Module 4 - Quantum (The Photoelectric Effect) Flashcards
What is the Photoelectric Effect
When EM radiation is shone on to a metal, electrons are released from the surface of the metal
What are photoelectrons
The electrons emitted via the photoelectric effect
Describe the gold-leaf electroscope demonstration
- The top part of the electroscope is made negative by touching it with a cathode
- This charge spreads to the stem and the gold leaf
- Because the leaf and the stem have the same charge they repel from each other
- When a certain wavelength of EM radiation is shone (UV), the photoelectric effect occurs
- The gold leaf falls back towards the stem showing that it has lost negative charge
What are the 4 observation of the Photoelectric effect
- Photoelectrons were emitted only if the incident radiation had a certain frequency
- If the incident radiation was above a certain frequency the effect occured instantanously
- Increasing the intensity only increases the number of electrons that was emitted
- Increasing the frequency of the photons increased the maximum Ek of the electrons after the effect took place
What is the threshold frequency (f₀)
The minimum frequency required for the photoelectric effect to take place, specific to each metal
What region in the EM spectrum did the threshold frequency for the gold leaf electroscope lie
UV
Explain why photoelectrons were emitted only if the incident radiation >= f₀
- The photoelectric effect is a one-to-one interaction between electrons and photons
- This means single lesser frequency photons, dont have enough energy to allow the electrons to escape
- so no photons regardless of intensity where absorbed by the electrons
Describe what happens when a photon is absorbed by an electron in a metal
- The quantum of energy is absorbed
- Some of the energy is used allow the electron to escape its atom
- Any remaining energy is transferred into the kinetic energy of the photoelectron
What is meant by the work function of a metal (Φ)
- The minimum quantity of energy required to free an electron from the surface of a metal
Formulate a CoE formula to describe the photoelectric effect
E = Φ + K.E.max
E - photon energy
Φ - Work function
KE - Maximum KE immediately after the photoelectron is released
Explain why if the incident radiation was above a certain frequency the effect occured instantanously
- The electrons couldn’t accumulate energy from multiple photons
- Because it is a one-to-one interaction meaning the photon is either absorbed or deflected
- This meant the electrons where released the instant they absorbed photons
What does the Work Function of a metal depend on and state the relationship
- The threshold freuqnecy:
Φ α f
Describe the relationship between the rate of photon transmission and intensity of the EM radiation, and state what it causes
- The gretaer the intensity the greater the rate of photon transmission
I α photon transmission - This causes a greater rate of photoelectron transmission
Describe the relationship between frequency and maximum kinetic energy
- The greater the frequency of radiation incident on the same metal the greater the leftover energy converted to Ek (as the work function is constant)
- f α Ekmax
How do you determine the threshold frequency of a metal given its work function
- The energy of a photon will be equal to the work function when there is no residual energy converted to Ek
- Therefore: hf = φ, where f is the threshold frequency
and so f₀ = φ/h
