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Ultimate Physics > 4.5 - Quantum > Flashcards

4.5 - Quantum Flashcards

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1
Q

When EM radiation interacts with matter, what does it interact as?

A

Photons

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2
Q

What are photons?

A

Discrete energy quanta (packets)

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3
Q

Why do we use electronvolts?

A

Because the energy of a photon is very small when measured joules, so electronvolts are more appropriate.

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4
Q

Define 1eV

A

The energy transferred when an electron travels through a potential difference of 1 volt.

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5
Q

How many joules in 1 eV?

A

1.60x10^-19

using W=VQ

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6
Q

How can we use LEDs to determine the Planck constant?

A

Vary the PD across an LED to determine the threshold PD required to turn it on. LED produces light of a specific colour so we know wavelength.

Each photon is emitted when a single electron loses energy. By equating energy of an individual electron in the LED with an individual photon produced, we can use eV = hc/λ

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7
Q

How can we improve the accuracy of the LED experiment?

A

Repeat the experiment with a variety of different coloured LEDs, each emitting different wavelengths of light.

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8
Q

What graph can be drawn after the LED experiment?

A

Draw V against 1/λ. The gradient will be equal to hc/e. c and e are known constants so we can calculate h from this.

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9
Q

Define the photoelectric effect.

A

The emission of electrons from the surface of a metal when EM waves, of greater frequency than the threshold frequency, are incident on the metal.

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10
Q

What is a gold leaf electroscope?

A

A negative zinc plate on top of a stem, with a negative piece of gold leaf attached to the stem.

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11
Q

What happens when UV light is shone onto the zinc plate of a gold leaf electroscope?

(Initially, the gold leaf and stem have the same charge so they repel)

A

Free electrons are released from the surface of the plate, and the negative charge is slowly lost so the leaf gradually falls back to the stem.

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12
Q

What happens if we shine visible light onto the zinc plate of a gold electroscope?

A

Electrons don’t get removed from the plate, regardless of the intensity.

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13
Q

Can the wave model of EM explain gold leaf electroscope observations?

A

No.

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14
Q

What does each electron on the surface of a metal require to escape?

A

A certain amount of energy.

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15
Q

What happens to the energy of a photon when it is incident on a metal surface?

A

It transfers its exact energy to one electron, with any excess energy transferred into kinetic energy.

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16
Q

Define the work function.

A

The minimum energy required to remove/emit a single electron from a metal surface.

17
Q

Why do we have a threshold frequency for EM radiation?

A

We have a minimum energy, and energy is directly proportional to frequency.

18
Q

How does the threshold frequency explain why visible light doesn’t produce the photoelectric effect (gold leaf) but UV does?

A

UV light has a frequency beyond the threshold frequency whereas visible light does not.

19
Q

What is the photoelectric equation?

A

hf = Φ + KEmax

Φ = work function of the metal
KEmax = maximum KE of the released electron
20
Q

In the photoelectric equation, why is KEmax the maximum kinetic energy of the released electron?

A

Some electrons may be closer to a nucleus, requiring more energy than the work function amount to be released, leaving less energy left over as kinetic energy.

21
Q

How does intensity come into play with the emission of electrons?

A

Changing intensity has no effect if the incident radiation has a frequency below the threshold frequency.

However, if it has a frequency above the threshold frequency, then increasing the intensity of the radiation will increase the rate of electron emission.

22
Q

Why does increasing intensity increase rate of electron emission (for radiation above threshold frequency)?

A

The increase in intensity increases the number of photons available to interact with the electrons.

23
Q

What is the only way of increasing the kinetic energy of ejected electrons?

A

By increasing the frequency further above the threshold frequency, so there is more energy left over to be converted to kinetic energy.

24
Q

Define the threshold frequency

A

The minimum frequency of the incident photon to release an electron from the surface of a metal.

25
Q

Define photoelectrons

A

Electrons emitted from the surface of a metal by the photoelectric effect.

26
Q

The wavelength associated with a particle is inversely proportional to what?

A

Its momentum.

27
Q

What is the de broglie equation?

A

λ = h/p =h/mv

28
Q

What can be used as evidence for wave-particle duality?

A

Electrons. They have mass and charge. They can be accelerated and deflected by magnetic and electric fields. However, they can be made to diffract.

29
Q

How can electrons be made to diffract?

A

When fired at a thin piece of polycrystalline graphite (material containing carbon atoms spread over many layers), they produce a diffraction pattern when hitting a screen.

30
Q

How can an electron move up an energy level?

A

It must gain the exact amount of energy to make a transition. It can do this by another electron colliding with it, or by absorbing a photon of the exact energy.

31
Q

What must be done by an electron when moving down a level?

A

It must lose the exact amount of energy when making the transition. This is released as a photon of energy equal to the energy lost.

32
Q

What equation is used for finding energy changes between energy levels?

A

ΔE = hf = E1 - E2

E1 = energy of starting level
E2 = energy of level the electron landed at
33
Q

Define excitation

A

An electron moving up one or more energy levels when it gains the exact amount of energy required.

34
Q

Define de-excitation.

A

When an electron gives out the exact amount of energy to move back down to its original energy level.

35
Q

Why do the ground state and energy levels leading up to ionisation have negative values of energy?

A

They are compared to the ionisation level. Energy must be given to electrons to move up a level.

36
Q

Transitions release a photon with a set amount of energy. This means what is also set?

A

Frequency and wavelength.

37
Q

If you know the energy change from transitioning from one energy level to another, what can you figure out?

A

The frequency of the light, allowing us to know its colour.

Ultimate Physics (26 decks)

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