We will review the development from classical physics to modern physics and confirm the achievements of physics used in everyday life.
Basic Knowledge
Development of physics:
① Classical mechanics (Newton, 17th century): Three laws of motion and universal gravitation
② Thermodynamics (19th century): Law of conservation of energy and entropy
③ Electromagnetism (Faraday and Maxwell, 19th century): Unification of electric fields, magnetic fields, and electromagnetic waves
④ Theory of relativity (Einstein, 1905): Special theory of relativity (E = mc²); 1915: General theory of relativity
⑤ Quantum mechanics (Planck, Bohr, Heisenberg, and Schrödinger, early 20th century): Phenomena at the atomic and molecular level
Relationship between modern technology and physics:
・Semiconductors: Quantum mechanics → smartphones and computers
・GPS: Corrected using relativity → car navigation systems
・MRI: Nuclear magnetic resonance → medical diagnosis
・Lasers: Stimulated emission (quantum mechanics) → optical communication and surgery
Theory of relativity (Einstein; E = mc²; constancy of the speed of light and curvature of spacetime)
Quantum mechanics (physics of the microscopic world of atoms and light; wave-particle duality)
E = mc² (equivalence of mass and energy; source of nuclear energy)
Photoelectric effect (a phenomenon in which light strikes a metal and releases electrons; evidence of the particle nature of light)
Semiconductor (a substance with electrical resistance intermediate between that of a conductor and an insulator; Si, Ge, and others)
Planck constant h (h = 6.63 × 10⁻³⁴ J·s; a fundamental quantum constant)
Further Exploration (Background and Significance)
E = mc² shows that energy E and mass m are equivalent. Since c = 3.0 × 10⁸ m/s, c² = 9 × 10¹⁶. Even a very small mass can produce enormous energy. When 1 g of uranium 235 undergoes nuclear fission, approximately 8 × 10¹⁰ J of energy (equivalent to 3000 kg of coal) is released.
Photoelectric effect: When light of a frequency above a certain value is shone on a metal, electrons are emitted. Einstein explained this as “particles of light (photons),” published a paper in 1905, and received the Nobel Prize in Physics in 1921. Photon energy E = hf (h: Planck constant, f: frequency).
Quantum computers use quantum “superposition” and “entanglement” and may solve certain problems exponentially faster than conventional computers. Applications in drug development, materials design, and code breaking are being researched.
- Classical mechanics → thermodynamics → electromagnetism → relativity and quantum mechanics
- E = mc²: Mass and energy are equivalent
- Photoelectric effect: Explained by Einstein using photons
- Photon energy: E = hf
- Quantum mechanics: Applied to semiconductors, lasers, and MRI
- GPS requires corrections based on the theory of relativity
- The development of science and technology is based on fundamental theories of physics
Points to Note (Common Confusions)
① The special theory of relativity (1905) and the general theory of relativity (1915) are different theories. Special relativity concerns uniform motion, while general relativity includes acceleration and gravity. ② The photoelectric effect was explained by Einstein (in the same 1905 year as relativity), and he received the Nobel Prize for this work. ③ Semiconductors are phenomena that can be understood only through quantum mechanics. They cannot be explained by classical physics. ④ E = mc² applies not only to nuclear energy but to all energy conversions, including chemical reactions; however, the mass change in chemical reactions is extremely small and cannot be measured.
Practice
- State the equation showing the relationship between energy and mass in Einstein’s special theory of relativity.
- Briefly explain the photoelectric effect.
- Name one modern technology in which quantum mechanics is applied.