Chapter 13: Quantum Physics Flashcards
How are EM waves released?
in discrete packets
quanta
energy from waves released as lumps
photon
discrete packets of waves
how does a photon act?
like a neutral particle
either transfer all OR none of its energy when it interacts with another particle
How do you calculate the energy of the photon?
E =hf
energy = planck’s constant x frequency
what does a higher frequency in a photon mean?
it has more energy
what happens when you accelerate an electron in a potential difference?
it transfers some electrical potential energy into kinetic energy
what is the energy transfer equation?
eV = 0.5 x m x v2
electronvolt
ke gained by an electron when accelerated through a PD of 1V
How do you find Planck’s constant?
You need a monochromatic light, set up circuit with a ammeter, variable resistor, voltmeter and battery
1) set Variable Resistor to R(max), so there is no current
2) Adjust until current just begins to flow through
3) record voltage across LED, this is the threshold Voltage
4) Record wavelength of LED
5) Disconnect so circuit can cool
6) Repeat for consistency, then repeat for range of LEDs
monochromatic light
single wavelength, single colour
threshold voltage
voltage needed to give an electron the same energy as a photon emitted by an LED
what happens at the threshold voltage?
Ke is equal to eV0, where V0 is the threshold voltage
What happens when you shine a high enough frequency onto a metal surface?
metal ejects electrons, usually in the UV range
Work function energy (φ)
energy needed for electrons to break metallic bonds and escape metal surface
photoelectrons
the electrons emitted from the metal surface after a high frequency is shone onto it
Photoelectric experiment (gold leaf electroscope)
1) zinc plate is given an negative charge
2) negatively charged repels negatively charge gold leaf, causing it to rise up
3) UV light is shone on the gold plate
4) Energy of light causes electrons to be lost from zinc by photoelectric effect
5) As all of the objects lose their negative charge, gold leaf is no longer repelled, falls back down
What conclusions can be made from the photoelectric experiment?
1) For a given metal, no photoelectrons emitted if radiation frequency is below the threshold
2) photoelectrons emitted from a variety of Ke (0 to max); max increases if frequency of radiation increases
3) photoelectrons emitted per second is directly proportional to intensity of radiation
what happens if the shone light on the metal has a low frequency?
it would take longer for the electrons to gain energy?
what happens if the intensity of the shone light increases?
Rate of photoemission increases
