More about photoelectricity Flashcards
What did Planck suggest about energy levels??
the energy of each vibrating atom is quantised
only certain energy levels are allowed
can only be multiples of a certain amount - the Planck constant
he imagined energy levels to be like the rungs of a ladder, with each atom absorbing or emmiting radiation when it moved up or down a energy level
What happens when a conduction electron absorbs a photon?
its kinetic energy increases by an amount equal to the energy of the photon
if the energy of the photon exceeds the work function of the metal, the electron can leave the metal
if the electron does not leave the metal, it collides repeatedly with other electrons and positive ions and loses its extra KE
What is a vacuum photocell?
a glass tube that contains a metal plate, referred to as the photocathode, and a smaller metal electrode (anode)
How does a vacuum photocell work?
when light above the threshold frequency is directed at the photocathode, electrons are emitted from the cathode and are attracted to the anode
this causes a photoelectric current
What is the equation for the number of photoelectrons per second?
photoelectric current / e (charge of an electron)
What is the photoelectric current proportional to?
photoelectric current is proportional to the intensity of the light incident on the cathode
the intensity is a measure of the energy per second carried by the incident light, which is proportional to the number of photons per second
What effect does the intensity of the light have on the max KE?
And what is the equation?
intensity does not effect the max KE
no matter how intense the light is, the energy gained is due to one photon only
Ekmax = hf - ϴ
ϴ = work function
What can be used to measure the max KE of the photoelectron emitted?
max KE of the photoelectrons emitted for a given frequency can be measured using a photocell
What does a graph of Ekmax against f look like?
a straight line in the form y = mx + c
y = Ek and x = f
gradient m = h
y intercept, c = -ϴ
the x intercept is equal to the threshold frequency
What effect does increasing the intensity have?
more photons
same energy
Why do emmitted electrons have range of kinetic energies up to a maximum value?
energy of photon is constant
electron has to overcome work function
deeper electrons require more energy
What is the physics definition of ionisation energy?
minimum energy to remove an electron from an atom in the ground state
Electromagnetic radiation is incident on the photoemissive surface
explain why there is a currently only if the frequency of the electromagnetic radiation is above a certain value
increase frequency, increase energy
increase energy of electrons, able to travel through circuit
above work function
What is the effect on current when the intensity of electromagnetic radiation is increased?
increase current
increased number of photon per second
increase no. of electrons liberated
current is rate of flow
The student increases the frequency of the electromagnetic radiation
explain the effect this has on the stopping potential?
increase energy of photons
increases stopping potential
because the energy of the emitted photons is increased
energy of stopping potential has to be equal to the energy of the emitted electrons
