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This unit covers special relativity, quantum basics and nuclear physics — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers special relativity, quantum basics and nuclear physics — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Special Relativity
Students must understand that the speed of light is constant for all observers and that time and length are not absolute. Time dilation and length contraction occur for objects moving at speeds close to the speed of light. Mass-energy equivalence (E = mc²) is a core testable concept.
Key Points
- The speed of light in a vacuum is constant: c = 3.0 × 10⁸ m/s, regardless of the observer's motion
- Time dilation: a moving clock ticks slower than a stationary one (moving observers experience less elapsed time)
- Length contraction: objects moving at relativistic speeds appear shorter in the direction of motion
- Mass-energy equivalence: E = mc², meaning mass can be converted to energy and vice versa
A spaceship travels at 0.9c relative to Earth. An astronaut on the ship measures a trip taking 10 years. How much time passes on Earth?
Because the astronaut is moving at relativistic speed, time dilation means more time passes on Earth than on the ship. Using the time dilation formula, t = t₀ / √(1 - v²/c²), Earth time = 10 / √(1 - 0.81) = 10 / √0.19 ≈ 22.9 years. On exams, you may be given the Lorentz factor γ directly; the key concept is that the moving observer ages less.
2 Quantum Basics
Students must know that light and matter exhibit both wave and particle properties (wave-particle duality). The photoelectric effect demonstrates that light is quantized into photons, each carrying energy E = hf. Electrons in atoms occupy discrete energy levels and emit or absorb photons only when transitioning between levels.
Key Points
- Photon energy: E = hf, where h = 6.63 × 10⁻³⁴ J·s (Planck's constant) and f is frequency
- Photoelectric effect: electrons are ejected from a metal only when photon frequency exceeds the threshold frequency, regardless of light intensity
- Wave-particle duality: electrons and photons exhibit both wave behavior (diffraction) and particle behavior (collisions)
- Atomic energy levels: electrons emit a photon when dropping to a lower level; photon energy equals the difference between levels (E = hf = ΔE)
Light of frequency 8.0 × 10¹⁴ Hz strikes a metal with a work function of 2.0 eV. Does the photoelectric effect occur, and what is the maximum kinetic energy of the ejected electron?
First calculate photon energy: E = hf = (6.63 × 10⁻³⁴)(8.0 × 10¹⁴) ≈ 5.3 × 10⁻¹⁹ J ≈ 3.3 eV. Since 3.3 eV exceeds the 2.0 eV work function, the photoelectric effect does occur. The maximum kinetic energy of the ejected electron is KE = 3.3 eV − 2.0 eV = 1.3 eV.
3 Nuclear Physics
Students must be able to interpret nuclear equations, identify types of radioactive decay (alpha, beta, gamma), and apply the concept of half-life. Conservation of mass number and atomic number must hold in all nuclear reactions. Nuclear fission and fusion both release energy because products have less mass than reactants.
Key Points
- Alpha decay: nucleus loses 2 protons and 2 neutrons (emits ⁴₂He); atomic number decreases by 2, mass number by 4
- Beta decay: a neutron converts to a proton, emitting an electron (β⁻); atomic number increases by 1, mass number unchanged
- Half-life: the time for half of a radioactive sample to decay; after n half-lives, amount remaining = N₀ × (1/2)ⁿ
- Binding energy: energy required to break a nucleus apart; greater binding energy per nucleon = more stable nucleus
A sample of a radioactive isotope has a half-life of 20 years. If you start with 80 g, how much remains after 60 years?
First determine the number of half-lives elapsed: 60 years ÷ 20 years per half-life = 3 half-lives. Apply the half-life formula: remaining mass = 80 × (1/2)³ = 80 × (1/8) = 10 g. On exams, always divide total time by the half-life first to find n before calculating.
Questions, answered.
What is Modern Physics?
Modern Physics is Unit 10 of Physics, covering special relativity, quantum basics and nuclear physics.
How to study for Physics Unit 10?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 25+ review questions across 5 different game modes.