P13 - Quantum Physics Flashcards
What did Planck discover?
.EM radiation can only exist in certain values.
.These are little packets called quanta.
.Einstein called them photons.
.Opposes theory of radiation being a continuous wave.
What is the equation for energy of a photon using frequency?
E = hf.
What is the equation for energy of a photon using wavelength?
E = hc/λ.
What is the value of Planck’s constant, h?
6.63 x 10^-34 Js.
How do you determine Planck’s constant using LED’s?
.Make a circuit with a variable resistor involved.
.Measure the threshold p.d. of the LED.
.Repeat for other colours.
.Plot p.d. against 1/λ. Take the gradient, multiply by e and divide by c to get Planck’s constant.
What was observed in the Photoelectric Effect?
.PE only emitted if radiation was above the threshold frequency.
.Emission is instantaneous when above the threshold frequency.
.Increasing intensity increases number of PE emitted, not their KE (frequency does this) and only when it is above the threshold frequency.
What did Einstein conclude from the Photoelectric Effect?
.Only explained with particle model.
.Need a specific amount of energy (work function) to escape metal.
.E = hf so if frequency must equal work function to be liberated.
.If the frequency was below the WF, intensity/ photon number has no effect.
.One-to-one interaction so no time delay.
What relationship relates the energy of a photon and electron
Ep = Ee.
What is the Photoelectric Equation?
hf = ϕ + Ke max.
Define the work function.
The work required to remove a surface electron.
Why does the Photoelectric Equation use max Ke?
Not all electrons are at the surface and require more work to remove them hence PE are emitted with different kinetic energies - but the max KE comes from surface electrons.
What happens when hf = ϕ?
Electrons are liberated but not emitted.
How does a gold-leaf electroscope show the Photoelectric Effect?
.Top plate is given a negative charge.
.The charge spreads.
.The leaf repels from the metal.
.When zinc with light with high frequency is shone this continues.
.When zinc with light with low frequency is shone they stop repelling.
.Electrons aren’t liberated so charge stops spreading and they don’t repel.
What did De Broglie propose?
.All subatomic particles can exhibit diffraction patterns.
.He showed this experimentally with electrons.
.Won the 1929 Nobel Prize.
How do you experimentally show particle-wave duality?
.Electron Gun - electrons undergo thermionic emission? and exhibit diffraction patterns on the glass (see textbook).
How are wavelength and momentum related?
λ = 1/p.
What is the wave-particle duality equation?
λ = h/p.
What is λ in terms of Ek?
λ = h/sqrt(2mEk).
