Topic 12: Quantum and nuclear physics (HL) Flashcards
What is the photoelectric effect?
The phenomena by which electrons are emitted from the surface of some metals when the surface is illuminated with EM radiation (usually UV)
How many electrons are emitted below the threshold frequency, f0?
No electrons are emitted
Which variable affects the maximum kinetic energy of the emitted electrons?
It depends on the frequency of the incident light
Which variable affects the number of the emitted electrons?
Depends on the intensity of the incident light
What is the delay between light hitting the surface and the emission of the electrons as a result?
No noticeable delay
What does the photoelectric effect show?
Why?
Wave-particle duality. Light is acting as a particle in this instance.
This effect could not happen with the view of light as a wave as any frequency would eventually bring enough energy to the metal plate i.e. there would not be a threshold frequency
What is the setup for the photoelectric effect experiment?
- A variable power supply (for accelerating p.d.)
- Voltmeter in parallel, micro-ammeter in series
- Cathode and anode in a vacuum
- (UV) Light transmitted onto the cathode
Explain why a variable p.d. is used?
The p.d. decelerates the electrons. At a certain value of potential, the stopping potential (VS) no more photocurrent is observed.
(Electrons are brought to rest before arriving at the anode)
What variable affects the stopping potential?
The frequency of the light - linear relationship
What can be calculated using the stopping potential?
The maximum kinetic energy of the electrons as p.d. is energy/charge (we know the charge)
Equation for KEmax of electrons
KEmax = VSe
where VS is the stopping potential
e is the electron charge
Equation for the maximum velocity of electrons
Vmax = √ ( 2 Vs e / m)
where Vs is the stopping potential
e is the electron charge
m is the electron mass
What is the work function?
the minimum energy required for an electron to overcome the attractive forces that act on it within the metal, allowing it to be emitted as a photoelectron
What variable sets the energy of a photon?
the frequency
(E = hf)
What variable sets the amount of photons per second?
the intensity
What happens to the energy absorbed by electrons if it is below the work function?
It will still gain the energy but will quickly be shared out to other electrons, so cannot by emitted
What happens to the energy absorbed that is over the work function?
It is converted into kinetic energy for the electron
Equation for energies in the photoelectric effect
KEmax = Ep - ϕ
Ep = ϕ + KEmax
hf = ϕ + Vse
What is the y-intercept on a energy-frequency diagram for the photoelectric effect?
Negative work function (-ϕ)
What proof do we have that light can behave like waves?
- Reflection
- Refraction
- Diffraction
- Interference
What proof do we have that light is a particle?
- Photoelectric effect
If light waves have particle properties, do other particles have wave properties?
Yes.
High speed electrons can be diffracted through polycrystalline graphite,
What happens to the electron diffraction pattern if the acceleration voltage is decreased?
Intensity decreases and separation increases
What happens to the electron diffraction pattern if the acceleration voltage is increased?
Intensity increases and separation decreases.