Chapter 3 - Quantum phenomena

Question 1

Briefly describe the photoelectric effect

Answer 1

When photons are absorbed by electrons on metal’s surface and if photon’s frequency is above the threshold frequency, then photoelectrons are emitted

Question 2

What are the conclusions made from the photoelectric effect?

Answer 2

• No photoelectrons emitted below threshold frequency
• photoelectrons emitted with variety of kinetic energies depending on frequency of photon
• kinetic energy of photoelectrons is unaffected by intensity of photons
• number of photoelectrons emitted per second is proportional to intensity of photons

Question 3

What does the photoelectric effect give evidence for?

Answer 3

Particle properties of light

• wave theory doesn’t explain why intensity of photon does’t increase kinetic energy of photoelectrons released, but particle theory does.
• Energy increases with intensity in waves

Question 4

Calculate the wavelength of a photon with energy 3.06x10^-19J?

Answer 4

E =hf
E = hc/wavelength
wavelength = hc/E
= 6.63x10^-34 x 3.0x10^8/3.06x10^-19
= 6.5x10^-7m

Question 5

Describe an experiment used to prove the photoelectric effect

Answer 5

zinc plate attached to top of an electroscope (box containing piece of metal with a strip of gold leaf attached). Zinc plate is negatively charged - repelling the gold leaf. UV light is shone onto plate - causing electrons to be lost via photoelectric effect. Zinc plate therefore loses negative charge and gold leaf is no longer repelled and so falls back down

Question 6

What is the work function of a metal?

Answer 6

Energy an electron in a metal needs to break the bonds holding it there, so it can leave the surface.

Minimum energy of photon that causes electron emission from metal surface

Question 7

Write the equation for threshold frequency using knowledge for work function?

Answer 7

threshold frequency = work function/ planck’s constant

Question 8

For a metal with work function of 7.2 x 10^-19 J, the minimum frequency of EM radiation needed for photoelectron to be released would be?

Answer 8

threshold frequency = work function/ planck’s constant
= 7.2x10^-19/ 6.63x10^-34
= 1.09 x 10^15 Hz

Question 9

What is the equation of kinetic energy for a photoelectron leaving metals surface?

Answer 9

Ek max = hf - work function

Question 10

How do you work out velocity of photoelectron released?

Answer 10

Vmax^2 = 2Ek max / mass of electron

Question 11

What is an electronvolt?

Answer 11

Kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt

1eV = 1.6x10^-19J

Question 12

What is excitation?

Answer 12

Movement of an electron to a higher energy level. Happens when an electron absorbs a photon

Question 13

What happens when electrons move down energy levels?

Answer 13

Releases a photon (equal to energy between 2 energy levels)

Question 14

Give equation for transition between 2 energy levels

Answer 14

change in energy = energy of initial energy level - energy of final energy level = photon energy (hf)

Question 15

What is ionisation?

Answer 15

When an electron is removed from an atom, forming an ion

Question 16

How does a fluorescent tube work?

Answer 16

Contain mercury vapour (conductive at low pressure). When a high voltage is applied - fast moving electrons are accelerated and by electron bombardment mercury atom’s electrons are excited . Mercury electrons are excited to a higher energy level - the electrons then de-excite and release high-energy photons (mainly UV) in stages- each have different wavelengths. The phosphur coating an inside of tube then absorbs these photons - exciting it’s electrons and then releasing photons of various wavelengths (form visible light ) as it cascades down the energy levels - when de-exciting

Question 17

What is the line emission spectrum and how does it work?

Answer 17

• Seen as bright lines against a black background, each line corresponding to a particular wavelength of light emitted by a source.
• works by diffracting light of different wavelengths at different angles

Question 18

What do line spectrums show?

Answer 18

Evidence that electrons in atoms exist in discrete energy levels as atoms can only emmit photons with energies equal to difference between energy levels

Question 19

What is line absorption spectrum?

Answer 19

When white light is passed through a cool gas. Photons of correct wavelengths are then absorbed by electrons to excite them to higher energy levels. Wavelengths are then missing from continuous spectrum of outside the gas - these are the absorbed wavelengths.

Question 20

What shows that light acts asa particle and what shows that it acts as a wave?

Answer 20

particle - photoelectric effect
wave - diffraction

Question 21

What is the de broglie equation?

Answer 21

de broglie wavelength = planck’s constant/ momentum

Question 22

What does the de broglie equation do?

Answer 22

• relates wave property (wavelength) to particle property (momentum)
• Showing particles can show wave like properties

Question 23

Describe how electrons where shown to have wave like properties?

Answer 23

electrons accelerated to high velocities in a vacuum and then passed through a graphite crystal. They diffract just like waves as they pass through spaces between atoms of crystal. They produce pattern of rings seen in electron diffraction tube

Question 24

Why does spread of lines in diffraction pattern increase as the velocity decreases

Answer 24

using de broglie’s equation, as the velocity decreases, momentum decreases, so the wavelength increases - meaning diffraction spacing increases.

Question 25

What effect does increasing frequency of light have on emitted electrons?

Answer 25

Increases energy of light, and therefore increases kinetic energy of photo electrons emitted

Question 26

What effect does increasing the intensity of the light have on emitted electrons?

Answer 26

More photons are striking the surface so amount of photoelectrons emitted increases

Question 27

Why might a photoelectron released not have maximum kinetic energy?

Answer 27

Photon and electron have a 1 on 1 interaction. Electron below the surface needs to do work to reach the surface

Question 28

How does photoelectric effect support the particle nature of EM waves?

Answer 28

light travels as photons. photons have energy that depends on frequency; if frequency below the threshold frequency, no emission even if intensity increased because the energy of the photon is less than the work function. Wave theory can not explain this as energy of wave increases with intensity. Lack of time delay and 1 on 1 interaction between photon and electron could not be explained by wave model

Question 29

At what energy is the ionisation level (gives ionisation energy) on an energy level diagram

Answer 29

0 eV/ 0J

At the top of the energy level diagram

Question 30

What happens when an atom in the ground stateis given more energy than it’s ionisation energy?

Answer 30

Electron in the ground state leave the atom with remianing energy as kinetic. Electrons in the higher energy levels fall to fill the vacancy in the lower levels - emitting photons

Question 31

What is ground state?

Answer 31

When electrons are in their lowest/minimum energy state/ most stable state

Question 32

Why is it easier to prove the wave-like nature of electrons, than protons?

Answer 32

Easier to accelerate electrons than protons/ easier to get wavelength the same as scattering object

Question 33

Explain how emission of light from the fluorescent screen shows that electrons incident on it are behaving as particles?

Answer 33

Electrons must provide enough kinetic energy to cause excitation - energy transfer in a 1 to 1 interaction (energy not provided over time like it would be in a wave). Electrons collide with atoms in fluorescent screen causing it’s electrons to excite, which then de-excite - releasing photons as they drop back down to the lower energy level - emmitting light.