Photoelectric effect and Energy levels Flashcards
What 3 particles are atoms of all elements made up of
Protons, neutrons and electrons
What is the specific charge of a proton
+1.60 x 10^-19 C
What is the specific charge of a neutron
0
What is the specific charge of an electron
-1.60 x 10^-19 C
Mass of a proton
1.673 x 10^-27
Mass of a neutron
1.675 x 10^-27
Mass of an electron
9.11 x 10^-31
Relative charge of a proton
+1
Relative mass of a proton
1
Relative charge of a neutron
0
Relative mass of a neutron
1
Relative charge of an electron
-1
Relative mass of an electron
1/2000
Why is a stable atom neutral
Since protons and electrons have equal and opposite charges and a stable atom has an equal number of both, the overall charge is neutral
Specific charge of a particle
Ratio of its charge to its mass
Specific charge =
charge / mass
What is the SI units for specific charge
C/kg
How to calculate the specific charge of a particle
- Determine the number of neutrons and protons
- Calculate the total mass by doing mass number x mass of 1 nucleon (1.67 x 10^-27)
- Calculate the total charge by doing number of electrons x ( -1.60 x 10^-19)
- Substitute values into the equation
What will the specific charge be is there is a gain in electrons
Negative
Nuclide
A group of atoms containing the same number of protons and neutrons
What does the A, Z represent in the notation
A - nucleon number/ mass number
Z - proton number
Nucleon number
Total number of protons and neutrons in the nucleus
Isotopes
An atom of the same element that has the same/ equal number of protons but a different number of neutrons
Why are isotopes unstable and what to they do as a result
They have an imbalance of neutrons and protons this means they constantly decay and emit radiation to achieve a more stable form
Isotopic data
Relative amounts of different isotopes of an element found within a substance
What is isotopic data used for
To identify an isotopic signature within organic and inorganic materials
Used in radioactive dating
What is an electronvolt used to represent
Very small energies because quantum energies tend to be much smaller than 1J
Equation for voltage in terms of energy and charge
V = E / Q
1 eV = how many joules
electron of charge 1.6x10^-19 x 1V = 1.6x10^-19J
What is the definition of an electronvolt
The energy gained by an electron travelling through a potential difference of one volt
What happens to a charge particle when it is accelerated through a PD
it gains KE
Equation for eV if an electron accelerates from rest
eV = 1/2mv^2 as eV=KE gained here
What is the photoelectric effect
The phenomena in which electrons are emitted from the surface of a metal upon absorption of EM radiation
What are electrons removed from a metal called
Photoelectrons
Why is the photoelectric effect important
It is evidence that light is quantised/carried in discrete packets
How do we know light is quantised or carried in discrete packets
Each electron can absorb only a single photon and this means ONLY the frequencies of light above a threshold frequency will emit a photoelectron
Threshold frequency
The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal
Threshold wavelength
The longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a metal
High frequency =
Low wavelength
Work function
Minimum energy required to release a photoelectron from the surface of a metal
How many photons can a single electron absorb
1
When can an electron escape from the surface of the metal
If it absorbs a photon which has an energy equal to the work function or higher
What colour of visible light has highest energy
Violet as it has highest frequency
What colour of visible light has the lowest energy
Red
Stopping potential
The potential difference required to stop photoelectron emission from occurring
Emitter plate
The photons arriving at the metal plate causing photoelectrons to be emitted
Collector plate
Electrons that cross the gap are collected at this other metal plate
How is a photoelectric current produced
There is a flow of electrons across the gap which results in an emf between the plates that causes a current to flow around the rest of the circuit and it therefore becomes a photoelectric cell with a photoelectric current.
What happens to the electrons which escape with enough KE
They can overcome the attraction between them and the emitter plate and can cross to the collector plates
Energy of an incident photon =
Work function + Maximum kinetic energy of the photoelectron
Energy of a photon =
Planck’s constant x frequency
What happens to the energy within a photon when it strikes the surface
It is transferred to the electron to release it from the surface of a metal and the remaining energy is converted to KE to the emitted photoelectron
Photoelectric equation
E= hxf = work function + KE max
Why is the KE of the photoelectrons independant of the intensity of radiation
Each electron can only absorb one photon
What is the KE dependant on
The frequency of the incident radiation
What is intensity a measure of
The number of photons incident on the surface of the metal
Where are the photoelectrons with max KE found
Surface of the metal as they dont require much energy to leave the material
What is photoelectric current
Number of photoelectrons emitted per second
What is photoelectric current proportional to
Intensity
Ionisation of an atom
The removal, or addition, of an electron from, or to, an atom when given sufficient energy
Excitation of electrons
When an electron is given enough energy to move up an energy level, but not enough to leave the atom
When does fluorescence occur
When an electron in an atomic orbital absorbs energy from an interaction with a photon or collision with another atom
What happens to the electrons when an electric current is passed through the vapour in a FL tube
They are excited and move to a higher energy level
Why do electrons de-excite
Higher energy level is unstable
What do the electrons do as they de-excite
Release some of the energy in the form of a UV photon
What causes the fluorescent glow
The UV light released excites the electrons in the phosphor coating and so visible light photons are released which causes the glow
What is a photon
A discrete packet of EM energy with no mass
How can electrons gain energy and move up the energy levels
If it absorbs energy by :
Collisions with other atoms or electrons
Absorbing a photon
A physical source e.g. heat
How can elements be identified by their line spectrum
No 2 elements can emit the same set of spectral lines
Difference between 2 energy levels =
Energy of lower level - Energy of the higher level
Describe how a fluoroscent tube works
The tube contains low presure mercury gas and thermionic emission causes electrons to be released from the cathode.
High voltage accelerates electrons from one end to the next
Electrons collide with the atoms and transfer some KE to excite the electrons in the shells to go in energy levels.
Then, they de-excite and emit a photon (mainly UV) due to high frequency
The UV is absorbed by the phosphorous paint coating, exciting an electron, and then de-excite, emitting visible light
State and explain the effected on the emitted electrons by increasing the frequency of light (2)
The max KE of the released electrons increases because by increasing the frequency it also increases the energy
State and explain the effect on the emitted electrons by increasing the intensity of light (2)
It will increase the number of electrons being emitted because there are now more photons striking the metal surface per second.
Explain what is meant by validated evidence (2)
Been proven corrected
Experiment/observation needs to be performed
Explain why the KE of the emitted electrons has a maximum value (2)
Same energy from photons
Energy required to remove electron varies
Explain why the emitted electrons have a range of kinetic energies up to a maximum value (4)
Photons have energy dependant on frequency
There is a 1 to 1 interaction between photon and electron
KE max = energy of photon- work function
More energy needed to remove deeper electrons
Describe the process by which mercury atoms become excited in a fluroscent tube (3)
Electrons flow through the tube, and collide with mercury atoms raising the electrons to a higher energy level
What is the purpose of the coating on the inside surface of the glass in a fluorescent tube (3)
Photons emitted from mercury atoms are high energy photons.
These photons are absorbed by the powder.
The powder emits photons in the visible spectrum.
Explain the difference between excitation and ionisation (3)
In either case, an electron receives energy
Excitation is when electrons absorb a photon and so gain enough energy to move up to a higher energy level.
Ionisation is when an electron receives enough energy to leave the atom
Explain why only photons of certain frequencies cause excitation in a particular atom (4)
Different atoms have different energy levels.
Electrons need to absorb a photon with enough energy to move up energy levels
High frequency = high energy
Electrons occupy discrete energy levels
How can an electron gain energy and move up energy levels
By absorbing energy either by:
Collisions with other atoms or electrons
Absorbing a photon
A physical source such as heat
Line Spectra
A phenomenon which occurs when excited atoms emit light of certain wavelengths which correspond to different colours.
The emitted light can be observed as a series of coloured lines with dark spaces in between.
Why can elements be identified by their line spectrum
Each element produces a unique set of spectral lines.
No two elements emit the same set of spectral lines
Emission spectra (when electron goes to lower energy level and emits a photon)
Contains a set of discrete wavelengths, represented by coloured lines on a black background
What is line spectra evidence of
Transitions between discrete energy levels in atoms
Absorption spectra (when atom becomes excited by absorption of a photon)
A continuous spectrum containing all the colours with dark lines at certain wavelengths.
What do the dark lines in an absorption spectra mean
They correspond exactly to the differences in energy levels in an atom
When excited electrons return to lower levels why are some wavelengths missing
The photons are emitted in all directions rather than in the original direction of the white light
difference between 2 energy levels ( E2 - E1) =
h x f
What is wave-particle duality of light
Light can behave as a particle (i.e. photon) and a wave
How does light act a a particle
Light interacts with matter such as electrons
Evidence for light acting as a particle
Photoelectric effect
How does light act as a wave
Light propagates through space as a wave
Evidence for light acting as a wave
Diffraction
Interference of light in Young’s Double Slit experiment
Explain the photon model of light
EM waves carry energy in discrete packets called photons
E = hf
Each electron can absorb only a single photon - this means only the frequencies of light above the threshold frequency will emit a photoelectron
How do electrons behave like waves
They can be diffracted, and produce a pattern of concentric rings
de broglie wavelength =
h / mv where mv is momentum
Electron diffraction pattern when at low accelerating voltage
Low speed therefore low KE
Longer wavelength and so more diffraction/greater radius
How does momentum affect wavelength
Smaller momentum = Longer wavelength because p=mv
Larger momentum =
Shorter wavelength
Process of the development of scientific theories
Evaluation of the theory
Peer review
Validation when there is enough evidence
What is checked during the peer review process
Validity
Originality
Significance
How to evaluate scientific claims
Check if :
It is repeatable and reproducible
How are KE and stopping voltage related
KE = eVs
As KE increases the stopping voltage also increases
