Electricity and Circuits

Semiconductors: Materials that conduct electricity under certain conditions (heat, light, impurities). Silicon (Si) is the most important semiconductor.

Why semiconductors matter for India:

• All modern electronics (smartphones, computers, solar cells, EVs) depend on semiconductor chips
• India has ~0% domestic semiconductor manufacturing (as of 2024) — imports ~₹5 lakh crore in electronics annually
• India Semiconductor Mission (ISM, 2021): ₹76,000 crore incentive programme to build semiconductor fabs in India
• First fab approved: Tata Electronics at Dholera (Gujarat) and Sanand (Gujarat); ISMC at Mysuru (Karnataka) — expected to begin production ~2026-27
• This is a critical geopolitical issue: US-China chip war; India wants to be a semiconductor manufacturing hub

Solar cells = semiconductor devices (silicon PV cells) that convert light directly to electricity (photoelectric effect)

Ohm's Law: V = I × R

• V = Voltage (Potential difference) — measured in Volts (V); named after Alessandro Volta (Italian)
• I = Current (flow of charge) — measured in Amperes (A); named after André-Marie Ampère (French)
• R = Resistance (opposition to current flow) — measured in Ohms (Ω); named after Georg Simon Ohm (German)

Practical relationships:

• Higher voltage → more current (if resistance constant)
• Higher resistance → less current (if voltage constant)
• Double the voltage → double the current; Double the resistance → half the current

India household standard: 230 V AC, 50 Hz (following the British colonial standard; the US uses 120 V at 60 Hz)

Electric Power (P = VI):

• Power = Voltage × Current
• Measured in Watts (W)
• 1 kilowatt-hour (kWh) = 1 unit of electricity — the unit your electricity bill charges for
• A 100W bulb running for 10 hours = 1 kWh = 1 unit consumed

Transmission loss — why Ohm's Law matters at grid scale:

• Transmission loss = I² × R (power lost as heat in wires is proportional to the square of current)
• Strategy: Transmit at very high voltage (400 kV, 765 kV) → same power at much lower current → dramatically less heat loss
• India's national grid transmits at 765 kV AC (the highest standard transmission voltage in India) and 400 kV AC for major inter-regional links
• AT&C losses (India's electricity distribution losses) are currently ~16–20% of all power generated — reducing these is a major policy priority under RDSS (Revamped Distribution Sector Scheme)

Electrochemical cell: Converts chemical energy to electrical energy through a redox (oxidation-reduction) reaction.

Components:

• Anode (negative terminal): Electrode where oxidation occurs (loses electrons) — electrons flow out
• Cathode (positive terminal): Electrode where reduction occurs (gains electrons) — electrons flow in
• Electrolyte: Liquid or paste that allows ions to move between electrodes (completing the internal circuit)

Dry Cell (Zinc-Carbon / Leclanché type):

• Anode: Zinc (outer casing — gets slowly consumed)
• Cathode: Carbon rod surrounded by manganese dioxide (MnO₂ — active cathode material)
• Electrolyte: Ammonium chloride paste
• EMF: 1.5 V per cell (standard AA/AAA/D batteries)
• Non-rechargeable — zinc is consumed; MnO₂ is reduced; cannot be reversed

Lithium-ion battery (Li-ion):

• Lithium compound electrodes; organic electrolyte
• Nominal voltage: 3.6–3.7 V per cell
• Rechargeable — the chemical reaction can be reversed by passing current the other way (charging)
• Energy density far superior to older battery types → powers phones, EVs, grid storage

Why batteries go flat: The chemical reactants (like zinc or lithium compounds) are consumed. In non-rechargeable cells, this is irreversible. In rechargeable Li-ion, electrical energy during charging restores the reactants.

DC (Direct Current):

• Current flows in one direction only
• Produced by: batteries, solar cells (photovoltaic output), fuel cells
• Used in: all electronics (phones, laptops, EVs), medical equipment
• Can only be transmitted over short distances without massive losses

AC (Alternating Current):

• Current periodically reverses direction at a fixed frequency
• Produced by: generators/alternators (all power plants — thermal, hydro, nuclear, wind)
• India's standard: 230 V, 50 Hz (50 complete cycles per second)
• Used in: entire power grid, household supply, industrial machinery

Why AC is used for long-distance transmission:

• AC voltage can be easily stepped up or down using transformers (a step-up transformer at the power plant raises voltage to 400 kV or 765 kV; a step-down transformer at your area substation reduces it to 230 V)
• High voltage → low current (for same power: P = VI) → very low I²R heat losses in transmission wires
• DC cannot be transformed this way (DC transformers are much more complex)

HVDC (High Voltage Direct Current): For very long distances (>700–800 km), HVDC becomes more efficient than HVAC. India has several HVDC lines:

• Talcher–Kolar HVDC line: ±500 kV; 1,450 km (Odisha → Karnataka via Andhra Pradesh and Tamil Nadu); 2,500 MW capacity; operated by Power Grid Corporation of India; commissioned 2003
• HVDC is also used for under-sea cables and for integrating asynchronous grids