Quantum Phenomena

Describe the Gold Leaf Electroscope Experiment

1. The electroscope is made of a metal plate, a rigid metal rod and a flexible piece of gold foil

2. When negatively charged, the foil is repelled away from the plate and rod due to electrostatic repulsion

3. But when high frequency light shines on the metal, electrons are released from the plate, reducing its negative charge, so the foil fall

What is a photon

A theoretical particle of light

Why does the gold leaf electroscope not work when the device is positively charged

• The positive charge is fixed in the metal and can’t be emitted

• Negative electrons are attracted back towards the positive plate

State the predictions, what happened and conclusions of wave theory

• Any frequency of light should eventually emit electrons → Only light above a certain (threshold) frequency caused electron emission → Energy (which must come in packets=photons) is proportional to frequency

• Higher light intensity should emit electrons with more energy → Higher intensity increased the number of electrons emitted → One photon transfers energy to one electron (1-1 interaction)

• Low intensity should cause a delay in electron emission → Electrons emitted instantly regardless of intensity → Electrons cannot store energy, must be delivered in one go/packet

Define work function ($\phi$)

The minimum energy required to remove an electron from the surface of a metal

State what must be true about the photons energy for the photoelectric effect to take place

The photons energy must be greater than or equal to the work function

Define threshold frequency ($f_0$)

The minimum frequency required for an electron to be removed from the surface of a metal

State how photon energy and the work function affect electron emission

• If photons energy is less than work function → No electrons emitted

• If photons energy equals work function → Electrons emitted with no kinetic energy

• If photons energy greater than work function → Electrons emitted with extra kinetic energy

State why different electrons leave a metal at different speeds (photoelectric effect)

Electrons deeper in the metal require more energy to escape, so have less kinetic energy left over, so travels at a lower speed.

State the equation for threshold frequency given E=hf

$$f_0 = \frac{\phi}{h}$$

Describe the maximum kinetic energy against frequency of photons graph

• Gradient = Plank’s constant (h) = Straight line/linear

• Y-intercept = negative of the work function

• X-intercept = Threshold frequency

Describe what piece of equipment can be used to determine the stopping potential of an electron

A vacuum photocell

• Emitted photoelectrons are emitted from a metal/emitter plate (smile) to the collector plate(eye)

• Increasing the opposing voltage eventually stops even the fastest photoelectrons → causing current to drop to zero. This voltage

• If 1V is required, the maximum kinetic energy is $1eV=1.6\times10^{-19}J$

State the equation for stopping potential, including units

$E_{ke}=eV_{s}$,

• e = Charge of electron ($1.6 \times 10^{-19} C$)

• $V_s$ = Stopping potential (Volts)

• $E_{ke}$= Kinetic energy (Joules)

Define an electron volt (eV)

The energy gained by an electron which is accelerated through a p.d. of 1V = $1.6×10^-19$

Define wave particle duality

The fact that light can behave as a wave and a particle, therefore particles should also behave like waves

Describe electron diffraction

• Electrons accelerated and passed through thin graphite screen

• Producing pattern of rings on fluorescent screen

• Electrons interfering constructively and destructively

• Proving electrons can behave like waves

State evidence of electrons/particles and light/photons behaving like waves and particles

Electrons

Particle: Deflection in a magnetic field ← curved track of electron

Wave: Electron diffraction ← Interference pattern seen

Light

Particle: Photoelectric effect ← Gold leaf experiment/threshold frequency

Wave: Diffraction ← Interference pattern seen/Young’s Double Slit experiment

State how electron speed affects the rings in electron diffraction

Electron speed increases:

1. Wavelength decreases

2. Less diffraction

3. Rings become closer together

Describe how an emission spectra is achieved

• Gas is heated → Emits light of specific wavelength (depends on element)

• Split into distinct lines (with prism/diffraction grating)

• Visible lines produced = emission spectra

Describe how an absorption spectra is achieved

• White light passed through cold gas

• Gas will absorb specific wavelengths of light

• Light disperses into a continuous spectrum (with prism/diffraction grating)

What wavelengths of light are absorbed when a gas gains energy

The same wavelengths of light that are absorbed by the gas when it loses energy, the wavelengths visible on the emission spectra or not visible on the absorption spectra

What is the lowest energy level of an atom called

The ground state (n=1)

State the level where electrons have just escaped the atom

Ionisation level ($n=\infty$ )

State what happens when electrons lose energy (in terms of energy levels)

When electrons lose energy they move down energy levels and emit a photon

State how electrons in an atom can gain energy

• The electrons can absorb a photon with the exact amount of energy required to move up one or more energy levels

• The electrons can absorb an electron at least enough kinetic energy to move one or more energy levels

Define ionisation energy

The minimum energy required to remove an electron from the ground state to and has just escaped the atom (ionisation level)

What are line/emission+absorption spectra evidence for?

• Evidence that different elements have different energy levels + line spectra

• Energy of each photon emitted/absorbed = difference between energy levels

• The energy of a photon is linked to it frequency and wavelength

Describe how fluorescent tubes work

• Filled with mercury vapour

• A high potential difference applied

• Causes free electrons to accelerate

• These fast-moving electrons collide with orbital electrons in the ground state of mercury atoms

• Excites the mercury atoms electrons to higher energy levels

• When the mercury electrons de-excite and return to lower energy levels, they emit high-energy UV photons

• The inner surface of the tube is coated with fluorescent material/coating

• UV photons excite electrons in ground state of the coating to higher energy levels

• The electrons in the coating de-excite and cascade back to lower energy levels and emit low-energy photons (in visible spectrum)

• These visible photons combine to produce light that appears white to our eyes