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MODERN PHYSICS Flashcards

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1
Q

What is the photoelectric effect?

A

When light shines on a metal surface and liberates electrons from it.

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2
Q

Why is the photoelectric effect significant in physics?

A

It showed that light behaves as particles (photons) and led to the development of quantum theory, challenging classical wave-based thinking.

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3
Q

What is the equation for the maximum kinetic energy of a photoelectron?

A

E k=hf−ϕ, where

E k is the photoelectron’s kinetic energy

h is Planck’s constant

f is the light’s frequency

𝜙 is the work function of the metal

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4
Q

What is the work function (Φ)?

A

The minimum energy required to liberate an electron from a metal surface.

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5
Q

What unit is commonly used for photoelectron energy?

A

Electron-volt (eV), where 1eV = 1.6x10^-19J

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6
Q

What is a phototube (or photocell)?

A

A vacuum tube that uses the photoelectric effect to create a photocurrent when light hits its cathode.

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7
Q

What causes the photocurrent in a photocell?

A

Only the light hitting the cathode causes the photocurrent—not voltage.

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8
Q

What is the stopping potential?

A

The voltage that stops photoelectrons from reaching the anode; at this point, photocurrent becomes zero.

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9
Q

What is the speed of visible light in a vacuum?

A

c= 3.0x10^8 m/s

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10
Q

What is the wavelength range of visible light?

A

400 nm (violet) to 800 nm (red).

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11
Q

What is the frequency range of visible light?

A

4x10^14HZ (red) to 8x10^14HZ (violet)

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12
Q

What is a quantum?

A

The smallest possible discrete unit of something—such as energy in the form of photons.

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13
Q

What is a photon?

A

A quantum of electromagnetic energy, behaving as both a particle and a wave.

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14
Q

What equation links energy of a photon with its frequency?

A

E=hf

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15
Q

What does increasing the frequency of light do to the photoelectrons?

A

Increases their kinetic energy, as E k=hf−ϕ,

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16
Q

What does increasing light brightness (intensity) do?

A

Increases the number of photons → more photoelectrons → higher photocurrent, but does not increase photoelectron energy.

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17
Q

What are the three main conclusions about brightness (intensity)?

A

Bright light → more photoelectrons per second (↑ photocurrent)

Photoelectrons have same energy as dim light of same frequency

Liberation is instantaneous, even with dim light

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18
Q

What is the threshold frequency (f0)

A

The minimum frequency of light needed to liberate electrons: hf0=ϕ

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18
Q

What are the three main conclusions about frequency (colour)?

A

If F<F0, no electrons ejected

Low f = low energy electrons

High f = higher energy electrons

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19
Q

What happen when light with f<f0 hits metal?

A

No photoelectrons are emitted—regardless of intensity.

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20
Q

What does the gradient of a photoelectric graph represent?

A

Planck’s constant h

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21
Q

What does the y-intercept of a photoelectric graph represent?

A

−ϕ (negative work function)

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22
Q

What does the x-intercept of a photoelectric graph represent?

A

Threshold frequency f0

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23
Q

What must you always check before using the gradient to calculate ℎ h from a graph?

A

That the y-axis is in joules, not volts or eV—convert if necessary.

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24
What are the control, independent, and dependent variables in a photoelectric experiment?
Control: Light frequency (monochromatic) Independent: Light intensity Dependent: Number and energy of photoelectrons
25
What does the photocurrent depend on?
Number of photoelectrons per second, which depends on light intensity.
26
How does frequency affect photocurrent?
It doesn’t—frequency affects energy, not number of photoelectrons.
27
What problem with classical physics did the Bohr model resolve?
Classical physics predicted that electrons orbiting the nucleus would continuously emit energy and spiral into the nucleus. The Bohr model resolved this by proposing that electrons occupy stable, quantised orbits (energy levels) where they do not emit radiation.
28
What are the key postulates of the Bohr model of the atom?
Electrons orbit the nucleus in fixed energy levels (quantised orbits). Electrons do not radiate energy while in a stable orbit. Electrons emit or absorb energy only when they transition between energy levels, with energy given by 𝐸=ℎ𝑓
29
How is light emitted according to the Bohr model?
When an electron drops from a higher energy level to a lower one, it emits a photon with energy equal to the difference between the two energy levels: Ephoton = Ehigh-Elow = hf
30
What is meant by a "quantised energy level"?
A quantised energy level means electrons can only occupy specific, discrete energy states—nothing in between. They can gain or lose only fixed amounts of energy (quanta).
31
What is the equation for the energy of a photon emitted or absorbed during a transition between two levels in a hydrogen atom?
E=hf = hc/lambda
32
What is the relationship between energy level transitions and the emission/absorption spectra?
Emission spectrum: Shows the wavelengths of light emitted when electrons fall to lower energy levels. Absorption spectrum: Shows the wavelengths of light absorbed when electrons are excited to higher energy levels. Each element has a unique spectrum due to its unique energy levels.
33
What are spectral lines?
Spectral lines are the specific wavelengths (or frequencies) of light emitted or absorbed by an atom due to electron transitions between energy levels. They appear as lines in an emission or absorption spectrum.
34
Why does hydrogen have a line spectrum and not a continuous spectrum?
Because the hydrogen atom has discrete energy levels. Electrons can only transition between these specific levels, resulting in the emission or absorption of photons with precise energies, not a continuous range.
35
What is the Lyman series in the hydrogen spectrum?
Transitions where electrons fall to the n=1 level. Emits ultraviolet radiation. Energies are highest because electrons drop to the lowest level.
36
What is the Balmer series in the hydrogen spectrum?
Transitions where electrons fall to the n=2 level. Emits visible light. Includes familiar spectral lines seen in hydrogen lamps.
37
What is the Paschen series in the hydrogen spectrum?
Transitions where electrons fall to the n=3 level. Emits infrared radiation.
38
How is the frequency of light related to the energy difference between two levels?
f=E/h
39
How is the wavelength of a spectral line calculated from an energy difference?
lambda = hc/E OR lambda = c/f
40
What are the units of energy in atomic transitions, and how are they converted?
Energy is often in electron-volts (eV). 1eV = 1.6x10^-19J To convert eV into J: MULTIPLY To convert J to eV: DIVIDE
41
What does the Bohr model explain well, and where does it fall short?
Explains well: Hydrogen’s emission and absorption spectra The concept of quantised energy levels Falls short: Cannot fully explain spectra of more complex atoms Doesn’t incorporate electron wave behaviour or probability clouds (fixed orbits are too simplistic)
42
What modern model replaced Bohr’s?
The Quantum Mechanical Model, which uses orbitals (probability regions) instead of fixed orbits. It incorporates wave-particle duality and Heisenberg’s uncertainty principle.
43
What is nuclear binding energy?
The nuclear binding energy is the energy required to break a nucleus into its individual protons and neutrons. It is also the energy released when a nucleus is formed from those nucleons (protons and neutrons). It represents how tightly bound the nucleus is.
44
What are nucleons?
Nucleons are the particles that make up the nucleus: protons and neutrons. Nucleon number = mass number (A).
45
What is mass defect (Δm)?
The mass defect is the difference between: - the total mass of all individual nucleons, and - the actual mass of the nucleus.
46
What is Δm
(mass of separate protons + neutrons) − (mass of nucleus)
47
What has missing mass been converted into?
This missing mass has been converted into BINDING ENERGY E=mc^2
48
Why does the nucleus weigh less than the sum of its parts?
Because the missing mass was converted into binding energy when the nucleus formed. This energy keeps the nucleus together.
49
What is the equation for binding energy using mass defect?
E=mc^2
50
What is the unit of binding energy?
Joules
51
How do you convert a mass defect from atomic mass units (u) into energy?
1u = 931.5 MeV SO E=Δm (in u) x 931.5 MeV
52
What’s the equation for binding energy per nucleon and why is it important?
Binding Energy per Nucleon = Total Binding Energy/Number of Nucleons (A) It shows how stable a nucleus is. Higher value = more stable nucleus.
53
Which element has the highest binding energy per nucleon?
Iron - 56 (Fe56)
54
What is the significance of the binding energy per nucleon curve?
It shows: Light nuclei (like H, He) can fuse to increase B.E./nucleon → Fusion releases energy Heavy nuclei (like U, Pu) can fission to increase B.E./nucleon → Fission releases energy Peak at Iron-56 means fusion/fission both move toward more stable products
55
What’s the difference between fusion and fission in terms of binding energy?
Fusion: Two light nuclei combine → product has higher B.E./nucleon → energy is released. Fission: A heavy nucleus splits into lighter nuclei → products have higher B.E./nucleon → energy is released. In both cases, mass is lost and converted to energy.
56
Why does fusion release energy?
Because the new nucleus formed has a higher binding energy per nucleon than the original nuclei. That difference in energy is released, often as kinetic energy or radiation.
57
Why does fission release energy?
Because the sum of binding energies of the fission products is greater than that of the original heavy nucleus. The extra binding energy is released as energy (E = mc²).
58
What does a larger binding energy per nucleon tell you about a nucleus?
It is more stable. It would require more energy to break apart. It has less potential energy, meaning it's at a lower energy state.
59
What is the role of E = mc² in nuclear reactions?
It explains that mass is not conserved in nuclear reactions—mass is converted into energy, and vice versa. That’s why small mass defects result in large energy outputs.
60
How do you answer a question comparing nuclear stability?
Calculate binding energy per nucleon for each nucleus The one with the highest BE/nucleon is more stable Justify using: “More energy required to break apart nucleus → more stable”
61
How do you handle “mass of proton = ..., mass of neutron = ..., nucleus mass = ...” type questions?
Add individual proton + neutron masses Subtract actual nucleus mass → mass defect Convert to energy using 𝐸=Δ𝑚𝑐^2 or ×931.5 if in u Divide by A if asked for per nucleon
62
What is mass defect
The mass defect is the difference between the total mass of the individual nucleons (protons and neutrons) that make up a nucleus and the actual mass of the nucleus.
63
Where does some of the mass go
BINDING ENERGY
64
What is binding energy?
The binding energy is the energy required to break a nucleus into its constituent protons and neutrons. It is also the energy released when a nucleus is formed from its separate nucleons. This energy is what keeps the nucleus stable, as it is the energy needed to overcome the attractive forces between the nucleons.
65
What is binding energy per nucleon
Binding energy per nucleon is the binding energy divided by the number of nucleons (protons and neutrons) in a nucleus. This value indicates how tightly the nucleons are bound in the nucleus. A higher binding energy per nucleon means a more stable nucleus.
66
What is fusion
Fusion is the nuclear process where two light nuclei (e.g., hydrogen isotopes) combine to form a heavier nucleus, releasing energy.
67
Why does fusion occur?
Fusion occurs because the total binding energy of the resulting nucleus is higher than the sum of the binding energies of the original nuclei.
68
What is fission?
Fission is the nuclear process where a heavy nucleus (e.g., uranium-235) splits into two lighter nuclei, releasing energy.
69
Why does fission occur?
Fission occurs because the total binding energy of the fission products is greater than the binding energy of the original nucleus

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