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Physics OCR A > Module 4: Electrons, Waves and Photons > Flashcards

Module 4: Electrons, Waves and Photons Flashcards

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1
Q

What is the value of e ?

A

1.6 x 10⁻¹⁹

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2
Q

Q= …

A
  • Q=ne
  • Q=It
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3
Q

What is the difference between conventional current and the actual flow of current?

A

Conventional current: +ve to -ve
Actual current: -ve to +ve

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4
Q

Define current.

A

Charge per unit time

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5
Q

Define voltage.

A

Work done per unit charge

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6
Q

Define resistance.

A

Voltage per unit current

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7
Q

Define the term e.m.f.

A
  • Energy transferred from chemical to electrical energy per unit charge
  • Work done by components on charge carriers per unit charge
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8
Q

Define the term p.d.

A
  • Energy transferred from electrical to other forms of energy per unit charge
  • Work done by charge carriers on components per unit charge
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9
Q

In a lamp, why does resistance increase as current increases?

A

More electrons flow per unit time and as a result electrons collide more frequently with metal cations, losing energy as it’s transferred to the ions, which vibrate more. This increases resistance.

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10
Q

What is Ohm’s law?

A

In an ohmic resistor, current is directly proportional to voltage, given that temperature is constant

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11
Q

Define power.

A

Rate of energy transfer

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12
Q

State Kirchoff’s first law.

A

Sum of current into a point is equal to the sum of current out of that point.

This is due to conservation of charge.

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13
Q

State Kirchoff’s second law.

A

Sum of voltage in is equal to the sum of voltage out around a closed loop.

Thisis due to conservation of energy

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14
Q

What type of material are LDRs and thermistors made of?

A

Semiconductors

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15
Q

As light intensity increases…

A

…resistance of LDR decreases

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16
Q

As temperature increases…

A

…resistance of thermistor decreases

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17
Q

What is the potential divider equation?

A

V₁:V₂ = R₁:R₂

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18
Q

What are the advantages of a potentiometer compared to a potential divider?

A
  • More compact, which makes them more portable
  • Can be constructed so that change in resistance is linear or logarithmic
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19
Q

What two constants does every battery have?

A

e.m.f and internal resistance of battery

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20
Q

What equation links e.m.f and internal resistance?

A

e.m.f = IR + Ir

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21
Q

What is the relationship between resistance and length of wire?

A

Resistance ∝ Length

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22
Q

What is the relationship between resistance and cross-sectional area of wire?

A

Resistance ∝ 1 / cross-sectional area

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23
Q

What is the relationship between resistance and resistivity?

A

Resistance ∝ Resistivity

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24
Q

What is the equation that links resistance and resistivity?

A

R = ⍴ L/A

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25
Q

Define number density.

A

Number of charge carriers per unit volume

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26
Q

What is the relationship between number density and resistivity?

A

Number density ∝ 1 / Resistivity

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27
Q

What is the relationship between current and mean drift velocity?

A

I = nAVe

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28
Q

What is the relationship between cross-sectional area and mean drift velocity?

A

Mean drift velocity ∝ 1 / cross-sectional area

This is because, to maintain the same current, electrons must move faster through narrower wires.

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29
Q

What is a progressive wave?

A

The transfer of energy through oscillations in a medium

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30
Q

Define oscillation.

A

Periodic displacement from an equilibrium point

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31
Q

Define a wave pulse.

A

A single disturbance travelling through a medium

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32
Q

What is the difference between a transverse and longitudinal wave?

A
  • Transverse: oscillations are perpendicular to direction of energy transfer
  • Longitudinal: oscillations are parallel to direction of energy transfer
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33
Q

What are compressions and rarefactions?

A
  • Compressions: Regions of higher pressure due to particles being close together
  • Rarefactions: Regions of lower pressure due to particles being further away from each other
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34
Q

Define displacement.

A

Distance from equilibrium position

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35
Q

Define amplitude.

A

Maximum displacement from an equilibrium position

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36
Q

Define wavelength.

A

Minimum distance between two points on a wave in phase

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37
Q

Define period.

A

Time taken for one oscillation

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38
Q

Define frequency.

A

Number of oscillations per unit time

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39
Q

Define wave speed.

A

Distance travelled by wave per unit time

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40
Q

What is phase difference?

A

The difference in displacement between particles on a wave, or the difference between the displacements of particles on two different waves, with each complete cycle or difference of one wavelength representing 360° or 2π radians

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41
Q

What does a phase difference of 2π mean?

A

The two particles/waves are in phase

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42
Q

What does a phase difference of π mean?

A

The two particles/waves are in anti-phase

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43
Q

What is the law of reflection?

A

θᵢ = θᵣ

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44
Q

What is a wave front?

A

A line joining points on a wave which are in phase

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45
Q

When a wave travels into a more optically dense medium…

A

It slows down and refracts towards from the normal as wavelength decreases

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46
Q

When a wave travels into a less optically dense medium…

A

It speeds up and refracts away from the normal as wavelength increases

47
Q

State Snell’s law.

A

n₁sinθ₁ = n₂sinθ₂ (n=1 in air)

48
Q

When does total internal reflection occur?

A

When light strikes the boundary between two media, and all the light is reflected back into the original medium due to the angle of incidence being greater than or equal to the critical angle

49
Q

What are the conditions for total internal reflection?

A
  • Original medium must have a higher refractive index (n)
  • The angle at which it strikes the boundary must be greater than or equal to the critical angle
50
Q

Define the intensity of a progressive wave.

A

The radiant power passing through a surface per unit area

51
Q

What equation links intensity, power and area?

A

I = P/A

52
Q

What is the relationship between intensity and amplitude?

A

Intensity ∝ Amplitude² (only true for 3D waves)

53
Q

What is the relationship between intensity and distance from source?

A

Intensity ∝ 1/distance from source² (only true for 3D point sources)

54
Q

What is the wavelength range for radio waves?

A

λ > 10⁻¹

55
Q

What is the wavelength range for microwaves?

A

10⁻³ < λ < 10⁻¹

56
Q

What is the wavelength range for infrared waves?

A

7 × 10⁻⁷ < λ < 10⁻³

57
Q

What is the wavelength range for visible light?

A

4 × 10⁻⁷ < λ < 7 × 10⁻⁷

58
Q

What is the wavelength range for ultraviolet radiation?

A

10⁻⁸ < λ < 4 × 10⁻⁷

59
Q

What is the wavelength range for x-rays?

A

10⁻¹³ < λ < 10⁻⁸

60
Q

What is the wavelength range for gamma rays?

A

λ < 10⁻¹⁰

61
Q

Why do the wavelength ranges of x-rays and gamma-rays overlap?

A

They are defined by their origin, not their wavelength.

  • x-rays: emitted from fast moving electrons
  • gamma rays: emitted from unstable atomic nuclei
62
Q

What is special about transverse waves?

A

They can be polarised

63
Q

Define diffraction.

A

The spreading out of a wave at a slit or around an object

64
Q

What changes when a wave is diffracted?

A

Nothing, its speed, wavelength and frequency remain the same

65
Q

What is polarisation?

A

When oscillations of a wave are confined to a single plane

66
Q

What happens when transverse waves are reflected off of a surface?

A

It’s partially polarised

67
Q

State the principle of superposition.

A

The principle of superposition states that when two or more waves meet at a same point, the net displacement is equal to the sum of the individual displacements of the waves at that point

68
Q

Define coherence.

A

When two waves emitted from two sources have a constant phase difference.

These two waves must have the same frequency and wavelength.

69
Q

What is complete constructive interference?

A

If two waves are in phase, their maximum displacements line up, increasing amplitude

70
Q

What is complete destructive interference?

A

If two waves are in anti-phase, one wave’s peaks line up with the other’s troughs, and if the magnitude of the displacements are equal, the net displacement at these points equal zero

71
Q

Define path difference.

A

The difference in the distance travelled by two waves from their source to a specific point

72
Q

What is the path difference at the central maxima?

A

0 m

73
Q

What is the path difference at the first order minima?

A

½ λ m

74
Q

What is the path difference at the first order maxima?

A

λ m

75
Q

What equation can be derived from the double slit experiment?

A

nλ/a = x/D

Where:
- n is the order of maxima
- λ is the wavelength of the wave
- a is the slit spacing
- x is the fringe spacing
- D is the distance to screen

76
Q

What are the advantages of using a diffraction grating?

A
  • Interference fringes are bright and sharp
  • Large angle between direction of incident light and fringes
77
Q

What is the net energy transfer of a standing wave?

A

0 J

78
Q

What is the wave speed of a standing wave?

A

0 ms⁻¹

79
Q

What is a node?

A

A point where displacement is always zero

80
Q

What is an antinode?

A

A point of greatest amplitude

81
Q

What is the distance between two adjacent nodes?

A

½ λ m

82
Q

What is the distance between an adjacent node and antinode?

A

¼ λ m

83
Q

How is a standing wave formed on a string?

A

A wave is sent from one fixed end of the string by a signal generator and is reflected at the other fixed end. These two waves are superposed, and due to them having the same frequency, at some points, they are in phase, and at others they are in anti-phase. This results in nodes at points of destructive interference and antinodes at point of constructive interference.

84
Q

What is first frequency that a standing wave forms at called?

A

Fundamental frequency

85
Q

What is the relationship between the wavelength of standing wave and the length of the string it’s formed on?

A

n(λ/2) = L

Where:
- n is the harmonic
- λ is the wavelength
- L is the length of the string

This only applies to none-node and antinode-antinode standing waves

86
Q

What is different about node-antinode standing waves?

A
  • They only form odd harmonics
  • n(λ/4) = L

Where:
- n is the harmonic
- λ is the wavelength
- L is the length of the string

87
Q

What is the value of Planck’s constant, h ?

A

≈6.63 x 10⁻³⁴

88
Q

What equation links the energy of a photon with its frequency?

A

E = hf

89
Q

Define the electron-volt (eV)

A

The energy transferred when one electron moves through a p.d. of one volt.

90
Q

What is special about photons?

A

They don’t have a charge or mass and they travel at light speed.

91
Q

In an LED at the threshold p.d. …

Equations

A

eV = hf

or

eV = hc/λ

92
Q

What is the photoelectric effect?

A

When light above a certain threshold frequency is shone on a metal surface, resulting in the emission of electrons.

93
Q

What is the difference between photoelectrons and normal electrons?

A

There is no difference, the “photo” in photoelectrons only indicates its origin.

94
Q

When and why does the gold leaf move up in a gold leaf electroscope.

A

When the metal plate of the electroscope comes into contact with something negative, the negative charge spreads out throughout the metal plate and gold leaf, causing them to repel each other.

95
Q

When and why does the gold leaf move down in a gold leaf electroscope.

A

If UV light (which is above the threshold frequency) is shone on the metal plate, electrons are emitted due to the photoelectric effect, resulting in the electroscope losing negative charge.

96
Q

What effect does increasing the frequency of incident radiation have on photoelectric emission?

A

The energy of each photon increases, which means that the maximum kinetic energy of each electron emitted increases.

97
Q

What effect does increasing the intensity of incident radiation have on photoelectric emission?

A

It increases the number of electrons emitted per unit time.

98
Q

What is meant by the “one-to-one interaction” between photons and electrons?

A

Only one photon with enough energy can knock off one electron.

This means that even if you have two photons with half the required energy, they couldn’t free an electron.

This explains why if the threshold frequency is not met, increasing intensity has no effect on the energy transfer.

99
Q

If the frequency of incident radiation is greater than the threshold frequency…

A

Photoelectric emission is instantaneous.

100
Q

What is the work function, 𝟇?

A

The minimum energy required to free an electron from the surface of the metal.

101
Q

Due to the principle of conservation of energy, what equation can be derived for the energy of a single photon?

A

hf = 𝟇 + Eₖₘₐₓ

102
Q

What energy transfer occurs when there is an accelerating p.d. ?

A

Electrical potential to kinetic

This is because the direction of acceleration of the electron is the same as the direction of its motion.

103
Q

What energy transfer occurs when there is a stopping p.d. ?

A

Kinetic to electrical potential

This is because the direction of acceleration of the electron is the opposite of the direction of its motion.

104
Q

What is thermionic emission?

A

The emission of electrons through the action of heat.

105
Q

What equation can be used to calculate the stopping potential of an electron?

A

½mv² = eVₛₜₒₚ

106
Q

What is the relationship between accelerating voltage and fringe spacing?

A

As accelerating voltage increases, fringe spacing decreases.

107
Q

What is the de Broglie equation?

A

λ = h/p, where p is the momentum

108
Q

What equation can be derived from the de Broglie equation?

A

Eₖ = h²/(2mλ²)

109
Q

Explain wave-particle duality.

A

Wave particle duality describes how light has wave behaviour (e.g. diffraction) and particle behaviour (e.g. photoelectric effect).

This explains why only light above a certain frequency results in photoelectric emission and why electron emission is instantaneous.

110
Q

What is meant by the de Broglie wavelength of an electron?

A

When electrons are observed to behave like waves, their wavelength depends on its speed/momentum.

111
Q

What are the conditions required for electrons to produce observable diffraction?

A

The gaps between carbon atoms in graphite must be comparable to the wavelength of the electron.

112
Q

Why is maximum kinetic energy of emitted electrons a maximum?

A

Most electrons in the metal require more than the work function to escape.

113
Q

What evidence is there for electrons behaving like waves?

A

When a beam of electrons passes through graphite, diffraction occurs and a ring pattern can be observed on fluorescent screen.

Physics OCR A (5 decks)

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