Electromagnetic radiation Flashcards
Electronvolt
The energy gained by an electron travelling through a potential difference of one volt
If an electron, with a charge of 1.6 × 10-19 C, travels through a potential difference of 1 V, the energy transferred is equal to:
E = Q V = (1.6 x 10^-19) C x 1V = 1.6 x 10^-19
therefore 1 eV = 1.6 x 10^-19 J
Electronvolt Relation to Kinetic Energy
When a charged particle is accelerated through a potential difference, it gains kinetic energy
If an electron accelerates from rest, an electronvolt is equal to the kinetic energy gained:
Electrons & energy levels
Electrons in an atom occupy certain energy states called energy levels
Electrons will occupy the lowest possible energy level as this is the most stable configuration for the atom
When an electron absorbs or emits a photon, it can move between these energy levels, or be removed from the atom completely
Excitation
When an atomic electron receives (exactly the right amount of) energy to move to a higher energy level. This is through either the collision of a free electron / by the absorption of a photon.
When an electron moves to a higher energy level, the atom is said to be in an excited state
De-excitation is when electrons can also move back down to a lower energy level.
To de-excite an electron to a lower energy level, it must emit a photon
Ionisation
Ionisation occurs when an electron receives enough energy to leave the atom
An electron can be removed from any energy level it occupies
the ionisation energy of an atom is the minimum energy required to remove an electron from the ground state of an atom
ground state
When electrons/atoms are in their lowest energy state
Fluorescent Tube
Fluorescent tubes are partially evacuated glass tubes filled with low-pressure mercury vapour with a phosphor coating on the glass
Fluorescence process
free electrons flow through tube (from cathode to anode) and collide with atomic electrons in mercury atom ✔
transferring kinetic energy to the atomic electrons ✔
causing atomic electrons to move to higher energy level (means atom is excited) ✔
excited atom (or electron) is unstable and de-excites to a lower energy level ✔
emitting a photon of energy equal to the energy difference between the levels ✔
in the ultraviolet region ✔
- The UV photons then collide with electrons in the atoms of the phosphor coating and excite them into a higher energy level
- As these phosphor electrons de-excite, they emit visible photons.
How does exitation by collision with free electrons differ from excitation by the absorption of a photon?
For a transition between levels an exact amount of energy is needed (because electrons occupy discrete energy levels) ✔ all the photon’s energy absorbed (in 1 to 1 interaction) ✔ electron can transfer part of its energy to cause a transition and then continues moving with a lower kinetic energy ✔
Line Spectra & Energy Levels
Energy levels can be represented as a series of horizontal lines
The line at the bottom with the greatest negative energy represents the ground state
The lines above the ground state with decreasing energies represent excited states
The line at the top, usually 0 eV, represents the ionisation energy
Line Spectra
Line spectra occur when excited atoms emit light of certain wavelengths which correspond to different colours
The emitted light can be observed as a series of lines with spaces in between
These series of lines are called line or atomic spectra
Each element produces a unique set of spectral lines
No two elements emit the same set of spectral lines, therefore, elements can be identified by their line spectrum
There are two types of line spectra: emission spectra and absorption spectra
Emission Spectra
When an electron transitions from a higher energy level to a lower energy level, this results in the emission of a photon
Each transition corresponds to a different wavelength of light and this corresponds to a line in the spectrum
The resulting emission spectrum contains a set of discrete wavelengths, represented by coloured lines on a black background
Each emitted photon has a wavelength which is associated with a discrete change in energy, according to the equation:
Difference in Discrete Energy Levels
Electrons with energy 13.0 eV collide with atoms of hydrogen in their ground state. What is the number of different wavelengths of electromagnetic radiation that could be emitted when the atoms de-excite? How would you show this on the diagram?
The highest energy state the hydrogen atom can reach is -0.85 eV so there are 6 possible transitions for the atom to return to the ground state
What is meant by the photoelectric effect?
Photons of light incident on the metal surface cause the emission of electrons (also known as photoelectrons) ✔
the electrons emitted are those near the surface of the metal ✔
threshold frequency
The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal
What is meant by “stopping potential”? How is it used to calculate the maximum kinetic energy of photoelectrons?
The potential at which no electrons travel across the photocell (the current becomes zero)
Maximum kinetic energy (J) = charge of electron (J) x stopping potential
This means that the maximum kinetic energy in eV is numerically equal to the stopping potential.
Two different metals emit photoelectrons when illuminated with the same source of electromagnetic radiation. Each metal has a different work function. Explain how the stopping potential is different for the metal with the higher work function. (3)
The stopping potential is related to the maximum kinetic energy of the photoelectrons: Ek(max)=eVs ✔
Maximum KE = energy of photon - work function ✔
So a higher work function means a lower maximum KE, so it has a smaller stopping potential ✔
Draw a graph showing how the maximum kinetic energy of photoelectrons changes with the frequency of EM radiation incident upon the metal.
What is the equation for the de Broglie wavelength?
de Broglie wavelength = the Planck constant / momentum
diffraction
When a beam of light passes through a narrow gap, it spreads out. This is called diffraction (see p.88). Diffraction can only be explained using waves. If the light was acting as a particle, the light particles in the beam would either not get through the gap (if they were too big), or just pass straight through and the beam would be unchanged.
