Olive Defence

Physics · AFCAT

PA09 — Modern Physics: Atomic, Nuclear & Electronics

✈ Physics – PA09 · AFCAT General Awareness AFCAT Level

Modern Physics covers the innermost structure of matter and the devices that power the digital world. For AFCAT, focus on knowing the three radiation types (α, β, γ), which atomic model explained hydrogen spectra (Bohr), and the truth table for basic logic gates — all direct recall questions.

📌 AFCAT Focus: Alpha (α) vs beta (β) vs gamma (γ) — charge, penetration, ionisation; Bohr model key postulates; half-life concept; fission (atom bomb, nuclear reactor) vs fusion (hydrogen bomb, Sun's energy); E = mc²; p-n junction diode (forward = conducts); AND, OR, NOT gate truth tables.

1. Atomic Models

Model	Scientist	Key Idea	Limitation
Plum Pudding	Thomson (1897)	Electrons embedded in a diffuse positive sphere like plums in a pudding	Could not explain Rutherford's alpha particle scattering experiment
Nuclear Model	Rutherford (1911)	Tiny, dense, positive nucleus at centre; electrons orbit at large distances	Could not explain atomic stability — electrons should spiral inward and emit radiation
Bohr's Model	Bohr (1913)	Electrons in fixed circular orbits (shells); no radiation while in orbit; photon emitted/absorbed when jumping between orbits	Could not explain spectra of multi-electron atoms

💡 Bohr's Key Postulates (AFCAT-level):
1. Electrons revolve in allowed circular orbits — stationary states — without radiating energy.
2. Energy is emitted/absorbed only when electron jumps between orbits: E = hf.
3. Bohr's model successfully explained the hydrogen emission spectrum (Balmer series). This is its most important achievement.

2. Radioactivity — Alpha, Beta & Gamma Radiation

Fig. 1 — Three Types of Radiation: Charge, Penetration Power, and Ionising Power

💡 Key Memory Aid: Penetrating power is the OPPOSITE of ionising power.
Alpha: most ionising, least penetrating (stopped by paper or skin).
Gamma: least ionising, most penetrating (needs several centimetres of lead). These two facts are directly tested in AFCAT.

3. Half-Life

Half-Life — AFCAT Calculation Type:

● Half-life (T½): time for half the radioactive atoms to decay
● Amount remaining after n half-lives: N = N₀ × (1/2)ⁿ

Example: T½ = 10 years; Initial = 80 g. How much remains after 30 years?
n = 30/10 = 3 half-lives. Remaining = 80 × (1/2)³ = 80 × 1/8 = 10 g

● After 1 T½: 50% remains | After 2 T½: 25% | After 3 T½: 12.5% | After 4 T½: 6.25%

4. Nuclear Fission vs Fusion

⚡ Nuclear FISSION

Heavy nucleus (U-235, Pu-239) SPLITS into lighter nuclei + neutrons + energy
Triggered by slow neutron hitting nucleus
Chain reaction: 2–3 neutrons released trigger further fissions
Atomic bomb (A-bomb): uncontrolled chain reaction
Nuclear reactor: controlled chain reaction → electricity
Moderator (heavy water) slows neutrons; Control rods (cadmium) absorb neutrons
E = mc² — mass defect converts to enormous energy

⚡ Nuclear FUSION

Light nuclei (hydrogen isotopes) COMBINE to form heavier nucleus + energy
Requires extremely high temperature (~10⁷ K) — thermonuclear
Releases MORE energy per unit mass than fission
Hydrogen bomb (H-bomb): uncontrolled fusion
Sun's energy: continuous fusion of H → He
Controlled fusion: future energy goal (ITER project)

5. Semiconductors & Logic Gates

Fig. 2 — Basic Logic Gates: AND, OR, NOT with Truth Tables (Most Tested in AFCAT)

Semiconductors & Diode — Quick Reference:

● n-type: Doped with pentavalent (5-valence) atoms (P, As, Sb) → extra free electrons as majority carriers
● p-type: Doped with trivalent (3-valence) atoms (B, Al, Ga) → holes as majority carriers

● p-n Junction Diode:
Forward bias (p to +, n to −): current flows (diode ON)
Reverse bias (p to −, n to +): no current (diode OFF)
● Rectifier: converts AC to DC using diodes (half-wave: 1 diode; full-wave: 4 diodes)

● Universal gates: NAND and NOR — any logic circuit can be built from NAND alone (or NOR alone)

📝 AFCAT PYQs — Modern Physics

Q1. Which radiation has the highest penetrating power? AFCAT PYQ

(a) Alpha (α)(b) Beta (β)(c) Gamma (γ)(d) All equal

✔ Answer: (c) Gamma (γ)

Gamma radiation (electromagnetic waves) has the highest penetrating power — it requires several centimetres of lead or thick concrete to stop. Alpha has the lowest penetrating power (stopped by a sheet of paper or skin). However, the order reverses for ionising power: Alpha > Beta > Gamma. Penetrating ∝ inversely to ionising — common AFCAT question.

Q2. The half-life of a radioactive substance is 5 years. If we start with 64 g, how much remains after 20 years? AFCAT PYQ

(a) 16 g(b) 8 g(c) 4 g(d) 2 g

✔ Answer: (c) 4 g

n = 20/5 = 4 half-lives. Remaining = 64 × (1/2)⁴ = 64 × 1/16 = 4 g. Step by step: 64→32→16→8→4. Each step is one half-life. This type of step-by-step calculation appears directly in AFCAT — the key is to find n first.

Q3. The energy released in the Sun is due to: AFCAT PYQ

(a) Nuclear fission(b) Chemical burning(c) Nuclear fusion(d) Radioactive decay

✔ Answer: (c) Nuclear fusion

The Sun produces energy through nuclear fusion — hydrogen nuclei fuse to form helium at extremely high temperatures (~15 million K) in the core. This is also the principle of the hydrogen bomb. Nuclear fission (splitting of U-235) is used in atomic bombs and nuclear power plants — not the Sun. This distinction is a top AFCAT question.

Q4. The output of an AND gate is 1 when: AFCAT PYQ

(a) Any one input is 1(b) Both inputs are 0(c) Both inputs are 1(d) One input is 0

✔ Answer: (c) Both inputs are 1

AND gate: output = 1 only when ALL inputs are 1. For inputs A=1, B=1: Y=1. For any other combination (0,0), (0,1), (1,0): Y=0. OR gate: output = 1 when ANY input is 1 (output 0 only when both are 0). NOT gate: inverts — 0 becomes 1, 1 becomes 0. These truth tables are directly asked in AFCAT.

Q5. Which atomic model successfully explained the hydrogen spectrum? AFCAT PYQ

(a) Thomson model(b) Rutherford model(c) Bohr model(d) Dalton model

✔ Answer: (c) Bohr model

Bohr's model (1913) introduced quantised (fixed-energy) orbits for electrons. When an electron jumps from a higher orbit to a lower orbit, it emits a photon of specific energy: E = hf. This predicted the exact wavelengths of hydrogen emission spectrum (Balmer series) — something Rutherford's model could not do. Bohr's is the only model that explained the hydrogen spectrum.

🧠 Quick Memory Chart — PA09 Modern Physics

☢ Radiation

α: +2 charge, heaviest, least penetrating
β: −1, medium
γ: neutral, most penetrating
Penetration ↑ = Ionising ↓
α stopped by paper; γ by lead

⚡ Fission vs Fusion

Fission: heavy nucleus splits (A-bomb, reactor)
Fusion: light nuclei join (H-bomb, Sun)
Fusion: more energy per kg
Moderator slows neutrons
E = mc²; Sun = fusion

📋 Logic Gates

AND: Y=1 only if ALL=1
OR: Y=0 only if ALL=0
NOT: inverts (0→1, 1→0)
NAND & NOR: universal gates
Half-life: N = N₀/2ⁿ

📝 Practice Exercise

E1. Which type of radiation is not deflected by electric or magnetic fields?

(a) Alpha(b) Beta(c) Gamma(d) Both alpha and beta

E2. A NOR gate output is 1 when:

(a) Both inputs are 1(b) Any one input is 1(c) Both inputs are 0(d) One is 0 and other is 1

E3. Nuclear fission is used in:

(a) Hydrogen bomb(b) Nuclear power reactor(c) Sun's energy production(d) Thermonuclear reaction

E4. The half-life of a substance is 10 years. After 40 years, fraction remaining from 100 g is:

(a) 50 g(b) 12.5 g(c) 25 g(d) 6.25 g

Answers: E1 → (c) Gamma [no charge → not deflected; α and β are charged → deflected by fields] | E2 → (c) Both inputs are 0 [NOR = NOT(OR); OR is 0 only when both=0; NOT(0)=1] | E3 → (b) Nuclear power reactor [controlled fission of U-235; H-bomb and Sun use fusion] | E4 → (d) 6.25 g [n=40/10=4; 100×(1/2)⁴ = 100/16 = 6.25 g]

📝 Mock Tests 🎯 Subject Quiz ✈️ Telegram