Kidz Olympiad🎉

Science Class 12 - Modern Physics Notes

Comprehensive study notes for Class 12 - Modern Physics olympiad preparation

Modern Physics

Welcome to the chapter on Modern Physics for Class 12. In this chapter, you will learn about the revolutionary concepts that changed our understanding of the physical world, including quantum mechanics, atomic models, nuclear physics, and the theory of relativity. By the end of this chapter, you will have a strong foundation in the principles and applications of modern physics.

Key Concepts

  • Photoelectric Effect: Emission of electrons from a metal surface when light falls on it.
  • Wave-Particle Duality: Light and matter exhibit both wave-like and particle-like properties.
  • Atomic Models: Evolution from Thomson’s model to Rutherford’s and Bohr’s atomic models.
  • Nuclear Physics: Study of the nucleus, radioactivity, and nuclear reactions.
  • Special Theory of Relativity: Einstein’s theory describing the relationship between space and time.

Photoelectric Effect

The photoelectric effect shows that light can eject electrons from a metal surface. The effect depends on the frequency of light, not its intensity. This experiment supported the particle nature of light and led to the concept of photons.

  • Einstein’s Equation: \( E = h\nu \), where \( h \) is Planck’s constant and \( \nu \) is the frequency.
  • Work function (\( \phi \)): Minimum energy needed to remove an electron from the metal.
  • Kinetic energy of emitted electron: \( KE = h\nu - \phi \)

Wave-Particle Duality

Light and matter can behave as both waves and particles. This is known as wave-particle duality.

  • de Broglie Wavelength: \( \lambda = \frac{h}{p} \), where \( p \) is momentum.
  • Electrons show diffraction and interference, just like light waves.

Atomic Models

  • Thomson’s Model: Plum pudding model, electrons embedded in a positive sphere.
  • Rutherford’s Model: Atom has a small, dense, positively charged nucleus; electrons revolve around it.
  • Bohr’s Model: Electrons move in fixed orbits with quantized energies; explains hydrogen spectrum.

Nuclear Physics

  • Radioactivity: Spontaneous emission of particles or radiation from unstable nuclei (alpha, beta, gamma decay).
  • Nuclear Fission: Splitting of a heavy nucleus into lighter nuclei, releasing energy (used in nuclear reactors).
  • Nuclear Fusion: Combining of light nuclei to form a heavier nucleus, releasing even more energy (occurs in stars).

Special Theory of Relativity

  • Postulates: Laws of physics are the same in all inertial frames; speed of light is constant in vacuum.
  • Time Dilation: Moving clocks run slower.
  • Length Contraction: Moving objects are measured shorter in the direction of motion.
  • Mass-Energy Equivalence: \( E = mc^2 \)

Applications of Modern Physics

  • Photoelectric cells (used in automatic doors, solar panels)
  • Nuclear power plants
  • Medical imaging (X-rays, PET scans)
  • Lasers and semiconductors

Practice Questions

  1. State Einstein’s photoelectric equation and explain each term.
  2. What is de Broglie wavelength? Calculate the wavelength of an electron with momentum \( 2 \times 10^{-24} \) kg·m/s.
  3. Describe two differences between nuclear fission and fusion.
  4. What are the main postulates of the special theory of relativity?
  5. Explain Bohr’s model of the hydrogen atom.

Challenge Yourself

  • Draw and label the energy levels in Bohr’s model for hydrogen.
  • Explain why the photoelectric effect cannot be explained by the wave theory of light.
  • Calculate the energy released when 1 gram of matter is converted into energy using \( E = mc^2 \).

Did You Know?

  • The GPS system in your phone uses corrections from Einstein’s relativity to work accurately!
  • The sun shines because of nuclear fusion reactions at its core.

Glossary

  • Photon: A particle of light with energy \( E = h\nu \).
  • Radioactivity: The process by which unstable nuclei emit radiation.
  • Relativity: The study of how space and time are linked for objects moving at constant speeds.
  • Quantum: The smallest possible discrete unit of any physical property.

Answers to Practice Questions

  1. Einstein’s photoelectric equation: \( KE = h\nu - \phi \), where \( KE \) is kinetic energy of emitted electron, \( h \) is Planck’s constant, \( \nu \) is frequency, and \( \phi \) is work function.
  2. de Broglie wavelength: \( \lambda = \frac{h}{p} \). For \( p = 2 \times 10^{-24} \) kg·m/s, \( \lambda = \frac{6.626 \times 10^{-34}}{2 \times 10^{-24}} = 3.313 \times 10^{-10} \) m.
  3. Fission splits heavy nuclei; fusion joins light nuclei. Fusion releases more energy than fission.
  4. Laws of physics are same in all inertial frames; speed of light is constant in vacuum.
  5. Bohr’s model: Electrons revolve in fixed orbits with quantized energies; explains hydrogen spectrum lines.

Modern physics helps us understand the universe at its smallest and largest scales. Keep exploring!

Quick Navigation