Comprehensive Physics Review – Electromagnetic Waves to Elementary Particles

200 question-and-answer flashcards spanning electromagnetic waves, quantum and nuclear physics, lasers, and elementary particles.

What must accelerate in order to produce electromagnetic waves?

Electric charges must accelerate.

In an electromagnetic wave, how are the E and B fields oriented relative to each other?

They are perpendicular to each other.

At what speed do electromagnetic waves propagate in free space?

At the speed of light, 3.00 × 10⁸ m s⁻¹.

Write the wave equation for the electric field of a sinusoidal EM wave.

E = E₀ sin(ωt – kx).

Why can electromagnetic waves travel through a vacuum while mechanical waves cannot?

EM waves do not require a material medium; they consist of self-propagating electric and magnetic fields.

State Maxwell’s theoretical relationship for the speed of light in free space.

c = 1 / √(ε₀ μ₀).

Numerically, what are the values of ε₀ and μ₀?

ε₀ = 8.854 × 10⁻¹² F m⁻¹ and μ₀ = 4π × 10⁻⁷ N A⁻².

What portion of the electromagnetic spectrum has wavelengths between 10⁻¹ m and 10⁻³ m?

Microwaves.

Give the frequency range of visible light.

Approximately 4 × 10¹⁴ Hz to 7.5 × 10¹⁴ Hz.

Which EM radiation has the shortest wavelength listed in the provided spectrum table?

Gamma rays, about 10⁻¹³ m to 10⁻¹⁰ m.

Which experiment first proved that accelerating charges radiate electromagnetic waves?

Hertz’s spark-gap radio wave demonstration.

Define the photoelectric effect.

Ejection of electrons from a metal surface when illuminated by electromagnetic radiation of sufficient frequency.

In photoelectric experiments, what is the stopping potential?

The reverse potential needed to reduce the photocurrent to zero by stopping the most energetic photoelectrons.

How does photocurrent vary with light intensity at constant frequency?

It is directly proportional to the intensity.

Does the stopping potential depend on intensity?

No, it depends only on the frequency of the incident radiation.

State Einstein’s photoelectric equation.

hf = KE_max + φ, where φ is the work function.

What is the work function?

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

Define threshold frequency.

The minimum frequency below which no photoelectric emission occurs, regardless of intensity.

Give the expression for the maximum kinetic energy of photoelectrons in terms of stopping potential.

KEmax = e Vs.

Planck’s constant has what accepted value?

h ≈ 6.63 × 10⁻³⁴ J s.

What type of spectrum is produced by hydrogen’s Balmer series?

Visible line spectrum.

For hydrogen, what is the ground-state energy?

−13.6 eV.

Write the formula for hydrogen’s energy levels.

Eₙ = −13.6 eV / n².

What is the radius of the first Bohr orbit?

0.0529 nm.

How is the angular momentum of an electron quantized in Bohr’s model?

L = n h / 2π.

What experimental evidence supports de Broglie waves?

Electron diffraction patterns (e.g., Davisson-Germer experiment).

State de Broglie’s relation.

λ = h / p.

Which instrument exploits short electron wavelengths for imaging?

The electron microscope.

What is the de Broglie wavelength of a 1 keV electron (non-relativistic)?

≈ 0.039 nm.

What is an electron volt?

Energy gained by an electron moving through 1 volt; 1 eV = 1.6 × 10⁻¹⁹ J.

Which two forces bind protons and neutrons inside nuclei?

The strong nuclear force and electromagnetic repulsion (balanced by strong force).

Define atomic number.

Number of protons in the nucleus, symbol Z.

Define mass number.

Total number of nucleons (protons + neutrons), symbol A.

What is mass defect?

The difference between the mass of a nucleus and the sum of masses of its separated nucleons.

How is nuclear binding energy related to mass defect?

E_b = Δm c².

One unified atomic mass unit equals how many MeV/c²?

931.5 MeV.

What is meant by binding energy per nucleon?

Average energy required to remove a single nucleon from the nucleus.

At approximately what mass number is binding energy per nucleon maximum?

Around A ≈ 56 (iron region).

Define half-life.

Time required for half of a radioactive sample’s nuclei to decay.

Write the decay law for number of nuclei remaining.

N(t) = N₀ e^(−λt).

How is decay constant λ related to half-life T₁/₂?

λ = ln 2 / T₁/₂.

What unit measures radioactive activity?

The becquerel (Bq) — one decay per second.

Name the three common types of radiation emitted in radioactive decay.

Alpha particles, beta particles, and gamma rays.

What is background radiation?

Natural ionizing radiation from cosmic rays, rocks, and atmosphere present even without artificial sources.

Which common isotope is used for thyroid scans?

Iodine-131.

Describe spontaneous emission in lasers.

Random photon emission from excited atoms returning to lower energy states.

What is stimulated emission?

Process where an incoming photon induces an excited atom to emit a coherent photon.

Define population inversion.

Condition where more atoms are in an excited state than in the ground state.

Which material serves as the gain medium in a ruby laser?

Cr³⁺-doped Al₂O₃ crystal (ruby).

Role of the optical resonant cavity in a laser?

To provide feedback and amplify coherent light through repeated stimulated emissions.

List two characteristics of laser light.

Highly monochromatic and highly coherent (also extremely low divergence).

What minimum wavelength emerges from an X-ray tube with 50 kV accelerating voltage?

λ_min = hc / eV ≈ 0.025 nm.

Explain bremsstrahlung.

Continuous X-ray emission produced when high-speed electrons decelerate in a target.

What creates characteristic X-rays?

Electron transitions between inner shells after ejection of K- or L-shell electrons.

State Bragg’s law for X-ray diffraction.

2 d sin θ = n λ.

What is the typical spacing d between atomic planes in crystals used for X-ray diffraction?

Of the order 0.1 nm (10⁻¹⁰ m).

Name three uses of X-rays.

Medical imaging, crystallography, and security scanning.

What is thermionic emission?

Release of electrons from a heated filament.

Purpose of control rods in a nuclear reactor?

Absorb neutrons to regulate chain reaction.

Define critical mass.

Minimum mass of fissile material needed to sustain a chain reaction.

Write a representative fission reaction for U-235 with one neutron.

²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3 n + energy (one of many possible fragment pairs).

Give an example fusion reaction in stars.

¹H + ¹H → ²H + e⁺ + νₑ + 0.42 MeV.

Which basic force is responsible for beta decay?

The weak nuclear force.

Identify the carriers (gauge bosons) of the weak force.

W⁺, W⁻, and Z⁰ bosons.

Which particle mediates the electromagnetic force?

The photon γ.

Name the particles that mediate the strong force in quantum chromodynamics.

Gluons.

How many quark flavors exist?

Six — up, down, charm, strange, top, and bottom.

What fractional charge does an up quark carry?

+⅔ e.

What is the quark content of a π⁺ meson?

u anti-d (u d̄).

Which lepton is associated with the muon neutrino?

The muon (μ⁻).

State two properties of neutrinos.

Neutral charge and extremely small rest mass.

Why are neutrinos difficult to detect?

They interact only via the weak force and have no electric charge.

What is meant by lepton number conservation?

Total lepton number (leptons – antileptons) remains constant in any interaction.

Give the baryon number of a proton.

+1.

What happens to baryon number in beta decay?

It is conserved; proton converts to neutron or vice versa with no net change.

Which process converts a neutron to a proton inside a nucleus?

Beta minus (β⁻) decay.

What particle pair is produced in pair production from a gamma photon?

An electron and a positron.

State the threshold energy for electron-positron pair production.

At least 1.022 MeV (2 mₑc²).

Define annihilation.

Process in which a particle and its antiparticle destroy each other, converting mass to photons.

Which decay mode emits a positron?

Beta plus (β⁺) decay.

What symbol represents an antiproton?

p̄.

Write the quark composition of a neutron.

u d d.

Which basic force explains planetary orbit attraction?

Gravitational force.

Relative to the strong force, how weak is gravity at subatomic scales?

About 10⁻³⁸ times weaker.

What is the approximate range of the weak force?

~10⁻¹⁷ m.

Give one example of a baryon other than proton or neutron.

Λ⁰ (lambda baryon).

Name a meson other than pions.

Kaon (K⁺, K⁻, K⁰).

What is Cherenkov radiation?

Blue glow produced when a charged particle moves through a medium faster than light travels in that medium.

Explain why alpha particles have discrete energies.

They result from transitions between specific nuclear energy states with fixed Q values.

Which radiation is most penetrating?

Gamma rays.

How is Bragg reflection order denoted?

By integer n in 2 d sin θ = n λ.

What is the main function of a moderator in thermal reactors?

To slow down fast neutrons to thermal energies to increase fission probability.

Typical moderator materials?

Graphite, heavy water (D₂O), or light water (H₂O).

State Einstein’s mass-energy equivalence equation.

E = m c².

What is the primary source of the Sun’s energy?

Proton-proton fusion chain reactions.

Give the approximate core temperature of the Sun.

~1.5 × 10⁷ K.

Which device combines energy levels and allowed transitions to predict spectral lines?

Energy level diagram.

For the hydrogen Lyman series, to which final level do electrons transition?

n = 1.

Which interaction conserves both charge and lepton number but changes quark flavor?

Weak interaction (e.g., beta decay).

Define activity in terms of decay constant and number of nuclei.

A = λN.