Chapter 3

Question 1

Describe the photoelectric effect

Answer 1

The emission of electrons from the surface of a material due to exposure of the material to electromagnetic radiation

Question 2

Briefly describe the key points in the experimental observations of the photoelectric effect

Answer 2

• There is a threshold frequency
• There is a variation of the threshold frequency
• There is an affect of the radiation intensity
• There is a time of emission
• There is a minimum KE of photoelectrons

Question 3

Observations of photoelectric effect: Expand on the “threshold frequency”

Answer 3

The photoelectric effect only happens if the frequency of the EM radiation is above a certain value

Question 4

Observations of photoelectric effect: Expand on the “variation of the threshold frequency”

Answer 4

Different materials require different amounts of energy to release electrons. TF varies with materials

Question 5

Observations of photoelectric effect: Expand on the “affect of radiation intensity”

Answer 5

The greater the intensity, the greater the number of electrons emitted, but only if the radiation is above the threshold frequency

Question 6

Observations of photoelectric effect: Expand on the “time of emission”

Answer 6

Electrons were emitted as soon as the material was exposed

Question 7

Observations of photoelectric effect: Expand on the “minimum KE of photoelectrons”

Answer 7

Depends only on the frequency of the EM radiation and the material exposed, not the intensity

Question 8

What are the problems with the wave theory which the photoelectric effect has?

Answer 8

Wave theory predicted that:

• No threshold frequency -> All frequencies of radiation should eventually cause emission
• Increasing intensity should increase emission of all frequencies -> Not just those >threshold
• Emission wouldn’t take place immediately -> weaker radiations would take longer to produce electrons

Question 9

What is Einstein’s explanation for the photoelectric effect and the clashes with the wave theory?

Answer 9

• EM radiation consists of photons
• Energy of photons depends on frequency of radiation ONLY. This energy is proportional to E=hf
• Photons interact with one to one interaction with electrons with the material
• If photon energy > certain amount, an electron would be emitted, and excess energy goes into electron KE

Question 10

What is the work function in: hf = Φ + EK max?

Answer 10

The minimum amount of energy needed for an electron to escape the surface

Question 11

What is stopping potential?

Answer 11

The minimum potential to stop photoelectric emission.

Question 12

What is an ion?

Answer 12

A charged atom

Question 13

How are ions created?

Answer 13

By adding or removing electrons

Question 14

How can ionisation be caused?

Answer 14

• Nuclear radiation
• Heating
• Passing an electric current through a gas

Question 15

What is an electron volt?

Answer 15

A unit of energy equal to work done when an electron is moved through a p.d of 1V

Question 16

What is ionisation energy?

Answer 16

The energy required to remove one electron from an atom

Question 17

What is excitation?

Answer 17

The promotion of electrons from lower to higher energy levels within an atom

Question 18

In excitation, where do electrons move in relation to the shell?

Answer 18

Electrons move from an inner to an outer shell

Question 19

What is some of the KE of an incoming electron used to do in excitation?

Answer 19

Used to move the electron to a higher energy level

Question 20

How many excitation states and energies to atoms have?

Answer 20

Multiple

Question 21

How are electrons bound to the nucleus of an atom?

Answer 21

EM attraction

Question 22

What is the ground state?

Answer 22

The energy level of an electron occupying the nearest possible position to the nucleus

Question 23

What is the lowest possible energy level for an electron called?

Answer 23

The ground state

Question 24

How many electrons can exist in the lowest possible energy level?

Answer 24

2

Question 25

How are energy levels measured?

Answer 25

In respect to ionisation energy, which is assigned 0eV

Question 26

If the ground state has an energy level, what is true about all other energy levels?

Answer 26

They are negative

Question 27

When are electrons considered to be in ionisation state?

Answer 27

When their energy levels are above the ground state but below the ionisation level

Question 28

How stable are excited states?

Answer 28

very unstable

Question 29

How long does it take an electron to fall back to a lower energy level?

Answer 29

about 10^-6s

Question 30

What happens when an electron falls through energy levels?

Answer 30

A photon of EM radiation is emitted

Question 31

What is de-excitation?

Answer 31

When an electron moves from a higher to lower energy level (and a photon is emitted)

Question 32

What is true about the energy of a photon emitted in de-excitation?

Answer 32

It is equal to energy lost by the electron

Question 33

What is a fluorescent tube?

Answer 33

A glass tube with a fluorescent coating on its inner surface. Contains mercury vapour at low pressure

Question 34

Why does light have a dual nature?

Answer 34

It can behave as a particle and a wave

Question 35

How is the wave-like nature of light observed?

Answer 35

Through diffraction

Question 36

How is the particle-like nature of light observed?

Answer 36

Photoelectric effect