Heat Transfer
Physics · ऊष्मा स्थानांतरण
📋Quick Overview
Heat is a form of energy that transfers from a hotter body to a colder body. SI unit of heat = Joule (J). There are THREE modes of heat transfer: Conduction, Convection, and Radiation. Understanding which mode works in which situation is key for exams.
📖Three Modes of Heat Transfer
| Mode | How It Works | Medium Needed? | Examples |
|---|---|---|---|
| Conduction | Heat passes through a material by molecule-to-molecule contact (molecules vibrate and pass energy to neighbor) | YES — needs solid medium (best in metals) | Touching a hot pan handle; iron rod heated at one end gets hot at other end; metal spoon in hot tea becomes hot |
| Convection | Heat transfers by actual movement of heated fluid (hot fluid rises, cold sinks — creates currents) | YES — needs fluid (liquid or gas) | Sea breeze/land breeze; boiling water (hot water rises, cold sinks); room heater warms air |
| Radiation | Heat transfers through electromagnetic waves — NO medium needed (works even in vacuum) | NO — works in vacuum too! | Sun's heat reaches Earth through vacuum; heat from fire; infrared lamps; feeling warmth near a bonfire |
📝Good & Bad Conductors
- •Good conductors (transfer heat easily): Silver (BEST conductor), Copper, Aluminum, Iron, Gold — all METALS
- •Bad conductors / Insulators: Wood, plastic, rubber, glass, air, wool, cotton, paper
- •That's why cooking utensils are metal (good conductor) with plastic/wooden handles (bad conductor)
- •Silver is the BEST conductor of heat (and electricity). Copper is second best.
- •Woolen clothes keep us warm because wool TRAPS AIR — and air is a bad conductor
📝Thermos Flask & Greenhouse Effect
- •Thermos Flask prevents ALL 3 modes: Double-wall glass with vacuum between them (stops conduction & convection), silver coating (reflects radiation). Invented by James Dewar.
- •Greenhouse Effect: Glass of greenhouse allows sunlight IN but traps heat inside (doesn't let infrared radiation escape). CO₂, methane, water vapor are greenhouse gases.
- •Global Warming is caused by excess greenhouse gases trapping too much heat in Earth's atmosphere
- •Black surfaces ABSORB more radiation; White/shiny surfaces REFLECT more radiation — that's why we wear white in summer!