0.2 Quantum Phenomena Flashcards
What is the photoelectric effect?
The photoelectric effect is a phenomenon in which, when certain types of light shines on a metal surface, electrons are ejected from it.
What did wave theory predict?
1) Any frequency (colour) of light should cause the photoelectric effect.
2) Increasing the intensity (brightness) of light should increase the energy of the electrons emitted.
3) It should take linger for electrons to be emitted when using low intensity light compared to high intensity light.
What actually happend (wave theory)?
1) Only above a certain ‘threshold’ frequency were electrons emitted.
2) As long as you are above the threshold frequency, increasing intensity increases the amount of electrons but their individual energy stayed the same.
3) The electrons were emitted instantly regardless of the intensity of light.
What was concluded (wave theory)?
1) Energy must be delivered in packets (particles), and must be proportional to the frequency of the wave. Electrons can’t store energy. Must be delivered all in one go.
2) There is a one-to-one ineteraction between a photon and an electron. The greater the intensity the more photons produced, therefore the greated the number of electrons emitted.
3) Energy must be delivered in packets in one go, rather than continuously. Electrons cannot store energy.
What is the work function?
The minimum amount of energy required for an electron to escape the surface of a metal.
What happens if hf < φ ?
- Nothing will happen,
- Increasing the intensity of the red light (producing more photons will also have no effect),
- They cannot store energy.
What happens if hf = φ ?
- An electron is released,
- But with no kinetic energy,
-The frequency of this photon is known as the ‘threshold frequency’,
-The minimum frequency needed for an electron to escape the surface of a metal is called threshold frequency (fo).
What happens if hf < φ ?
- Electron is released,
- It leaves faster as the extra energy is transformed to kinetic energy,
- We don’t release extra electrons as it is still only a 1 to 1 interaction.
Why do you see electrons released with a range of speed?
Because not all electrons are on the surface of the metal.
Those at deeper levels require more energy to esccape.
What would happen to the electrons if you double the intensity of light without changing the intensity of light without changing frequency?
- Doubling the intensity of the light will double the number of photons incident on the surface of the metal.
- Therefore DOUBLE the number of electrons are emitted.
- But the Ek(max) will not change as each photon still carries the same amount of energy.
What is intensity?
The amount of energy arriving at every second per unit area.
How does the vaccum photocell (happy cyclops) work?
1) Photons release photoelectrons from the ‘smile’
2) They travel to the ‘eye’
3) Completing the circuit and creating a current
What happens when you add a power supply to a vaccum photocell?
1) When you increase the voltage the ‘eye’ becomes more negative and the ‘smile’ more positive.
2) This makes it harder for the electrons to escape; they are attracted back to the positive smile, and are repelled from the negative eye.
3) Initially only the electrons with small amount of Ek are pushed back.
4) Eventually if you turn up the voltage high enough all the electrons are pushed back and none cross the gap.
5) The current falls to zeron when even the electron with max kinetic energy are stopped.
How do you calculate the maximum kinetic energy of electrons using their charge and the voltage that stopped all of them?
eVs = Ek(max)
charge of an electron x stopping potential = Max kinetic energy
How can you make a gas glow?
By heating up / exciting the gas, it can cause it to glow.
How can you heat a gas to cause it to glow?
By passing a very high current through them, the high current is made up of fast-moving electrons.
