photoelectric effect + electron diffraction Flashcards
State what is meant by the photoelectrons.
electrons emitted due to light
Explain the theoretical vs the experimental view of photoelectrons’ KE.
- theoretically, we observe photoelectrons with a range of different energies
- experimentally, maximum kinetic energy of photoelectrons.
State the effect of light intensity on maximum kinetic energy of photoelectrons.
max EK is independent of light intensity.
State the two hypotheses’ made by wave theorists about the photoelectric effect.
→ lag time between light source being turned on & photoelectrons being emitted
→ EK max is dependant on light intensity
State the effect of increasing light intensity on the photoelectric effect.
no. of photoelectrons emitted per second increases
State what determines the maximum kinetic energy of photoelectrons.
frequency.
For a given metal plate, when will photoelectrons be emitted?
only if the frequency of the light source is above a certain value
For a given metal plate, when will photoelectrons be emitted?
→ frequency > threshold frequency
→ if photon energy > work function
State what is meant by the work function.
the minimum energy required to remove an electron from a metal surface Φ
What is a photon?
a packet of energy that makes up light
State the relationship between photon energy and frequency.
increasing frequency increases photon energy
State how to calculate the energy of a photon.
E = hf
→ E is energy in joules
→ h is Planck’s constant in Js
→ f is frequency in hertz
State what is meant by threshold frequency.
the minimum frequency of incident electromagnetic radiation required for an electron to overcome the work function
Explain how to find the threshold frequency.
1) using E = hf, set E to work function
2) rearrange for f(min) and solve
Use the photon theory of light to explain why increasing light intensity increases the number of electrons emitted per second.
→ increasing intensity means more photons interact with more electrons
→ hence more photoelectrons are emitted
State the equation that links maximum EK of photoelectrons and photon energy.
hf = work function + KE(max)
State the relationship between wavelength and photon energy. State the equation.
→ E inversely proportional to λ
→ E = hc/λ
State what is meant by the stopping potential
the minimum potential difference that prevents photoelectrons from crossing the gap between two metal plates
State the relationship between potential difference and the force experience by photoelectrons.
greater the potential difference, greater the force of repulsion
Describe & explain an experimental set up to stop photoelectric emission.
→ potential difference put across two plates
→ top plate is negative and so repels photoelectrons
What is the work done on photoelectron travelling between two plates equal to?
initial kinetic energy of photoelectron
State an equation used to calculate the work done on a photoelectron.
work done = electron charge x stopping potential
In a vacuum photocell setup, when will the ammeter show a non-zero reading?
when there is a flow of electrons which will happen if:
→ frequency > threshold frequency
→ potential difference < stopping potential
State the relationship between stopping potential and frequency. explain why
greater the frequency, greater the stopping potential
→ higher photon energy therefore higher max kinetic energy
What did Milikan’s results provide?
→ experimental evidence for the relationship between frequency and stopping potential
→ confirmed Einstein’s photoelectric equation
→ gradient of line can be used to find Planck’s constant
Which value can be calculated using Milikan’s results? How?
Planck’s constant, given by the gradient of a freq. vs stopping potential graph
State the meaning of the term photoemissive.
a material is photoemissive if it is able to emit electrons due to incident radiation
State how to convert from electron volts to joules.
multiply by electron charge
What is one electron volt equal to? (in words)
the work done on an electron as it moves through a potential difference of one volt
State how Louis de Broglie produced a diffraction pattern to observe electron diffraction.
→ a beam of electrons is directed at a thin graphite film
→ inside a vacuum tube
Describe the diffraction pattern observed by Louis de Broglie and what it proved.
→ pattern observed as series of concentric rings
→ similar to diffraction grating pattern
→ showed that e- behaved as waves
State and explain the effect of increasing accelerating voltage on the diameter of rings produced in electron diffraction.
→ increasing accelerating voltage increases kinetic energy
→ wavelength decreases
→ diameter decreases
Explain why graphite is ideal for electron diffracion.
→ crystalline structure
→ gaps between atoms act as slits allowing e- to spread out
Describe the experimental setup to demonstrate electron diffraction.
→ e- are accelerated in an electron gun to a high potential
→ directed through thin film of graphite
→ e- diffract from gaps between carbon atoms
→ produce a circular pattern on a fluorescent screen made from phosphor
State the effect of changing momentum, in terms of wavelength, on the amount of diffraction.
→ higher momentum = shorter wavelength
→ small λ means light spreads out less
→ hence a smaller radius is produced
