A-Level A-Level Physics — Questions and Answers
Question 1: What is the photoelectric effect and what does it prove?
- Electrons orbit the nucleus in fixed energy levels
- When light of sufficient frequency hits a metal surface, electrons are emitted, proving light behaves as particles (photons) (Correct answer)
- Electrons can be shared between atoms
- Light always behaves as a wave
Correct answer: When light of sufficient frequency hits a metal surface, electrons are emitted, proving light behaves as particles (photons)
The photoelectric effect: photons must have energy ≥ the work function to eject electrons. The emission depends on frequency, not intensity, proving light has particle-like properties (photons with E = hf). This challenged classical wave theory.
Question 2: What is the difference between elastic and inelastic collisions?
- Both conserve kinetic energy
- Elastic collisions conserve both momentum and kinetic energy; inelastic collisions conserve momentum but not kinetic energy (some KE is lost as heat or sound) (Correct answer)
- Inelastic collisions conserve kinetic energy
- Elastic collisions occur only in gases
Correct answer: Elastic collisions conserve both momentum and kinetic energy; inelastic collisions conserve momentum but not kinetic energy (some KE is lost as heat or sound)
In elastic collisions, kinetic energy and momentum are both conserved (e.g., billiard balls). In inelastic collisions, momentum is conserved but kinetic energy is not — some is converted to heat, sound, or deformation.
Question 3: What does the de Broglie equation describe?
- The energy of photons
- The wave-like behaviour of matter: λ = h/mv (wavelength of a particle) (Correct answer)
- The speed of electrons in an orbit
- The energy levels in a hydrogen atom
Correct answer: The wave-like behaviour of matter: λ = h/mv (wavelength of a particle)
The de Broglie equation λ = h/p = h/mv links the wavelength of a particle to its momentum. It demonstrates wave-particle duality: all matter has associated wavelength. At low mass/high speed (electrons), the wavelength is measurable.
Question 4: What is Ohm's Law?
- Power = voltage × current
- Voltage = current × resistance (V = IR), for ohmic conductors at constant temperature (Correct answer)
- Resistance = voltage + current
- Current = resistance ÷ voltage
Correct answer: Voltage = current × resistance (V = IR), for ohmic conductors at constant temperature
Ohm's Law states V = IR: the potential difference across a conductor is directly proportional to the current through it, provided temperature and other physical conditions remain constant. Materials that obey this are 'ohmic conductors'.
Question 5: What is a standing wave and what conditions are needed to form one?
- A wave that travels faster than sound
- A stationary pattern formed by superposition of two identical waves travelling in opposite directions, forming nodes and antinodes (Correct answer)
- A wave with no frequency
- A wave only found in solids
Correct answer: A stationary pattern formed by superposition of two identical waves travelling in opposite directions, forming nodes and antinodes
Standing waves form when two waves of equal frequency and amplitude travel in opposite directions and superpose. They have fixed nodes (zero displacement) and antinodes (maximum displacement). They form in resonating systems like strings, pipes, and cavities.
Question 6: What is the significance of the Hubble constant?
- It measures the density of the universe
- It relates the recession velocity of galaxies to their distance (v = H₀d), showing the universe is expanding (Correct answer)
- It describes the rate of nuclear decay
- It measures the speed of light
Correct answer: It relates the recession velocity of galaxies to their distance (v = H₀d), showing the universe is expanding
The Hubble constant H₀ relates the recession velocity of galaxies to their distance: v = H₀d. Hubble's observation (1929) that more distant galaxies recede faster demonstrated that the universe is expanding, supporting the Big Bang theory.
A-Level A-Level Physics
What is the photoelectric effect and what does it prove?