Quantum phenomena Flashcards
Describe the gold leaf electroscope parts
A metal plate attached to a right metal pole. Attached to this is a piece of gold foil.
What causes the flexible gold foil to repel away from the metal pole?
Adding extra electrons
-This makes it negatively charged
-So both the pole and foil are negatively charged
-So it repels away
What happens when light of a certain frequency is shone into the metal plate?
The gold foil falls back down
Why does the gold foil fall back down when light of a certain frequency is shone on it?
-The light causes electrons to be released from the metal
-So apparatus becomes negatively charged
-So the repulsion weakens
What happens if you charge the gold leaf electroscope experiment positively?
If the metal is positively charged then the negative electrons are attracted back to the plate so it’s harder for them to escape.
What did wave theory predict about frequency?
Wave theory predicted that any frequency of light should cause the photoelectric effect
How does frequency actually effect the photoelectric effect?
Only above a certain threshold frequency electrons are emitted
What did wave theory predict about light intensity?
Wave theory predicted increasing light intensity would increase the energy of electrons emitted
How does light intensity actually effect the photoelectric effect?
As long as you’re in the threshold frequency increasing intensity increases the amount of electrons. Energy stays the same.
What does the existence of threshold frequency show?
Energy must be delivered in packets and must be proportional to the frequency of the wave.
All delivered in one go as electrons can’t store energy
What is the photoelectric effect equation?
hf=ϕ+Ekmax
How does a vacuum photocell work?
As you increase the voltage the eye becomes more negative and the smile becomes more positive
-So it’s harder for electrons to escape
Vacuum photocell equation
eVs=Ekmax
What happens if you heat up (excite) a gas?
You can cause it to glow.
How can you heat up (excite) a gas?
You can heat them up by passing a very high current through them.
The high current is made of fast moving electrons.
How do you see the multiple photons in a glowing gas?
Splitting light using a prism / diffraction grating
What happens if you de excite (cool) a gas and pass white light through?
Certain photons are blocked
Energy level rules
-Electrons can move up/down
-If they absorb energy they move up
-If they emit energy they move down
-Electrons cannot exist inbetween levels
What is energy level 1
The ground state
Electrons are most stable here
Electrons have the least energy
What is the ionisation level?
When electron has completely left the atom
How can electrons gain energy?
Absorbing a photon or get hit by another electron
What allows an electron to absorb a photon?
An electron can only absorb a photon of it gives it exactly the right amount of energy to move up 1 or more whole levels
Ionisation energy
The minimum energy needed to remove an electron from the atom from the ground state
Explain why an electroscope won’t show the photoelectric effect if it’s positively charged
Electrons would be attracted back by the positive charge
So the gold foil won’t stop repelling
What is the threshold frequency evidence for?
Light can exist in discrete energy packets called photons
As the electrons need all energy to be transferred at the same time to be emitted
Explain why increasing the intensity of the light increases the amount of photoelectrons emitted?
Increasing the intensity increases the number of photons striking the plate per second.
For each photon an electron is emitted.
So the number of photoelectrons released per second increases.
Define the work function of a metal
The minimum energy needed for an electron to be released from the surface of the metal
Define the threshold frequency
The minimum frequency needed for an electron to be released from the surface
When electrons are excited to a higher energy level explain why different frequencies are possible
Electrons cascade rather than going straight to the ground state
Describe how the concept of energy levels is useful in the explanation of line spectra
Only certain energy changes allowed
A line corresponds to a transition between levels
Each transition corresponds to a definite wavelength
What happens in an atom when line spectra are produced?
Electrons move up and down energy levels
By emitting or absorbing a definite photon energy
Explain what is meant by the duality of electrons
Electrons sometimes behave like particles and sometimes behave like waves
Evidence that demonstrates electrons have particle properties
Deflection in E field
State what is meant by the wave particle duality of electromagnetic radiation
EM radiation sometimes behaves like waves and sometimes behaves like particles
Explain why the kinetic energy of emitted electrons has a maximum value
hf is energy received from photons
Energy required to remove the electron varies
So KE of electrons will vary
Evidence for light behaving like particles
Photoelectric effect
Existence of threshold frequency
Evidence for light behaving like waves
Diffraction
Interference pattern seen
Evidence for electrons behaving like particles
Deflection in magnetic field
Curved track of electron
Evidence for electrons behaving like waves
Electron diffraction
Interference pattern seen
How does the wavelength relate to the diffraction gap
Same order of magnitude
What does the mercury electron emit when it returns to ground state
Ultraviolet photon
Why must a photon have an exact amount of energy whereas a free electron only needs a minimum amount of kinetic energy
For a transition an exact amount of energy is needed
All of the photons energy is absorbed (1:1 interaction)
Electrons can transfer part of their energy and have a lower speed
How does the phosphorus coating work in fluorescent tubes?
-UV photons excite electrons in ground state of coating to higher energy level
-Electrons cascade back down releasing lower energy photons
How do fluorescent lamps work?
-High pd accelerates electrons across tube
-These electrons collide with electrons in the ground state of the mercury
-The mercury electrons gain energy and move to higher levels
-When the move back down UV photons released
-Phosphorous coating turns then into visible light photons
