4.5 - Quantum Physics

Question 1

What is Annihilation?

Answer 1

1. When a particle and its corresponding antiparticle collide, they annihilate (destroy) each other.
2. Their masses are converted into pure energy, producing a pair of gamma photons.
3. The energy carried away by the gamma photons must equal the total energy of the particles to begin with (kinetic energy plus rest mass energy).
4. So each gamma photon must carry away at least the rest mass energy of one particle.

Question 2

What is Pair Production?

Answer 2

1. Pair production is the opposite of annihilation.
2. Pair production is when one high energy photon spontaneously turns into a particle-antiparticle pair.
3. The energy of the photon must be at least the total rest mass energy of the particle-antiparticle pair it creates.

Question 3

What is Threshold Frequency?

Answer 3

Electrons are only emitted from a metal when the light hitting it is above a certain frequency. Below this frequency, no electrons are emitted.

Question 4

What is the relationship between No. of electrons and intensity?

Answer 4

When the light (EM radiation) is above the threshold frequency of the metal, the number of electrons emitted is proportional to the intensity of the light. (Directly Proportional)

Question 5

Explain why the maximum kinetic energy of emitted electrons being independent of intensity in the photoelectric effect provided evidence that light was not a wave.

Answer 5

According to the wave theory of light, the intensity should determine the energy transferred to the electrons each second.
So we would expect that the maximum kinetic energy of emitted electrons would increase with intensity.
Instead, the maximum kinetic energy was independent of intensity and increased with frequency.

Question 6

What is ionisation?

Answer 6

When an electron is being completely removed from an atom?

Question 7

Describe Einstein’s Photoelectric effect theory to explain these observations?

Answer 7

1.Individual photons are absorbed by individual electrons ( in
the metal surface)/ one to one interaction/AW
2.Only photon with energy above the work function energy will
cause photoelectron emission/idea of threshold frequency
3.Photon energy is proportional to frequency
4.(therefore) blue photons with higher f/shorter λ will cause
photoemission but red photons will not.
5.hf – ϕ = KEmax is the equation resulting from conservation of
energy or resulting from the meaning of each term
6.A wave model does not explain instantaneous emission

Question 8

Equation linking charge of electron (e), Stopping Voltage (Vs) and Energy (KE)

Answer 8

eVs = KE

Question 9

Diffraction grating equation

Answer 9

d sin (theta) = wavelength

Question 10

How can u demonstrate the photoelectric effect

Answer 10

1. A (clean) zinc plate mounted on the cap of a gold-leaf electroscope.
2. Plate initially charged negatively
3. A u-v lamp shining on plate
4. The gold leaf collapses as the charge leaks away from the plate (when ultra-violet light is incident on the zinc plate)
5. so experiment indicates the emission of negative charge/electrons

Question 11

What is the photon model?

Answer 11

When two or more progressive waves meet and overlap, they superpose, producing a single
wave. The principle of superposition states when two waves meet at a point, the resultant
displacement of the wave at that point is equal to the sum of the displacements of the individual
waves. The resultant displacement depends on if there is constructive interference (where the
individual displacements are both in the same direction, and add together) or destructive
interference (where the individual displacements are in opposite directions, and counteract each other).

Question 12

What is an electron Volt?

Answer 12

Using the formula W=VQ, the work done is equal to the charge on an electron multiplied by the
p.d., 1 volt, so an electronvolt = 1.60×10-19 joules.

Question 13

What is the De broglie wavelength

Answer 13

De Broglie realised that all matter can
exhibit both wave and particle properties, and that the wavelength associated with a particle is
inversely proportional it its momentum, 𝑝𝑝. This relationship is given by the de Broglie equation

Question 14

Evidence for Wave-particle duality

Answer 14

electrons can also be used as evidence for wave-particle duality. Electrons are classified as particles, with mass and charge. They can be accelerated and deflected by magnetic and electric fields, which is behaviour associated with particles. However, they can also be made to diffract. When a beam of
electrons is fired at a thin piece of polycrystalline graphite (a material containing carbon atoms spread over many layers) the electrons are diffracted by the gaps between atoms, and produce
a diffraction pattern when they hit a screen. This diffraction is a property of waves.

Question 15

What are photons?

Answer 15

Photons are particles of light. They are sometimes referred to as a quantum of
energy of EM radiation. ‘A quantum’ in this context just means a set (finite) amount.