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unit 12 notes Physics Oxford IB Diploma Programme
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these are detailed notes of unit 12: quantum physics from the ibdp physics hl course
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Quantum and nuclear physicsFailures of classical physics:
1. Black body curves:
➔ Very hot object should emit an infinite amount of energy.
➔ Energy is not emitted as a continuous wave, but as a stream of photons with energy
ℎ𝑐
𝐸= λ
.
2. Model of the atom: the nucleus is at the center of the atom, and electrons are orbiting
around it.
➔ As electrons accelerate, they give out energy. Then, since the electrons is constantly
accelerating, it must be constantly giving out energy. Then, it must be constantly
spiral towards the nucleus and annihiliate.
➔ For an electron to spiral it must gradually give off energy. And it can only give out
energy in the form of photons, as discrete packets of energy, with a certain
ℎ𝑐
wavelength, 𝐸 = λ
.
3. Photoelectric effect: shining a light of a certain frequency on a metal plate (usually
zinc). Nothing happens if you shine visible light, but if you shine UV, instead,
electrons will be emitted. All the electrons have the same kinetic energy, which
depends on the frequency of the light.
➔ If light was a continuous wave, even with a very low frequency, the intensity of the
light could be increased. Because wave intensity is proportional to the amount of
energy of the wave, above a certain intensity electrons could be released.
➔ Light is not a continuous waves. Instead, it is made of photons, each with an energy
of 𝐸 = ℎ𝑓. When one of these photons hits the plate, it either doesn’t have enough
energy to fully ionise the electron, or it does, and if the frequency is the same, the
energy of the photon is the same. If the energy of the photon is the same, and the
amount of energy needed to ionise the electrons from the zinc is the same, then
there’s always the same amount of energy left over, that is the energy of the each
electron.
Photoelectric effect
, Method: light of a specific wavelength is incident on a sheet of zinc. The velocity of the
emitted electrons is measured by changing the potential difference of two plates, to measure
the kinetic energy of electrons.
Results:
- Increasing the intensity of light just means that more electrons are emitted, but all of
them will have the same kinetic energy.
- Threshold frequency; the minimum frequency of light required to remove an electrons
from the surface of a metal.
- Increasing the frequency of the incident light increases the 𝐸𝑘 of the emitted
electrons.
- Visible light results in no electrons emission.
- With UV light, electrons are emitted immediately.
Gold leaf electroscope is used to demonstrate the photoelectric effect and to explain the
results in terms photons of light.
1. Scrub the zinc plate on both side with iron wool, to remove the sulfur oxide layer.
2. Place the zinc sheet above the gold leaf electroscope.
3. For the demonstration to work, the zinc plate must be negatively charged, which can
be done by charging by induction
4. When shining UV light on the plate, the gold leaf immediately starts to fall, meaning
that the UV light is discharging the elctroscope. Since we started with a negative
charged plate, the UV light must be releasing electrons.
5. The frequency of the light makes a difference, not the intensity nor how long the light
is let to shine.
6. The velocity of an individual electron in measured by creating an electric field
between the photoelectric plate and the collecting plate. The potential difference
between the plates is varied until the electrons are just stopped, where all of the
kinetic energy is converted to potential energy.
𝐸𝑘 = 𝐸𝑝 = 𝑞𝑉𝑒, where q is the charge on the electron at potential 𝑉𝑒.
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