Quantum Flashcards
What is the photoelectric equation?
hf=ф+KEmax
Definition of threshold frequency
The minimum frequency of electromagnetic radiation that would cause the emission of an electron
Definition of hf
Energy of a photon in Joules
Definition of KEmax
The maximum kinetic energy of an electron in Joules
Definition of work function (ф)
Minimum energy required to cause the emission of an electron from a metal surface
How does the intensity of the light affect the Kinetic Energy of the electrons
It doesn’t as intensity only affects the rate at which photons are emitted, the actual interaction of photons and electrons occurs on a one to one basis
Definition of 1eV
1 electron volt is 1.6x10^-19
- An electron volt is the energy acquired by an electron when accelerated through a p.d. of 1 volt
Describe the photoelectric effect
The photoelectric effect is the emission of electrons from the surface of a metal when electromagnetic radiation above the threshold frequency is incident upon it.
State de Broglie’s equation
λ=h/p or λ=h/mv
What is a photon?
A quantum of light energy/a packet of energy
If the accelerating potential difference in an electron gun is increased, qualitatively explain the effect on the interference pattern of electrons passing through a thin piece of polycrystalline graphite:
If V increases then v increases since eV=1/2mv^2, if v increases then p increases since p=mv, if p increases then de Broglie wavelength decreases since 𝛌=h/p, if 𝛌 decreases then fringe spacing decreases.
What is the name given to electrons emitted due to the photoelectric effect?
Photoelectrons
How does the photon model explain why photoelectrons are emitted immediately once the incident light is above threshold frequency in the photoelectric effect?
One electron interacts with one photon, so if an electron gains enough energy to emitted from one photon it will be emitted immediately.
How does the photon model explain why KEmax of emitted photoelectrons does not change when intensity of incident light increases?
Increasing intensity increases rate of arrival of photons but not energy of individual photons (since E=hf), each photon has one-to-one interaction with an electron, so rate of photoelectron emission increases but not KEmax.
What is the relationship between intensity of incident light and rate of photoelectron emission (as long as frequency of the light is above threshold frequency)?
Rate of emission of photoelectrons is directly proportional to intensity, because intensity is directly proportional to the rate of arrival of photons.
Name two wave behaviours that electrons have been observed to exhibit:
-> Diffraction
-> Interference
How does the photon model explain why KEmax of emitted photoelectrons increases with increasing frequency of incident light (independent of intensity)?
Photons have one-to-one interaction with electrons, so if frequency increases (above threshold frequency) excess energy gained by photoelectron goes into electron’s kinetic energy store, increasing KEmax.
Describe what is meant by wave-particle duality:
All particles can behave as waves (with de Broglie wavelength 𝛌=h/p) or as particles, and waves can behave as waves or particle (e.g. the photon model)
How does the photon model explain why no photoelectrons are emitted if the frequency of the incident light is below the threshold frequency?
An electron in the metal can only absorb energy from a single photon, gaining energy hf, the electron is only emitted if this energy is greater than the work function of the metal.
Define the electronvolt
One electronvolt is the energy transferred to or from an electron when it moves through a potential difference of 1 volt.
How can electron diffraction be observed?
By aiming a beam of electrons at a thin slice of polycrystalline graphite in a vacuum and observing the path of the electrons on a fluorescent screen.
For KEmax against f graph for incident light on a particular metal surface, what do the y intercept and gradient represent?
y-intercept (of the line of best fit) is the negative of the work function, gradient is the Planck constant.
What does the photoelectric effect give evidence for?
The particle nature of light (the photon model)
Convert 1eV to J
1eV = 1.6x10^-19J
Describe the photon model of light:
Electromagnetic radiation has a discrete quanta of energy, given by E=hf
Give the quantities and their units for the equation E=hf
-> E = photon energy (J)
-> h = Planck constant (6.63 x 10^-34Js)
-> f = frequency of electromagnetic radiation (Hz)
What effect does doubling the accelerating potential difference have on the de Broglie wavelength of an electron fired from an electron gun?
eV = 1/2mv^2 = p^2/2m
so V is proportional to p^2
and 𝛌 = h/p, doubling V increases p by a factor of squroot2 so decreases 𝛌 by a factor of squroot2
