Module 4 -Section 3 - The Photolelectric Effect

Question 1

What happens if you shine EM waves of a high enough frequency onto the surface of a metal ?

Answer 1

If you shine EM waves of a high enough frequency onto a surface of a metal - then the metal will instantaneously eject electrons - for most metals this frequency falls in the UV range

Question 2

What is the photoelectric effect ?

Answer 2

It is the effect when electrons are ejected from the surface of metals when a EM wave of a high enough frequency is shone at it

Question 3

How does the photoelectric effect work ?

Answer 3

So firstly, when light photons are shone at the surface of a metal - the photon gives energy to the electrons in the metal to overcome the ‘work function’ which is the energy needed to release the electron from the metal —> the remainder of the energy of the photon goes to the kinetic energy store of the electron which makes it eject from the metal

So firstly you have to overcome the work function and when this happens the bonds holding the electron to the metal break and this allow the energy to be transferred to the Ke store of the electrons which emits it from the surface

Question 4

What are the electrons called that are emitted from the surface of the metal ?

Answer 4

They are called photoelectrons

Question 5

How can you demonstrate the photoelectric effect ?

Answer 5

You can demonstrate the photoelectric effect with a gold-leaf electroscope

So firstly you have a block which has a zinc tube through the middle of it and a gold leaf it stitched to the tube and then coming out of the block you have a zinc circle thing and because the zinc is negatively charged the gold leaf is repelled and the gold leaf is moved up away from the tube inside the block

• the circle thing is called an electroscope plate

Then the zinc plate is exposed to ultraviolet light and the photoelectric effect causes its free electrons to be ejected - this causes the zinc to loose its negative charge - the gold leaf it no longer repelled and so it drops down

Question 6

What are the 3 conclusions from the gold-leaf electroscope experiment ?

Answer 6

Conclusion 1 - For a given metal - no photoelectrons are emitted if the EM radiation has a frequency below a certain value - this is called the threshold frequency

Conclusion 2 - The photoelectrons are emmited with a variety of kinetic energies ranging from zero to the maximum value - This value of maximum KE increases with the frequency of the radiation - and is unaffected by the intensity of the radiation — so when you increase the frequency of the radiation the maximum KE of the electrons increases but not the amount of photoelectrons released

Conclusion 3 - The number of photoelectrons emitted per second is directly proportional to the intensity of the radiation

Question 7

If a wave has more energy it doesn’t mean it has more intensity

Question 8

What does intensity increase in the photoelectric effect and what does frequency increase ?

Answer 8

Intensity is directly proportional to the amount of photoelectrons ejected and the frequency of the radiation is directly proportional to the maximum kinetic energy value of a electron

Question 9

Why can’t the photoelectric effect be explained by the wave theory ?

Answer 9

because according to the wave theory - the higher the intensity of the wave, the more energy it should transfer to each electron therefore the kinetic energy should increase with intensity however the kinetic energy of the electron only depends on the frequency of the wave and not the intensity —> the intensity is directly proportional to the amount of electrons ejected

There is also no explanation for the threshold frequency —> according to the wave theory, the electrons should be ejected eventually, no matter what the frequency is.

Question 10

How does the photon model explain photoelectric effect ?

Answer 10

According to the photon model:
- when light hits the surface of a metal - the metal is bombarded by photons - if one of these photons is absorbed by a free electron then the electron will gain the energy equal to hf —> and this just means the energy of a photon as E = hf —> so when an electron absorbs a photon it gains the energy of a photon —> first to overcome the work function and then it goes into the ke store of the photon and this ejects the electron —> this is true because if you have a photon with 10J of energy then 5J will go into overcoming the work function and 5J would go into the Ke store of the photoelectron however some energy could be dissipated into the surroundings

Question 11

What happens before an electron can leave the surface of a metal ?

Answer 11

The electron needs enough energy to break the bonds holding it there and this is the work function energy - its the energy needed for an electron to break free from the bonds holding it there - and the value of the work function depends on the metal

Question 12

Why is the there different threshold frequencies for different metals and different work functions for different metals ?

Answer 12

The threshold frequency is different for every metal because different metals have different atomic structures - some atoms have more or less shells —> so the first electron to get ejected from the atom would be the one on the outermost shell as it’s easier to loose - further from the nucleus therefore the atoms with more shells therefore the outermost electron is further from the nucleus are the ones that would have a smaller threshold frequency as it would require less energy to break the electron from the bonds holding it back

Question 13

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

Answer 13

Then an electron will get emitted and this is because the electron gains more energy than the work function and therefore the remaining energy is transferred into the Ke store of the electron

Question 14

What happens if the energy gained by an electron on the surface of the metal is not greater than the work function ?

Answer 14

Then the metal will heat up but no electrons will be ejected and this is because the photon did it provide the electron with enough energy to break free from the bonds holding it there

Question 15

Whats the equation for threshold frequency ?

Answer 15

f = phi/h

Question 16

Photon energy =

Answer 16

Work function + kinetic energy of electron

Question 17

Whats the energy transferred to an electron from a photon ?

Answer 17

Hf —> this is because the energy of a photon is hf

Question 18

Why do the electrons have a range of kinetic energies when they get emitted form the surface of the metal ?

Answer 18

This is because the energy that a electron will be carrying when it leaves the metals surface will be hf - minus any energy its lost on the way out - energy that has been dissipated to the surroundings - there could be many ways that this energy is dissipated

Question 19

Whats Einstein’s photoelectric energy ?

Answer 19

Hf = work function + 1/2 mv*2

Question 20

How do photons work ?

Answer 20

Photons only go to one electron - they only interact with one particle at a time

Question 21

Why the kinetic energy of the electron independent of the intensity of the radiation ?

Answer 21

This is because electrons can only absorb one photon at a time therefore a higher intensity just means more photons hitting the metals surface per second - which means more photoelectrons being emitted but it doesn’t mean greater KE of the electrons

Question 22

What is photoelectric emission directly proportional to ?

Answer 22

It’s directly proportional to the intensity of radiation provided which is above the threshold frequency - as more photons per second means more collisions which means more electrons getting ejected

Question 23

Whats threshold frequency ?

Answer 23

It’s the minimum amount of frequency that is needed for the photoelectric effect to happen