Chapter 13 - Quantum Physics Flashcards

1
Q

What are photons?

A

Photons are packets of discrete EM energy

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2
Q

Why were photons developed?

A

Developed as wave theory of light could not explain photoelectric effect

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3
Q

What is the energy of a photon dependant on?

A

Energy of a photon is dependant on its frequency

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4
Q

What is the equation for the energy of a photon?

A

E = hf or E = hc / y

Energy = Planck Constant x Frequency

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5
Q

What is the value of Planck Constant?

A

6.63 x 10-34

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6
Q

What is the relationship between energy of photon and wavelength?

A

Energy of a photon is inversely proportional to its wavelength

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7
Q

Why do X-rays damage the cells of bodies?

A

X-rays have more energy than radio waves which Is why X-rays damage the cells of bodies

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8
Q

Where are electron volts used?

A

Electron volts are used to measure energy at the quantum scale

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9
Q

What is an electron volt?

A

1 electron volt is the energy transferred to or from an electron when it moves through a PD of 1V

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10
Q

What is the value of an electron volt?

A

1.60 x 10-19

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11
Q

How do you calculate the number of electron volts?

A

Energy in J / 1.60 x 10-19

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12
Q

What happens when a diode has its threshold voltage across it?

A

LED allows currents to flow and transfer energy to EM radiation/photos of a specific wavelength

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13
Q

What is the property of a laser beam?

A

Laser beams consist of photons of the same frequency

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14
Q

What is the formula for the power of a beam?

A

P = nhf

Power of beam = Number of photons per second x Planck Constant x Frequency

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15
Q

What can be assumed about the transfer of energy from electrons to photons?

A

E = Ve = hf

This is a very unlikely assumption

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16
Q

How can Planck Constant be estimated?

A

By using a circuit and LEDs of different threshold frequencies
A single LED can be used but gives a less accurate value

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17
Q

What is the circuit used to approximate Planck Constant?

A

Potential divider circuit with LED in parallel with voltmeter across it
Safety resistor

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18
Q

Why is a potential divider circuit used?

A

Potential dividers are better than a series circuit as it provides an adjustable PD

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19
Q

How much energy is transferred in an LED at the threshold PD?

A

The energy transferred in the LED is approximately equal to the energy of a single photon it emits

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20
Q

What is the formula for the energy of emitted photons in the LED experiment?

A

Threshold PD x Charge on Electron = Energy of Emitted Photon

Ve = hc / y

21
Q

How is a graph formed from the LED investigation to estimate Planck Constant?

A

Using the data from the different wavelength LEDs, plot a graph of threshold PD against 1/wavelength
The gradient is hc / e where ‘c’ and ‘e’ are constants

22
Q

What is the photoelectric effect?

A

The photoelectric effect is the emission of electrons from the surface of the metal when it is illuminated by EM radiation above threshold frequency

23
Q

What is the photoelectric effect dependant on?

A

It is dependant on being over the threshold frequency and not on the intensity

24
Q

What is the result of the photoelectric effect?

A

Instantaneous emission of electrons

25
Q

How does increasing intensity affect the photoelectric effect?

A

Increasing intensity of incident radiation increases the number of electrons emitted per second, not their KE

26
Q

How is the kinetic energy of electrons increased in the photoelectric effect?

A

Kinetic energy is only increased by increasing the frequency of incident radiation

27
Q

What are the steps for the photoelectric effect to take place?

A

Single electron absorbs energy of single photon
If energy of single photon exceeds work function, electron escapes
Energy in excess of work function absorbed by electron becomes KE

28
Q

How does increasing intensity affect number of emitted electrons?

A

Increasing intensity
Increases photons per unit of time
Increases photon-electron interactions
Increasing number of emitted electrons

29
Q

What is the work function?

A

The work function is the minimum amount of energy needed by an electron to escape a metal

30
Q

What does photoelectric effect depend on in terms of energy?

A

Emissions can only take place if energy of photons exceeds work function (hf > O)

31
Q

Why do few electrons achieve max KE?

A

Most electrons require more energy than work function as they need to overcome attractive forces deeper leaving less energy for KE

32
Q

What is a vacuum photocell?

A

A vacuum photocell is a device which makes use of the photoelectric effect to generate a small current

33
Q

What does a vacuum photocell consist of?

A

It consists of an evacuated glass tube with a photoemissive plate at one end, and another metal plate at the opposite end

34
Q

What is a photoemissive plate?

A

A photoemissive plate is a metal that releases electrons via the photoelectric effect

35
Q

What is the formula for current using the photoelectric effect?

A

Current = Number of photoelectrons emitted per second x Elementary Charge

36
Q

What does it mean if the applied PD runs forward or backward in photoelectric circuit?

A

Forward: Photoemissive plate is cathode and opposite plate is anode
Backward: Photoemissive plate is anode and opposite is cathode

37
Q

What is the difference between applying the PD forward or backwards in photoelectric circuit?

A

Forward: Electrons gain energy and most will cross
Backwards: Electrons lose KE and only most energetic electrons can cross

38
Q

How does increasing bias potential impact the photoelectric current?

A

Electrostatic forces decelerate more electrons so fewer have enough KE. The current registered gets smaller and smaller

39
Q

How is maximum kinetic energy estimated?

A

Photocell
Adjusting bias potential between the cathode and anode until no current is registered

40
Q

What is the graph using Einstein’s photoelectric equation used to estimate Planck’s Constant?

A

Graph of KEMAX against f
KEMAX = hf1 - O

Gradient = Planck Constant
Y intercept = Work Function
X intercept = Threshold Frequency

41
Q

What properties of light show that it has wave-particle duality?

A

Diffraction shows light has wave-like nature
Photoelectric effect shows light has particle-like nature

42
Q

What is de Broglie’s formula?

A

Wavelength = Planck Constant / Momentum

y = h / p

43
Q

When are electrons diffracted?

A

Electrons can only be diffracted by metal lattices where the atomic spacing is comparable to the size of electron’s de Broglie wavelength

44
Q

Why do electrons form ring interference patterns?

A

The structure of the metal it is fired through creates a circular hole for the electrons to transmit
This creates a ring-shaped pattern

45
Q

How can an electron’s velocity be increased in an electron gun?

A

By increasing the accelerating potential difference between the filament and node

46
Q

What is Einstein’s photoelectric effect equation?

A

hf = KEMAX + O

47
Q

Why do certain materials diffract electrons?

A

Polycrystalline graphite has carbon atoms spaced so close (10-10m) the gap between is similar to the wavelength of electrons and so electrons diffract

48
Q

What are the two ways to estimate Planck Constant?
Which is better?

A

LED Potential Divider Circuit
Vacuum Photocell (Better)