C13 Quantum Physics

Question 1

Max Planck’s quantum model (1900)

Answer 1

• EM radiation has a particulate nature, being tiny packets of energy rather than a continuous wave.
• Einstein called these packets ‘photons’.

Question 2

Formula to calculate the energy of a photon

Answer 2

E = hf
f = frequency of the EM radiation
h = Planck’s Constant

Question 3

State Planck’s Constant

Answer 3

6.63 x 10^-34

Question 4

How to calculate energy of photon in terms of its wavelength and c

Answer 4

E = hc / λ

Question 5

Visible Light, from longest to shortest wavelength, and energy

Answer 5

Red (Lowest energy)
Orange
Yellow
Green
Blue
Indigo
Violet (Highest Energy)

Question 6

EM Spectrum from largest to smallest wavelength

Answer 6

Radio waves (Lowest Energy)
Microwaves
Infrared
Visible Light
Ultraviolet
X-ray
Gamma ray (Highest Energy)

Question 7

Define the energy of 1 eV (electrovolt)

Answer 7

The energy transferred to or from an electron when it moves through a potential difference of 1V.

Question 8

1 eV to 1J, converting between eV and J

Answer 8

1eV = 1.60 x 10^-19
W = VQ (Q = e)

• To convert from J to eV, divide by e
• To convert from eV to J, multiply by e

Question 9

Determining Planck’s Constant

Answer 9

• Use a voltmeter to measure minimum p.d required to turn on LED (with black tube placed over).
• If we already know wavelength of the LED’s photons, we can determine Planck’s constant.
• eV = hc / λ
• Gather data with multiple different-wavelength LEDs (this changes threshold p.d) for accuracy.
• Plot V against 1/ λ, gradient = Planck’s constant.

Question 10

Describe Photoelectric Effect

Answer 10

• When a photon of sufficient energy (above work function of metal) interacts with the surface of a metal, and causes a photoelectron to be emitted from its surface.
• Emission is almost instantaneous.

Question 11

Gold Leaf Electroscope

Answer 11

• Touching top plate briefly with negative electrode charges electroscope, depositing electrons on plate and stem. Leaf stands up as it is repelled.
• UV radiation shines on a clean plate of zinc places on top, gold leaf falls back down toward stem and electroscope loses electrons and its charge.

Question 12

Key observations from photoelectric effect

Answer 12

• Photoelectrons are only emitted if incident radiation was above threshold frequency (F0) for each metal.
• If incident radiation > F0, emission is instantaneous.
• If incident radiation > F0, increasing intensity of radiation does not affect max KE of photoelectrons, only means more will be emitted.
• The only way to increase max KE of photoelectrons is to increase frequency of incident radiation

Question 13

Implications of the photoelectric affect in 1905

Answer 13

• The brightness of light had no effect on emission of electrons. Only frequency.
• It could not be explained by the current wave model of light, which said rate of energy transferred is dependant on brightness. Instead it was replaced by the photon model.
• Einstein proposed EM radiation as a stream of photons, not continuous waves.

Question 14

Work Function

Answer 14

• Minimum energy required to free an electron from the surface of the metal.

Question 15

How does the photon model explain the instantaneous release of photoelectrons?

Answer 15

• Electrons cannot accumulate energy from multiple photons, only one-to-one interactions are possible.
• As long as incident frequency > F0, photoelectrons are instantly emitted.

Question 16

KE of photoelectron

Answer 16

This depends on how much energy is left over after the electron is freed.

Question 17

Photoelectric Equation

Answer 17

hf = ϕ + KEmax
energy of a single photon = minimum energy required + maximum possible energy

Question 18

Experiment to demonstrate wave-particle duality of electrons

Answer 18

• Use electron gun to fire electrons at a thin piece of polycrystalline graphite, carbon atoms with a gap roughly similar to wavelength of electrons.
• They will diffract and form a pattern.
• Demonstrates particle nature as the p.d accelerates then, and wave nature as they diffract.

Question 19

de Broglie equation

Answer 19

λ = h / p
p = momentum of particle
can be applied to all particles, e.g. neutrons, protons
however, as particles become larger wave properties are harder to observe.

Question 20

Mass of electron

Answer 20

9.11 x 10^-31