Quantum Physics Flashcards by Rosie Johnson

How does electromagnetic energy exsist?

Only in certain values - it appeared to come in little packets/ quantas

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What is the photon model?

EM radiation comes in tiny packets of energy, rather than one continuous wave

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What are the packets that EM radiation come in called?

Photons

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What are the equations for the energy of a photon?

E = hf
E = hc /λ

E = energy of photon J
h = Plancks constant
f = frequency Hz
c = speed of light 3 x 10^8 ms^-1

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What is the value of Plancks constant?

6.63 x 10^-34 Js

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At subatomic level, what unit for energy is used?

The electronvolt

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What is 1 electron volt equal to?

1.6 x 10^-19 J

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How do you convert from electron volts to joules?

multiply by 1.6 x 10^19

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What happens when a large enough potential difference is applied across an LED?

It emits photons that all the have the same wavelength and frequency.

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When an LED just begins to glow, what is the energy lost by each electron converted to?

The energy of a single photon

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What is the equation for energy lost by each electron?

E = eV

E = energy lost by electron
e = elementary charge
V = potential difference against the LED

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What is the equation for energy lost by electron = energy of photon?

eV = hc/λ

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What equation can be used to estimate Plancks constant?

eV = hc/λ

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How do you determine Plancks constant using LEDs?

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What is the photoelectric effect?

The phenomena in which electrons are emitted from the surface of a metal upon the absorption of electromagnetic radiation

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What are the electrons emitted during the photoelectric effect known as?

Photoelectrons

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What does the photoelectric effect provide evidence for?

The particle nature of light

How is the photoelectric effect shown with the gold leaf electroscope?

Briefly touching the top plate with the negative electrode from a high-voltage power supply will charge the electroscope
Excess electrons are deposited onto the plate and stem of the electroscope
Any charge developed on the plate at the top of the electroscope spreads to the stem and the gold leaf
As both the stem and gold leaf have the same charge, they repel each other, causing the leaf to lift away from the stem
If UV radiation is then shone onto the zinc surface, the the leaf will slowly fall back towards the stem
This shows that the electroscope has gradually lost its negative charge because the incident radiation has caused the free electrons to be emitted from the zinc plate

What are the 5 main observations of the photoelectric effect?

Placing the UV radiation source closer to the metal plate causes the gold leaf to fall more quickly
Using a higher frequency light source does not change how quickly the gold leaf falls
Using a filament light source causes no charge in the gold leafs position
Using a positively charged plate causes no change in the gold leafs position
Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal

What is the explanation for the observation ‘Placing the UV radiation source closer to the metal plate causes the gold leaf to fall more quickly’?

Placing the UV source closer to the plate increases the intensity of incident radiation on the surface of the metal
Increasing the intensity, or brightness, of the incident radiation increases the number of photoelectrons emitted per second
Therefore, the gold leaf loses negative charge quicker

What is the explanation of the observation ‘Using a higher frequency light source does not change how quickly the gold leaf falls ‘?

The maximum kinetic energy of the emitted electrons increases with the frequency of the incident radiation
In the case of the photoelectric effect, energy and frequency are independent of the intensity of the radiation
So, the intensity of the incident radiation effects how quickly the gold leaf falls, not the frequency

What is the explanation of the observation ‘Using a filament light source causes no charge in the gold leafs position’ ?

If the incident frequency is below a certain threshold no electrons are emitted, no matter the intensity of the radiation
A filament lamp source has a frequency below the threshold frequency of the metal, so no photoelectrons are released

What is the explanation of the observation ‘Using a positively charged plate causes no change in the gold leafs position’ ?

If the plate is positively charged that means there is an excess of positive charge on the plate
Electrons are negatively charged so they will not be emitted unless they are on the surface of the metal
Any electrons emitted will be attracted back by positive charges on the surface of the metal

What is the explanation of the observation ‘Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal’?

A single photon interacts with a single electron
If the energy of the photon is equal to the work function of the metal, photoelectrons will be released instantaneously

Explain the one to one interaction of photons and surface electrons

A single photon delivers its energy to a single surface electron. Each surface electron can ONLY interact with a single photon. This means the number of photoelectrons emitted is exactly equal to the number of photons incident on the surface in which the photoelectric effect is taking place

What is the energy of a single photon equal to?

Minimum energy required to free a single electron from the metal surface + maximum kinetic energy of the emitted electron

What is the work function and kinetic energy equation?

hf = Φ + KEmax h = Plancks constant f = frequency Hz Φ = work function J KEmax = maximum kinetic energy

What does the work function and KEmax equation show?

- If the incident photons do not have high enough frequency and energy to overcome the work function, then no electrons will be emitted - hf0 = Φ where f0 = threshold frequency; here photoelectric emission only just occurs - KEmax depends only on the frequency of the incident photon, and not the intensity of the radiation - The majority of photoelectrons will have kinetic energies less than KEmax

What would the gradient and y-intercept of a graph of KEmax against frequency be equal to?

gradient = h y-intercept = -Φ

What is threshold frequency?

The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal

What is threshold wavelength?

The longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a metal

What is the work fuction?

The minimum energy required to release a photoelectron from the surface of a metal

When can an electron escape the surface of a metal?

If it absorbs an energy equal to or higher than the value of the work function

What is the maximum KE of photoelectrons independent of?

The intensity of the incident radiation (KE is only dependant on the frequency of the incident radiation)

What is the rate of emission of photoelectrons (photoelectric current) directly proportional to?

The intensity of the incident radiation - Intensity is proportional to the number of photons striking per second - Photoelectrons absorb a single photon only so its also proportional

What are electron diffraction tubes used to investigate?

The wave properties of electrons

How do electron diffraction tubes work?

- The electrons are accelerated in an electron gun to a high potential (such as 5000 V) and are then directed through a film of graphite - The electrons diffract from the gaps between carbon atoms and produce a circular pattern on a fluorescent screen made from phosphor

In electron diffraction, what does increasing the voltage between the anode and cathode do?

Causes the energy, and hence speed, of the electrons to increase. The kinetic energy of the electrons is directly proportional to the voltage across the anode-cathode KE = 1/2 mv^2 = eV

What happens if an electron gun fires at a thin piece of polycrystalline graphite?

- It has carbon atoms arranged in many different layers - The electrons pass between the individual atoms in the graphite - The gap between the atoms is so small that is it similar to the wavelength of the electrons - Therefore, the electrons diffract as waves and form a diffraction pattern seen on the end of the tube - The diffraction pattern is observed on the end of the screen as a series of concentric rings - It is observed that a larger accelerating voltage reduces the diameter of a given ring and vice versa

What are the De Broglie wavelength equations?

λ = h / p λ = h / mv E = p^2 / 2m. (from KE equation) λ = h / √ 2mE λ = de broglie wavelength p = momentum m = mass v = velocity E = kinetic energy