⚛️ Physics — Modern Physics
1. Atom and Subatomic Particles
DISCOVERY OF SUBATOMIC PARTICLES
- Electron: Discovered by J.J. Thomson (1897)
- Proton: Discovered by Ernest Rutherford (1919) by bombarding nitrogen with α-particles
- Neutron: Discovered by James Chadwick (1932) by bombarding beryllium with α-particles
PARTICLE PROPERTIES
| Particle | Charge | Mass |
|---|---|---|
| Electron | −1.6 × 10⁻¹⁹ C | 9.11 × 10⁻³¹ kg |
| Proton | +1.6 × 10⁻¹⁹ C | 1.673 × 10⁻²⁷ kg |
| Neutron | 0 | 1.675 × 10⁻²⁷ kg |
ATOMIC TERMS
- Atomic Number (Z): Number of protons = Number of electrons (in neutral atom)
- Mass Number (A): Number of protons + Number of neutrons
- Isotopes: Same Z, different A (e.g., ¹H, ²H, ³H)
- Isobars: Same A, different Z (e.g., ²²Na, ²²Ne)
- Isotones: Same number of neutrons (e.g., ¹⁴C, ¹⁶O)
2. Radioactivity
BASICS
- Discovered by Henry Becquerel (1896)
- Spontaneous disintegration of unstable nuclei
- Three types of radiation: α, β, γ
TYPES OF RADIATION
| Type | Nature | Effect on Nucleus |
|---|---|---|
| α-particle | Helium nucleus (²He⁴) | Z decreases by 2, A decreases by 4 |
| β-particle | Electron (₋₁e⁰) | Z increases by 1, A unchanged |
| γ-rays | Electromagnetic waves | Z and A unchanged |
RADIOACTIVE DECAY LAW
- N = N₀·e^(−λt) (λ = decay constant)
- Half-life: T = 0.693/λ
- Average life: τ = 1/λ = 1.44T
- Activity: R = R₀·e^(−λt)
- SI unit: Becquerel (Bq) | 1 Curie = 3.7 × 10¹⁰ Bq
APPLICATIONS OF RADIOISOTOPES
- Carbon dating: Age of archaeological objects (Willard Libby)
- Geological dating: Using ²³⁸U → ²⁰⁶Pb
- Medical: Co-60 (cancer), I-131 (thyroid), P-32 (leukemia)
- Agriculture: P-32 for studying phosphorus uptake
3. Nuclear Reactions
NUCLEAR FISSION
- Splitting of heavy nucleus into lighter nuclei
- Example: ²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + 3¹n + Energy
- Used in: Nuclear reactors, Atom bombs
- Controlled chain reaction: Nuclear reactor
- Uncontrolled chain reaction: Atom bomb
NUCLEAR REACTOR PARTS
- Fuel: ²³⁵U, ²³⁸U, ²³⁹Pu
- Moderator: Heavy water, graphite, beryllium oxide (slows neutrons)
- Coolant: Cold water, liquid oxygen (removes heat)
- Control rods: Cadmium, Boron (absorb neutrons)
NUCLEAR FUSION
- Combining of lighter nuclei to form heavier nucleus
- Requires very high temperature (~10⁷ K) and pressure
- Used in: Hydrogen bomb
- Source of Sun's energy
NUCLEAR FORCES
- Strongest force in nature
- Short range (~10⁻¹⁵ m)
- 100 times stronger than electrostatic force
- 10³⁸ times stronger than gravitational force
- Exchange of meson particles (Yukawa)
4. Photoelectric Effect
BASICS
- Emission of electrons from metal surface when light of suitable frequency falls on it
- Explained by Einstein using Planck's quantum theory
EINSTEIN'S PHOTOELECTRIC EQUATION
- Ek = hν − φ
- φ = hν₀ (Work function = minimum energy to eject electron)
- Threshold frequency (ν₀): Minimum frequency to eject photoelectrons
- Threshold wavelength (λmax): Maximum wavelength
LAWS OF PHOTOELECTRIC EFFECT
- Rate of emission ∝ Intensity of incident light
- Max kinetic energy ∝ Frequency (independent of intensity)
- No emission below threshold frequency
- No time lag between incidence and emission
5. Electromagnetic Spectrum
WAVES AND THEIR USES
| Type | Applications |
|---|---|
| Radio & Microwaves | Radio/TV communication, microwave oven |
| Infrared | Muscular strain treatment, night vision, weather forecasting |
| Visible Light | Human vision, photography |
| Ultraviolet | Sterilisation, water purification, detecting forged documents |
| X-rays | Medical diagnosis, detecting fractures, cancer treatment |
X-RAYS
- Discovered by Röntgen (1895)
- Produced in Coolidge tube
- Wavelength: 0.1 Å to 100 Å
- Bragg's Law: 2d·sin θ = nλ
- Soft X-rays: Longer wavelength | Hard X-rays: Shorter wavelength
6. Devices and Technology
RADAR
- RAdio Detection And Ranging
- Uses radio waves to detect objects (aeroplanes, ships, missiles)
- Uses: Weather detection, metal/oil exploration
MEDICAL IMAGING
- CT Scan: X-ray based 3D imaging for internal structures
- MRI: Uses magnetic field and radio frequency pulses
- Breath Analyser: Detects alcohol using infrared radiation
- Thermographic Scanner: Detects fever using infrared
SEMICONDUCTOR DEVICES
- p-n Junction Diode: Forward bias (p to +), Reverse bias (p to −)
- LED: Light Emitting Diode (GaAsP, GaP)
- Zener Diode: Used as voltage regulator
- Photo Diode: Generates current when light falls on it
- Transistor: Combination of two p-n junctions
- IC: Integrated Circuit (multi-functional device)
7. Solar System and Universe
PLANETS (Distance from Sun)
- Terrestrial (Inner): Mercury, Venus, Earth, Mars
- Jovian (Outer): Jupiter, Saturn, Uranus, Neptune
- Largest planet: Jupiter
- Hottest planet: Venus
- Red planet: Mars
OTHER CELESTIAL BODIES
- Asteroids: Between Mars and Jupiter; largest is Ceres
- Meteoroids: Fragments in space; enter Earth as meteors (shooting stars)
- Comets: Frozen gases and rocks; tail points away from Sun
- Black Hole: Region with immense gravity; nothing can escape
- Galaxy: Our galaxy is Milky Way (250 billion stars)
- Big Bang Theory: Universe expanding since ~15 billion years ago
Summary: Modern Physics at a Glance
| Topic | Key Formula | Key Point |
|---|---|---|
| Half-life | T = 0.693/λ | τ = 1.44T |
| Photoelectric | Ek = hν − φ | Threshold frequency |
| Mass-Energy | E = Δmc² | Einstein's equation |
| Nuclear Fission | ²³⁵U + ¹n → | Nuclear reactor, atom bomb |
| Bragg's Law | 2d·sin θ = nλ | X-ray diffraction |
| Nucleon Force | Meson exchange | Strongest force |
KEY TAKEAWAYS FOR UPSC
- Electron: J.J. Thomson (1897); Proton: Rutherford (1919); Neutron: Chadwick (1932)
- Isotopes: Same Z, different A; Isobars: Same A, different Z
- Radioactivity: Discovered by Becquerel; α, β, γ emissions
- Half-life: T = 0.693/λ; Average life: τ = 1/λ
- Nuclear Fission: Splitting; Nuclear Fusion: Combining
- Photoelectric effect: Explained by Einstein
- X-rays: Discovered by Röntgen; Bragg's law for diffraction
- Largest planet: Jupiter; Hottest planet: Venus
High-Yield Interactive UPSC Quiz (Physics — Modern Physics)
Q1. Who discovered the electron?
Correct Answer: B. J.J. Thomson discovered the electron in 1897.
Q2. What is the half-life formula?
Correct Answer: B. Half-life T = 0.693/λ, where λ is the decay constant.
Q3. Which particle is emitted in beta decay?
Correct Answer: B. Beta particles are electrons (₋₁e⁰).
Q4. What is the work function in photoelectric effect?
Correct Answer: B. Work function (φ) is the minimum energy required to eject an electron from a metal surface.
Q5. What is the SI unit of radioactivity?
Correct Answer: B. The SI unit of radioactivity is Becquerel (Bq).
Q6. Which planet is known as the Red Planet?
Correct Answer: B. Mars is known as the Red Planet due to its reddish appearance.
Q7. What is the source of the Sun's energy?
Correct Answer: B. The Sun's energy comes from nuclear fusion reactions.
Q8. What is the function of a moderator in a nuclear reactor?
Correct Answer: B. A moderator slows down neutrons to sustain chain reaction.
Q9. Which element is used as a control rod in nuclear reactors?
Correct Answer: C. Cadmium and Boron are used as control rods as they absorb neutrons.
Q10. What is the Einstein's mass-energy equivalence formula?
Correct Answer: B. Einstein's mass-energy equivalence is E = mc².