🔬 Unit 3 – Part A (2-Mark Q&A)

1) Any two uses of Li-ion battery

• Portable electronics: phones, laptops, cameras, drones.
• Electric mobility & storage: EVs/HEVs, e-bikes, home/solar batteries.

2) Principle of a fuel cell

Direct conversion of chemical energy → electrical via redox at separate electrodes; continuous operation if fuel/oxidant supplied.

H₂–O₂ (acidic) cell: Anode: H₂ → 2H⁺ + 2e⁻ , Cathode: ½O₂ + 2H⁺ + 2e⁻ → H₂O , Overall: H₂ + ½O₂ → H₂O + energy

3) What is a moderator?

Material that slows fast neutrons → thermal neutrons to enhance fission probability. Examples: light water, heavy water (D₂O), graphite, Be.

4) Critical mass

Minimum mass of fissile material needed to sustain a self-maintaining chain reaction. Depends on density, geometry, purity, and neutron reflectors.

5) Lead–acid accumulator

Rechargeable battery: PbO₂ (pos), Pb (neg), electrolyte H₂SO₄.

Discharge: PbO₂ + Pb + 2H₂SO₄ → 2PbSO₄ + 2H₂O (≈2.0 V per cell).

Charge: Reverse of the above reaction.

6) Why fission & fusion release large energy?

Due to mass defect and rise in binding energy per nucleon; tiny Δm converts to energy by E = mc².

7) Battery classification with examples

• Primary (non-rechargeable): Dry cell, alkaline MnO₂–Zn.
• Secondary (rechargeable): Lead–acid, Li-ion, Ni-MH.

8) Role of moderator in a reactor

Reduces neutron energy to thermal range → increases fission cross-section of U-235/Pu-239, stabilizing the chain reaction; should have low absorption cross-section.

9) Advantages of Li-ion cell

• High energy density & voltage (≈3.6–3.7 V/cell).
• Low self-discharge, no memory effect, fast charge, long cycle life.
• Wide range of chemistries (LFP, NMC, NCA) for safety/performance.

10) Example for nuclear fission & fusion

Fission: ¹²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + 3 ¹n + energy

Fusion: ²H + ³H → ⁴He + ¹n + energy

11) Microbial fuel cell (MFC) – short note

Bacteria at the anode oxidize organics → release e⁻/H⁺; e⁻ travel through circuit, H⁺ through membrane; at cathode typically O₂ is reduced to H₂O. Generates power from wastewater/biomass.

12) Applications of solar cells

• Grid-tie/rooftop PV, solar farms.
• Remote power: satellites, telecom towers, rural lighting.
• Charging for gadgets, solar pumps, street lights, traffic signs.

13) Primary vs Secondary battery

Feature	Primary	Secondary
Reusability	Single-use	Rechargeable
Reaction	Irreversible	Reversible
Examples	Dry cell, Alkaline	Lead–acid, Li-ion, Ni-MH

14) What is a solar cell?

PN-junction device converting light → electricity (photovoltaic effect).

Current equation: I = I_ph − I₀\{e^{qV/kT} − 1\}; efficiency η = P_out/P_in.

15) A fission reaction of ²³⁵U

²³⁵U + ¹n → ²³⁶U* → ¹⁴¹Ba + ⁹²Kr + 3 ¹n + energy (many product pairs possible; ²³⁶U* is excited intermediate).

16) How wind energy is generated

Wind turns rotor blades → shaft → generator; power captured scales as P ∝ ½ρ A v³ (air density ρ, swept area A, wind speed v).

17) Any two examples of secondary cells

• Lead–acid accumulator.
• Lithium-ion cell (e.g., LFP/NMC).

18) Fast breeder reactor

Reactor using fast neutrons that produces more fissile fuel than it consumes (breeding ratio > 1). Example pathway: ²³⁸U(n,γ) → ²³⁹U → ²³⁹Np → ²³⁹Pu.

19) Examples of EV batteries

• Li-ion chemistries: NMC/NCA (high energy), LFP (safer, long life).
• Ni-MH (older hybrids), emerging solid-state (solid electrolyte).

20) Role of moderator (again)

Ensures neutrons reach thermal energies without being absorbed; chosen for high scattering and low absorption cross-sections (e.g., D₂O, graphite).