Introduction
Electricity and Magnetism are closely linked phenomena tested throughout Railway technical and
non-technical exams — this chapter covers Ohm's Law, circuit fundamentals, and the electromagnetic
relationship that powers most modern technology, including trains themselves.
Basic Electrical Quantities
| Quantity | Symbol | SI Unit |
|---|
| Electric Current | I | Ampere (A) |
| Voltage (Potential Difference) | V | Volt (V) |
| Resistance | R | Ohm (Ω) |
| Electric Power | P | Watt (W) |
| Electric Charge | Q | Coulomb (C) |
Ohm's Law
📌 Key Formula: V = IR (Voltage = Current × Resistance) — at constant temperature, current through a conductor is directly proportional to the voltage across it.
Q. A circuit has a resistance of 5 Ω and a current of 2 A flowing through it. Find the voltage.
V = IR = 2 × 5 = 10 Volts
Series vs Parallel Circuits
| Feature | Series Circuit | Parallel Circuit |
|---|
| Current | Same through all components | Divides across each branch |
| Voltage | Divides across each component | Same across each branch |
| Total Resistance | R = R1 + R2 + R3... | 1/R = 1/R1 + 1/R2 + 1/R3... |
| If one component fails | Entire circuit breaks | Other branches keep working |
⚠️ Common Trap: Household electrical wiring uses PARALLEL circuits — this ensures that if one appliance/bulb fails, the others continue to function normally, and each device gets the full supply voltage.
Electric Power
| Formula |
|---|
| P = VI (Power = Voltage × Current) |
| P = I²R |
| P = V²/R |
Magnetism — Basic Concepts
| Concept | Description |
|---|
| Magnetic Poles | Every magnet has a North and South pole; like poles repel, unlike poles attract |
| Magnetic Field | Region around a magnet where magnetic force can be experienced |
| Magnetic Field Lines | Travel from North pole to South pole outside the magnet |
Electromagnetism — The Link Between Electricity and Magnetism
📌 Key Principle (Oersted's Discovery): A current-carrying conductor produces a magnetic field around it — this is the foundational link between electricity and magnetism, discovered by Hans Christian Oersted in 1820.
| Device | Principle |
|---|
| Electromagnet | A coil of wire carrying current, often wound around an iron core, generates a strong temporary magnetic field |
| Electric Motor | Converts electrical energy into mechanical (rotational) energy using electromagnetic force |
| Electric Generator | Converts mechanical energy into electrical energy via electromagnetic induction |
Electromagnetic Induction
Discovered by Michael Faraday — a changing magnetic field induces an electric current in a nearby conductor. This principle powers generators, transformers, and is the basis for regenerative braking used in modern electric trains.
Conductors vs Insulators
| Type | Property | Example |
|---|
| Conductor | Allows electric current to flow easily | Copper, silver, aluminium |
| Insulator | Resists/blocks the flow of electric current | Rubber, glass, wood, plastic |
✅ Exam Focus: Ohm's Law (V=IR) and numerical application · Series vs parallel circuit differences (household wiring uses parallel) · Power formulas (P=VI, P=I²R) · Electromagnet, motor, and generator working principles · Faraday's electromagnetic induction.