Quantum

Question 1

Photons

Answer 1

discrete energy quanta (‘packets’)
The energy, E, of a photon is directly proportional to the frequency, f, of the electromagnetic radiation

Question 2

Electronvolt

Answer 2

• Photon energy is better measured in electronvolts (eV) due to its small value in joules.
• 1 eV equals the energy transferred when an electron moves through a 1-volt potential difference, equivalent to 1.60×10-19 joules.
• When accelerating electrons, the kinetic energy (KE) is measured in electronvolts (eV).

Question 3

Photoelectric effect

Answer 3

• Electromagnetic radiation hitting a metal surface releases electrons, known as the photoelectric effect.
• Each electron needs a certain energy to escape, called the work function.
• Photons transfer their energy to electrons, with any excess becoming kinetic energy.
• The work function determines the minimum photon energy needed to release an electron.
• UV light, with its higher frequency, can cause the photoelectric effect, while visible light cannot.

Question 4

Work function

Answer 4

• The work function (ϕ) is the minimum energy needed to release an electron, and KEmax is the maximum kinetic energy of the released electron.
• Electron release isn’t affected by radiation intensity but by frequency.
• If below the threshold frequency, no electrons release, regardless of intensity.
• Above it, increasing intensity boosts the emission rate, as more photons interact with electrons.
• Increasing kinetic energy requires raising the frequency above the threshold.

Question 5

De broglie

Answer 5

• Light exhibits wave-particle duality, seen in diffraction/superposition as waves and the photoelectric effect as photons.
• De Broglie proposed that all matter shows this duality, with a wavelength inversely proportional to momentum, described by his equation.
• As mass increases, wavelength decreases, making wave-like properties harder to observe.
• Momentum is given by the equation: p = √2mE

Question 6

Wave-particle duality

Answer 6

• Electrons exhibit wave-particle duality, similar to electromagnetic radiation.
• Classified as particles due to their mass and charge, electrons can be accelerated and deflected by fields.
• However, electrons can also diffract, demonstrating their wave-like behavior.
• When fired at graphite, electrons diffract at atom gaps, showcasing their wave-like behavior.

Question 7

Gold-leaf electroscope

Answer 7

• The photoelectric effect can be demonstrated using a gold-leaf electroscope experiment.
• A negatively charged zinc plate repels the negatively charged gold leaf, causing it to rise.
• When UV light hits the zinc plate, electrons are lost via the photoelectric effect, removing the negative charge.
• This causes the gold leaf to fall back down.

Question 8

Electron diffraction pattern

Answer 8

• Electrons diffract around atoms of graphite, creating circles of constructive and destructive interference on a fluorescent screen made from phosphor
• A larger accelerating voltage reduces the diameter of a given ring.

Question 9

Energy levels

Answer 9

• An electron can be excited to a higher energy level by either a free electron colliding with it or by absorbing a photon of energy equal to the difference in energy.
• It will emit a wavelength at every different level it drops

Question 10

Photoelectric graph

Answer 10

The key elements of the graph of maximum kinetic energy KEmax against frequency :

• The work function Φ is the y-intercept
• The threshold frequency is the x-intercept
• The gradient is equal to Planck’s constant
• There are no electrons emitted below the threshold frequency

KE max = hf - Φ