4.5 - Quantum Flashcards
When EM radiation interacts with matter, what does it interact as?
Photons
What are photons?
Discrete energy quanta (packets)
Why do we use electronvolts?
Because the energy of a photon is very small when measured joules, so electronvolts are more appropriate.
Define 1eV
The energy transferred when an electron travels through a potential difference of 1 volt.
How many joules in 1 eV?
1.60x10^-19
using W=VQ
How can we use LEDs to determine the Planck constant?
Vary the PD across an LED to determine the threshold PD required to turn it on. LED produces light of a specific colour so we know wavelength.
Each photon is emitted when a single electron loses energy. By equating energy of an individual electron in the LED with an individual photon produced, we can use eV = hc/λ
How can we improve the accuracy of the LED experiment?
Repeat the experiment with a variety of different coloured LEDs, each emitting different wavelengths of light.
What graph can be drawn after the LED experiment?
Draw V against 1/λ. The gradient will be equal to hc/e. c and e are known constants so we can calculate h from this.
Define the photoelectric effect.
The emission of electrons from the surface of a metal when EM waves, of greater frequency than the threshold frequency, are incident on the metal.
What is a gold leaf electroscope?
A negative zinc plate on top of a stem, with a negative piece of gold leaf attached to the stem.
What happens when UV light is shone onto the zinc plate of a gold leaf electroscope?
(Initially, the gold leaf and stem have the same charge so they repel)
Free electrons are released from the surface of the plate, and the negative charge is slowly lost so the leaf gradually falls back to the stem.
What happens if we shine visible light onto the zinc plate of a gold electroscope?
Electrons don’t get removed from the plate, regardless of the intensity.
Can the wave model of EM explain gold leaf electroscope observations?
No.
What does each electron on the surface of a metal require to escape?
A certain amount of energy.
What happens to the energy of a photon when it is incident on a metal surface?
It transfers its exact energy to one electron, with any excess energy transferred into kinetic energy.
Define the work function.
The minimum energy required to remove/emit a single electron from a metal surface.
Why do we have a threshold frequency for EM radiation?
We have a minimum energy, and energy is directly proportional to frequency.
How does the threshold frequency explain why visible light doesn’t produce the photoelectric effect (gold leaf) but UV does?
UV light has a frequency beyond the threshold frequency whereas visible light does not.
What is the photoelectric equation?
hf = Φ + KEmax
Φ = work function of the metal KEmax = maximum KE of the released electron
In the photoelectric equation, why is KEmax the maximum kinetic energy of the released electron?
Some electrons may be closer to a nucleus, requiring more energy than the work function amount to be released, leaving less energy left over as kinetic energy.
How does intensity come into play with the emission of electrons?
Changing intensity has no effect if the incident radiation has a frequency below the threshold frequency.
However, if it has a frequency above the threshold frequency, then increasing the intensity of the radiation will increase the rate of electron emission.
Why does increasing intensity increase rate of electron emission (for radiation above threshold frequency)?
The increase in intensity increases the number of photons available to interact with the electrons.
What is the only way of increasing the kinetic energy of ejected electrons?
By increasing the frequency further above the threshold frequency, so there is more energy left over to be converted to kinetic energy.
Define the threshold frequency
The minimum frequency of the incident photon to release an electron from the surface of a metal.
Define photoelectrons
Electrons emitted from the surface of a metal by the photoelectric effect.
The wavelength associated with a particle is inversely proportional to what?
Its momentum.
What is the de broglie equation?
λ = h/p =h/mv
What can be used as evidence for wave-particle duality?
Electrons. They have mass and charge. They can be accelerated and deflected by magnetic and electric fields. However, they can be made to diffract.
How can electrons be made to diffract?
When fired at a thin piece of polycrystalline graphite (material containing carbon atoms spread over many layers), they produce a diffraction pattern when hitting a screen.
How can an electron move up an energy level?
It must gain the exact amount of energy to make a transition. It can do this by another electron colliding with it, or by absorbing a photon of the exact energy.
What must be done by an electron when moving down a level?
It must lose the exact amount of energy when making the transition. This is released as a photon of energy equal to the energy lost.
What equation is used for finding energy changes between energy levels?
ΔE = hf = E1 - E2
E1 = energy of starting level E2 = energy of level the electron landed at
Define excitation
An electron moving up one or more energy levels when it gains the exact amount of energy required.
Define de-excitation.
When an electron gives out the exact amount of energy to move back down to its original energy level.
Why do the ground state and energy levels leading up to ionisation have negative values of energy?
They are compared to the ionisation level. Energy must be given to electrons to move up a level.
Transitions release a photon with a set amount of energy. This means what is also set?
Frequency and wavelength.
If you know the energy change from transitioning from one energy level to another, what can you figure out?
The frequency of the light, allowing us to know its colour.
