Chapter 3 Flashcards
Explain the photoelectric effect
When the plate is charged the hold lead rises, if uv light is incident on the plate the leaf falls, but if red light is incident on the plate it doesn’t fall.
Why does the photoelectric effect not make since if light is only a wave
If it was just a wave, energy would continuously transfer to the electrons so they would eventually gain enough energy to leave the surface regardless of the incident lights frequency, but in this we see electrons emitted without delay and only with an incident light above a certain frequency.
What did Einstein propose about the photoelectric effect?
One electron absorbs one photon of energy, E=hf, the electron gains enough energy to leave the surface above a threshold frequency.
Equation for the photoelectric effect
E = hf - work function
Kinetic energy of emitted photoelectron = hf of incident photon - the work function
Define work function of a metal
The energy required to remove an electron from the surface of a metal
What happens in the hf of the incident photon is less that the work function of the metal?
the electron still gains energy but quickly loses its extra kinetic energy through collisions.
What is the photoelectric current proportional to?
It is proportional to the intensity of the incident light, provided the frequency of the photons is above the threshold frequency.
Equation involving stopping potential
E (kmax) = hf - work function - qV s (electron charge x stopping potential)
If the surface of a metal is positively charged what is the extra work needed to free the electron from the surface equal to
qVs
What are the 2 things that can happen when a electron collides with an atom?
It can:
- Excite a shell electron
- Ionise it
How can we increase the kinetic energy of the electrons emitted from a filament?
By increasing the potential difference between the filament and the anode
How can we measure the ionisation energy?
Measure a current, once electrons have enough kinetic energy to ionise the gas atoms, the current drastically increases
Ionisation energy = electron charge x tube potential difference.
What happens when an atom absorbs energy?
It becomes excited and an electron moves to a shell at a higher energy level.
What are the 2 ways an atom can absorb energy?
Via collision (most commonly electrons)
Via photons
What happens after an electron moves to a higher energy level?
A vacant place is left in the lower shell so the electron de-excites back down to the lower shell , emitting the energy as a photon
Requirement for a photon to ionise fully
If their energy is equal to/ greater than the ionisation energy
Requirement for a photon to excite
If the photon energy is exactly the correct amount to change energy level (as photons can’t be partially absorbed)
What are the started units used for in a fluorescent tube?
To heat the filaments sufficiently to allow the mains potential difference to ionise the mercury vapour.
Steps of the lighting of a fluorescent tube
1) Electrons ionise and excite the Mercury
atoms
2) Mercury de-excitation emits UV photons
3) The fluorescent coating absorbs the UV
photons and excites
4) The de-excitation of the fluorescent
coating emits visible photons.
How is a line emission spectrum formed?
It is formed by a tube of glowing gas showing discrete lines of colour
What is the difference between the line spectra of different elements
Each element/ chemical has its own line spectrum that can be used to identify it
What do the lines of a line spectrum represent?
They show a specific photon emitted from specific electron de-excitations
What is formed when white light is passed through a prism and why?
A continuous spectrum is formed as it transmits all wavelengths of white light
What is an example of light behaving as a wave?
Diffraction
- Light spreads after a gap, spreads greatest when the wavelength is about the same size as the gap width
What is an example of light behaving as a particle?
The photoelectric effect
- Electrons are only emitted if the incident photons are above a threshold frequency
If waves can behave as particle (light waves) then what this tell us about what particles do?
It suggests that particles should be able to behave as waves
What did de Brogue say about particles wavelength when acting as a wave (1923) and what is the equation?
He said paryicles behave like waves with a characteristic “de Brogue” wavelength
Wavelength = h/p
h/mv
p= momentum
