Chapter 3 Flashcards
Describe the photoelectric effect
The emission of electrons from the surface of a material due to exposure of the material to electromagnetic radiation
Briefly describe the key points in the experimental observations of the photoelectric effect
- 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
Observations of photoelectric effect: Expand on the “threshold frequency”
The photoelectric effect only happens if the frequency of the EM radiation is above a certain value
Observations of photoelectric effect: Expand on the “variation of the threshold frequency”
Different materials require different amounts of energy to release electrons. TF varies with materials
Observations of photoelectric effect: Expand on the “affect of radiation intensity”
The greater the intensity, the greater the number of electrons emitted, but only if the radiation is above the threshold frequency
Observations of photoelectric effect: Expand on the “time of emission”
Electrons were emitted as soon as the material was exposed
Observations of photoelectric effect: Expand on the “minimum KE of photoelectrons”
Depends only on the frequency of the EM radiation and the material exposed, not the intensity
What are the problems with the wave theory which the photoelectric effect has?
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
What is Einstein’s explanation for the photoelectric effect and the clashes with the wave theory?
- 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
What is the work function in: hf = Φ + EK max?
The minimum amount of energy needed for an electron to escape the surface
What is stopping potential?
The minimum potential to stop photoelectric emission.
What is an ion?
A charged atom
How are ions created?
By adding or removing electrons
How can ionisation be caused?
- Nuclear radiation
- Heating
- Passing an electric current through a gas
What is an electron volt?
A unit of energy equal to work done when an electron is moved through a p.d of 1V
What is ionisation energy?
The energy required to remove one electron from an atom
What is excitation?
The promotion of electrons from lower to higher energy levels within an atom
In excitation, where do electrons move in relation to the shell?
Electrons move from an inner to an outer shell
What is some of the KE of an incoming electron used to do in excitation?
Used to move the electron to a higher energy level
How many excitation states and energies to atoms have?
Multiple
How are electrons bound to the nucleus of an atom?
EM attraction
What is the ground state?
The energy level of an electron occupying the nearest possible position to the nucleus
What is the lowest possible energy level for an electron called?
The ground state
How many electrons can exist in the lowest possible energy level?
2
How are energy levels measured?
In respect to ionisation energy, which is assigned 0eV
If the ground state has an energy level, what is true about all other energy levels?
They are negative
When are electrons considered to be in ionisation state?
When their energy levels are above the ground state but below the ionisation level
How stable are excited states?
very unstable
How long does it take an electron to fall back to a lower energy level?
about 10^-6s
What happens when an electron falls through energy levels?
A photon of EM radiation is emitted
What is de-excitation?
When an electron moves from a higher to lower energy level (and a photon is emitted)
What is true about the energy of a photon emitted in de-excitation?
It is equal to energy lost by the electron
What is a fluorescent tube?
A glass tube with a fluorescent coating on its inner surface. Contains mercury vapour at low pressure
Why does light have a dual nature?
It can behave as a particle and a wave
How is the wave-like nature of light observed?
Through diffraction
How is the particle-like nature of light observed?
Photoelectric effect
