PRELIMS

Electricity

Static Electricity and Charge

• Benjamin Franklin named the two types of charges as Positive and Negative.
• Law of Charges: Like charges repel; opposite charges attract.
• Surface Density: Charge per unit area ( ).
  • Inversely proportional to the radius of curvature; hence maximum at pointed parts.
• Hollow Conductor: Electric field intensity inside is zero.
  • Safety Note: A car or bus acts as an electrostatic shield (hollow conductor) during lightning.

Materials and Conductance

• Conductor: Allows charge flow (e.g., silver, copper, earth). Silver is the best conductor.
• Insulator (Dielectric): Does not allow flow (e.g., wood, paper, mica, glass).
• Coulomb's Law: .
• Electric Potential: Work done in bringing a unit positive charge from infinity to a point. Scalar; SI unit is Volt.
• Electric Capacity: . Unit: Farad (F).

Electrochemical Cells

Converts chemical energy to electrical energy. First discovered by Alexandro de Volta (1794).

1. Primary Cells (Irreversible/Non-rechargeable):
   • Voltaic Cell: Zinc rod (cathode) and Copper rod (anode) in .
   • Leclanche Cell: Carbon rod (anode) and Zinc rod (cathode) in . EMF = 1.5 V. Used for intermittent work (electric bell).
   • Dry Cell: Zinc vessel (cathode) with and carbon rod (anode). EMF = 1.5 V.
2. Secondary Cells: Rechargable (must be charged from an external source first).

Current Electricity Basics

• Electric Current (I): Rate of flow of charge. Scalar; SI unit is Ampere (A).
  • 1 Ampere = electrons flowing per second.
• Resistance (R): Opposition to flow. Unit: Ohm ( ).
• Dry Cell: A primary cell.
  • Anode: Zinc container.
  • Cathode: Carbon (Graphite) rod.
  • Electrolyte: Paste of Ammonium Chloride ( ) and Zinc Chloride ( ).

4. Current Electricity & Ohm's Law

• Electric Current: Rate of flow of charge ( ). Unit: Ampere.
• Ohm's Law: (Potential difference is proportional to current at constant physical conditions).
• Resistance (R): . Unit: Ohm ( ).
  • Specific Resistance ( ): Depends on the nature of the material and temperature.
  • Temperature Effect: Resistance of metals increases with temperature.

5. Combination of Resistances

• Series: (Current remains same).
• Parallel: (Potential difference remains same).
• Note: Domestic wiring is always in parallel.

6. Measuring Instruments

• Galvanometer: Detects the presence of small electric currents.
• Ammeter: Measures current; has low resistance; connected in series.
• Voltmeter: Measures potential difference; has high resistance; connected in parallel.

7. Electric Power & Heating Effect

• Power (P): . Unit: Watt.
• Joule's Law of Heating: .
• Fuse Wire: Made of an alloy of Lead and Tin.
  • Has low melting point and high resistance.
  • Connected in series for protection.
• Electric Bulb: Filament made of Tungsten (high melting point ). Filled with Argon/Nitrogen.

8. Electrical Devices

• Transformer: Based on mutual induction. Changes AC voltage (Step-up/Step-down). Works only on AC.
• Dynamo: Converts Mechanical energy Electrical energy (EM induction).
• Electric Motor: Converts Electrical energy Mechanical energy.
• Rectifier: Converts AC to DC.
• Inverter: Converts DC to AC.

9. AC vs DC

• Direct Current (DC): Flows in one direction only; magnitude constant with time. Sources: Cells, batteries, solar cells.
• Alternating Current (AC): Periodically reverses direction; varies sinusoidally with time.
  • Indian Household Supply: 230 V, 50 Hz.
  • Advantage: Can be easily stepped up/down using a transformer, enabling efficient long-distance transmission (hence used for grid power).

10. Household Wiring and Safety Devices

• Fuse: Thin wire (low melting point, high resistance alloy of lead-tin) that melts and breaks the circuit when current exceeds a safe limit, protecting appliances from overheating/short-circuits.
• MCB (Miniature Circuit Breaker): Automatic switch that trips on overload/short-circuit; unlike a fuse, it is reusable (can be reset).
• Earthing (Grounding): Connects the metallic body of an appliance to the earth via a low-resistance wire, providing a safe path for leakage current and preventing electric shock.
• Three-pin Plug: Live (brown), Neutral (blue), Earth (green/yellow) wires; the earth pin is thicker and longer to connect first.

UPSC Relevance

The fundamental laws of electricity are common in GS Paper I.

• Conceptual Focus: Why it's safe to stay inside a car during a storm (hollow conductor shield).
• Common Trap: Ammeter vs Voltmeter placement (Series vs Parallel) and their ideal resistances; Fuse (sacrificial) vs MCB (resettable).
• Technology: Transformers (AC vs DC usage) and the components of primary cells (Dry cells/Leclanche).
• Safety: Role of earthing in preventing electric shock — a recurring "science in daily life" question.