsteins photon model of light

Question 22

what did einstein suggest

Answer 22

electromagnetic waves (and the energy they carry) exist in discrete packets called photons

Question 23

what is the formula for the energy carried by one of these photons

Answer 23

E = h x f = hc/(labda)

Question 24

what did einstein see the photons as

Answer 24

as having a one-on-one particle like interaction with an electron in a metal surface

Question 25

what would a photon do

Answer 25

transfer all its energy to one, specific electron

Question 26

according to photon model, what happens when light hits the surface

Answer 26

the metal is bombarded by photons

Question 27

according to photon model, what happens if one of these photons collides with a free electron

Answer 27

the electron will gain energy equal to h x f

Question 28

what does an electron need to do before it can leave the surface of the metal

Answer 28

enough energy to break the bonds holding it there

Question 29

what is this energy called

Answer 29

the work function Φ

Question 30

what does its value depend on

Answer 30

the metal

Question 31

what does the photon model do

Answer 31

explain the threshold frequency and maximum kinetic energy

Question 32

when is an electron emitted

Answer 32

if the energy gained by an electron on the surface of a metal, from a photon, is greater than the work function

Question 33

what if the energy isnt

Answer 33

the metal will heat up but no electrons will be emitted

Question 34

what is the formula for threshold frequency

Answer 34

f = Φ/h

Question 35

what is the energy transferred to a photon

Answer 35

h x f

Question 36

what is the value of the kinetic energy the electron will be carrying when it leaves the surface of the metal

Answer 36

(h x f ) - any energy lost in its way out

Question 37

what do electrons deeper down the metal do and what does that explain

Answer 37

lose more energy than electrons in the surface which explains the range of energies

Question 38

what is the minimum amount of energy it can lose

Answer 38

the work function Φ

Question 39

what is the formula for maximum kinetic energy of a photoelectron

Answer 39

h x f = Φ + Ek(max) where Ek(max) = 1/2 x m x v(max)^2

Question 40

what is the kinetic energy of the electrons independent of

Answer 40

the intensity as they can only absorb one photon at a time

Question 41

what is intensity

Answer 41

the number of photons per second on an area

Question 42

what does increasing the intensity do

Answer 42

more photons per second on an area - each photon has the same energy as before

Question 43

what gives the maximum kinetic energy

Answer 43

stopping potential

Question 44

how can maximum kinetic energy be measured

Answer 44

using the idea of stopping potential

Question 45

how are emitted electrons made to lose their energy

Answer 45

by doing work against an applied potential difference

Question 46

what is the stopping potential, Vs

Answer 46

the potential difference needed to stop the fastest moving electron, with Ek(max)

Question 47

what is the work done by potential difference that stops the fastest moving electron equal to

Answer 47

the energy they were carrying

Question 48

write out the formula of this

Answer 48

e x Vs = Ek(max) where e is the charge of an electron (1.60 x 10^-19)

Question 49

what is work done

Answer 49

potential difference x charge