Explain what the photoelectric effect is
The process by which photoelectrons are emitted off the surface of a metal upon the absorption of electromagnetic radiation above a threshold frequency.
This is because, with enough energy, the bonds holding the electrons in place break.
Define the threshold frequency
The minimum frequency of incident electromagnetic radiation required for photoelectrons to be emitted from the surface of a metal
Define the threshold wavelength
The longest wavelength of incident em radiation required for photoelectrons to be emitted from the surface of a metal
the threshold frequency and wavelength are inversely proportional because of the wave equation
Define work function, giving a formula between work function and threshold frequency
- The minimum energy of incident em radiation required for photoelectrons to be emitted from the surface of a metal
- since E = hf for a photon,
work function = h x threshold frequency
What is the formula to find energy transferred to the electron
One electron absorbs one photon and all of the photons energy is transferred to the electron
= hf
Why is there a max kinetic energy of the emitted electrons
The energy required for electrons to be emitted varies with how deep within the metal the electrons lie
Electrons with Ekmax are therefore on the surface of the metal
Explain the concept of stopping potential
- It is the pd required to stop photoelectron emission from occurring
- This can be shown in a photoelectric circuit, made up of 2 metal plates, a collector plate and emitter plate
- shining photons at the emitter plate causes electrons to be emitted which cross the gap and are collected at the other plate
- If you add a power supply and increase the voltage, the emitter plate becomes more positive because it is connected to the positive terminal of the power supply
- Therefore, electrons that are emitted will be more attracted back towards the emitter plate and the electrons require more energy to cross the gap
- eventually there will be a voltage where no electrons can cross and this is the stopping potential
- At the Vs, the energy required to cross the gap = the max Ek of electrons
What is the formula for finding the stopping potential
V= W/Q
so Vs = Ekmax/e
where e is the charge of an electron
How does the photoelectric effect provide evidence that light behaves as a particle
- If light behaves as a particle, it is carried in discrete packets (quantised)
- This is proved in the photoelectric effect as each electron can only absorb the energy of 1 discrete packet ie photon
Define intensity
The total power delivered per unit area
It is related with the number of photons striking the metal plate
How can the stopping potential change
- Increasing the intensity does not change the max kinetic energy of the electrons so Vs is the same
- Increasing the frequency of photons increases the Ekmax of electrons so increases Vs
- Increasing the work function decreases the Ekmax so decreases the Vs
What is the relationship between intensity and number of photoelectrons emitted
As intensity increases, more photons hit the metal plate, so more electrons can absorb a photon so more electrons can be emitted
How can the photoelectric effect disprove the wave theory of light
- wave theory says that intensity of light is directly proportional to energy carried
- Therefore the kinetic energy should increase with intensity, which does not happen in the photoelectric effect
- Also, with a wave, over time the energy transferred to the electrons would gradually increase and therefore eventually, the electron would be emitted no matter the frequency of the light, which does not happen in the photoelectric effect
Why do the electrons have kinetic energy when they leave the surface of the metal and explain how this gives the photoelectric equation
- If the photon that it absorbed has an energy greater than the work function, the excess energy is the kinetic energy of the electron leaving the metal
- Therefore, the work function + the max kinetic energy = the energy of the photon
- E=hf=Φ +Ekmax
How are electrons located within an atom
In discrete energy levels
How can electrons move up an energy level
If another electron collides with the orbital electron or the orbital electron absorbs a photon, they can gain enough enough to move up energy levels
This is known as excitation.
Define ionisation
When an electron gains enough energy to leave the atom entirely. The required energy is called the ionisation energy
Why does de-excitation occur
- It is unstable for electrons to be in a higher energy level so they return back to their original energy level, the ground state. It will release the energy it gained in the form of a photon
- E=hf = the energy difference between energy levels
Explain how a fluorescent tube works
- A voltage accelerates free electrons through the tube, which collide the mercury vapour in the tube
- the electrons in the vapour excite and then de-excite, releasing UV photons
- The electrons in the phosphorus coating of the tube absorb the uv photons and excite and de-excite, releasing visible light photons
Define the electron volt explain why we use it, and how many joules is 1 eV
1 electron volt is the energy gained by an electron when accelerating from rest through a potential difference of 1 volt
- It is used because the energy difference between energy levels is very small
- 1eV = 1.6x10^-19 J
What is a line spectra
A diagram which shows the specific wavelengths of light that have been absorbed/emitted during excitation and de-excitation
How do line spectra provide evidence that electrons travel between discrete energy levels
The wavelengths of the light, and therefore positions on the spectra, are fixed values. This means the energy absorbed/emitted is a fixed value, which means the energies of the different energy levels are fixed values, meaning they are discrete
Describe the difference between an absorption and emission spectra
- An absorption spectrum is a continuous spectrum with gaps at the wavelengths where photons have been absorbed
- An emission spectrum is a black screen with colour at the wavelengths where photons have been emitted
How can emission spectra be formed
Splitting the visible light from a fluorescent tube using a diffraction grating or prism
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