Electricity and Magnetism — TNPSC Group II/IIA Science
Overview
Electricity and magnetism are fundamental to modern life. TNPSC Group 2/2A questions test Ohm's Law, circuit concepts, electromagnetic principles, and the physics behind everyday appliances. This chapter covers the essentials concisely.
Electric Charge and Current
| Concept | Details |
|---|---|
| Electric Charge | Property of matter; +ve (proton) and −ve (electron); measured in Coulombs (C) |
| Electric Current | Flow of electric charge; I = Q/t; unit: Ampere (A) |
| Potential Difference (Voltage) | Work done per unit charge; V = W/Q; unit: Volt (V) |
| Resistance | Opposition to current flow; R = V/I; unit: Ohm (Ω) |
Conventional current: Flow of positive charges (opposite to electron flow)
Ohm's Law
Statement: Current (I) flowing through a conductor is directly proportional to potential difference (V) across it, at constant temperature.
| Quantity | Symbol | Unit |
|---|---|---|
| Voltage (Potential Difference) | V | Volt (V) |
| Current | I | Ampere (A) |
| Resistance | R | Ohm (Ω) |
Ohm's Law graph: V vs I is a straight line through origin for ohmic conductors Non-ohmic conductors: Diode, LED, transistor — don't follow Ohm's Law
Resistors in Series vs Parallel
| Feature | Series | Parallel |
|---|---|---|
| Connection | End to end | Side by side |
| Current | Same in all | Different in each |
| Voltage | Divides | Same across all |
| Total Resistance | R = R₁ + R₂ + R₃... | 1/R = 1/R₁ + 1/R₂... |
| Total resistance | Greater than individual | Less than smallest |
| If one fails | All stop | Others continue |
| Home use | — | Household wiring |
Electric Power
- Power = Voltage × Current = VI = I²R = V²/R
- Unit: Watt (W); 1 W = 1 J/s
- Commercial unit: Kilowatt-hour (kWh) = 1 unit of electricity = 1000W for 1 hour
- Example: A 100W bulb burning for 10 hours consumes 1 unit (kWh)
Heating Effect of Electric Current
- When current flows through a resistance, heat is produced
- Joule's Law: Heat produced = I²Rt (proportional to square of current)
- Applications: Electric iron, heater, toaster, incandescent bulb, electric kettle
AC and DC
| Feature | AC (Alternating Current) | DC (Direct Current) |
|---|---|---|
| Current direction | Changes periodically | One direction only |
| Frequency (India) | 50 Hz (50 cycles/sec) | — |
| Voltage (India) | 230V | 1.5V–12V typically |
| Transformation | Can be stepped up/down by transformer | Cannot easily be transformed |
| Generation | By AC generators | By batteries, solar cells |
| Long-distance transmission | More efficient | Inefficient (high losses) |
| Examples | Home electricity supply | Batteries, DC motors, electronics |
Magnetism
Types of Magnetic Materials
| Type | Behaviour | Examples |
|---|---|---|
| Ferromagnetic | Strongly attracted to magnets; can be magnetised | Iron, Nickel, Cobalt |
| Paramagnetic | Weakly attracted | Aluminium, Platinum, Manganese |
| Diamagnetic | Weakly repelled | Bismuth, Copper, Zinc |
Earth's Magnetism
- Earth behaves like a giant magnet
- Magnetic North Pole ≠ Geographic North Pole (about 11° apart)
- Magnetic declination: Angle between geographic and magnetic meridian
- Compass needle: Points to magnetic north
Electromagnetic Induction (Michael Faraday, 1831)
Faraday's Laws
1. When magnetic flux through a conductor changes, an EMF is induced 2. Magnitude of induced EMF is proportional to rate of change of magnetic flux
Applications
| Device | Principle |
|---|---|
| Electric Generator | Rotating coil in magnetic field → converts mechanical energy to electrical |
| Electric Motor | Current-carrying coil in magnetic field → converts electrical to mechanical |
| Transformer | Mutual electromagnetic induction → step up/down AC voltage |
| Induction cooktop | Electromagnetic induction heats metal vessels directly |
Generators
- AC Generator (Alternator): Uses slip rings; produces alternating current
- DC Generator (Dynamo): Uses commutator (split ring); produces direct current
- Power plants: Turbines rotate → AC generators → electricity
Transformer
- Steps up or steps down AC voltage
- Step-up transformer: More secondary turns; higher output voltage (used in transmission)
- Step-down transformer: Fewer secondary turns; lower output voltage (used in homes)
- Turns ratio: V₁/V₂ = N₁/N₂
- Cannot transform DC
Everyday Appliances and Electricity
| Appliance | Principle | Effect Used |
|---|---|---|
| Electric bulb (incandescent) | Filament glows when heated | Heating effect |
| LED bulb | Electroluminescence | Light emission |
| Electric iron | Resistance heating | Heating effect |
| Electric fan | DC or AC motor | Magnetic effect |
| Refrigerator | Compressor motor + evaporation | Magnetic effect + cooling |
| Transformer | Electromagnetic induction | Mutual induction |
| Electric bell | Electromagnet | Magnetic effect |
| Fuse | Melts at high current; protective | Heating effect |
| MCB (Miniature Circuit Breaker) | Switches off at high current | Protection |
Electric Fuse
- Thin wire of low melting point (tin-lead alloy)
- Melts when current exceeds safe limit
- Short circuit: When live wire touches neutral → very high current → fuse blows
- MCB is modern replacement for fuse
Household Electricity in India
| Aspect | India Standard |
|---|---|
| Voltage | 230 V (single phase); 415 V (three phase) |
| Frequency | 50 Hz |
| Socket type | Type D (round 3-pin), Type M (large round 3-pin) |
| Colour coding | Live: Red/Brown, Neutral: Black/Blue, Earth: Green/Yellow |
Safety Measures
- Earth wire: Provides safe path for leakage current → prevents electric shock
- ELCB (Earth Leakage Circuit Breaker): Detects earth leakage; cuts off supply
- Never touch switches with wet hands
- Use rubber gloves/slippers when working with electricity
Key Facts for TNPSC Group II/IIA
- Ohm's Law: V = IR; current proportional to voltage at constant temperature
- Power = VI = I²R; unit Watt; commercial unit kWh (1 unit)
- Series circuit: Same current; voltages add; total R increases
- Parallel circuit: Same voltage; currents add; total R decreases; used in homes
- Faraday (1831): Electromagnetic induction — basis of all generators and transformers
- AC motor and DC motor convert electrical energy to mechanical energy
- Transformer: Only works with AC; cannot transform DC
- India's domestic supply: 230V AC at 50 Hz
- Heating effect of current: Used in electric iron, heater, incandescent bulb
- Fuse: Melts at excess current; protects circuit; MCB is modern equivalent
Previous Year MCQ Patterns
Q: According to Ohm's Law, if resistance doubles and voltage remains constant, what happens to current? a) Current doubles b) Current becomes four times c) Current halves d) Current remains the same
Q: Which of the following converts electrical energy to mechanical energy? a) Electric generator b) Electric motor c) Transformer d) Battery
Q: Household electrical circuits are connected in: a) Series b) Parallel c) Both series and parallel d) Neither
Frequently Asked Questions
Ohm's Law states that the current flowing through a conductor is directly proportional to the potential difference (voltage) across it, provided the temperature and other physical conditions remain constant. V = IR (Voltage = Current × Resistance). SI unit of current is Ampere (A), voltage is Volt (V), resistance is Ohm (Ω).