Heat
Overview
Tea is hot, ice is cold — but how is 'hot' measured? Temperature. And how does heat travel from one place to another — conduction, convection, radiation: three ways. Let us learn.
Typed Notes
Heat vs Temperature
Heat — a form of energy (like the flow of water). Temperature — the measure of that heat (like the level of water). A large vessel (more water) and a small vessel can be at the same temperature — but the large one holds more heat. The instrument used to measure temperature is a thermometer: it contains mercury or alcohol — when heated it expands — read the value on the scale. Clinical thermometer (35–42°C — for the human body), has a kink (bend) — so mercury does not go back — shake it after reading. Laboratory thermometer (−10 to 110°C) — no kink, used for water and stove temperatures.
Three Ways Heat Travels
(1) Conduction — in solids: particles at the hot end pass heat to the next particles — like a chain (a metal spoon in a flame — the handle gets hot). Metals are good conductors. (2) Convection — in liquids and gases: hot particles become lighter and rise, cold particles sink — they circulate (water boiling: the bubbling of boiling water; AC in a room is placed low because cool air sinks and warm air rises). (3) Radiation — without any medium, directly: the Sun's heat travels 15 crore km — without any air! That is why we also feel heat near a stove.
Sea Breeze and Land Breeze
Daytime: land heats up quickly, sea heats up slowly — air over land becomes hot and rises, cool air from the sea takes its place — sea → land: sea breeze — brings coolness. Night: land cools down quickly — the reverse happens: land → sea: land breeze. This is the biggest daily example of convection — the weather of coastal cities (Mumbai, Chennai) stays balanced because of this.
Conductors, Insulators and the Science of Clothing
Conductor — allows heat to pass through quickly: all metals (that is why cooking vessels are metal, but pan handles were earlier made of wood or bakelite — an insulator). Insulator — blocks heat: wood, plastic, wool, cloth, air. Wool is warm because the air trapped inside it (an insulator) — keeps the body's heat from escaping. Dark colours absorb heat, white reflects it — that is why we wear white or light colours in summer and dark colours in winter. Thermos flask — insulator on both sides — keeps tea hot and water cold.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Heat is an energy that flows from hot to cold; Temperature is the measure of hotness or coldness — measured with a thermometer.
- 02Thermometer: Clinical (35°C–42°C; has a kink — mercury does not go back, holds the reading) and Laboratory (−10°C–110°C).
- 03Conduction: in solids — heat passes from particle to particle (metal spoon gets hot). Convection: in liquids and gases — particles themselves move around (water boiling, sea breeze). Radiation: without any medium — directly (heat from the Sun).
- 04Conductor — allows heat to pass through (metal); Insulator — blocks heat (wood, plastic, wool, air). That is why pan handles are made of wood and cooking vessels are made of metal.
- 05Sea breeze (daytime — from sea to land), Land breeze (night — from land to sea) — examples of convection. Dark colours absorb heat, white reflects it — that is why we wear white in summer and dark colours in winter.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is the difference between heat and temperature?
What is the difference between a clinical and a laboratory thermometer?
Give one example each of conduction, convection and radiation.
How are sea breeze and land breeze formed?
Why do woollen clothes feel warm in cold weather?
Why do we wear white clothes in summer and dark clothes in winter?
Heat = energy (hot → cold); temperature = measure (thermometer). Three ways: conduction (solid-metal), convection (liquid-gas — sea breeze), radiation (without medium — Sun). Conductor (metal) allows heat through, insulator (wood-wool) blocks it. Dark absorbs, white reflects.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.In what range does a clinical thermometer measure body temperature?
Q2.How does heat from the Sun reach us?
Q3.Wool feels warm because...
Q4.What is the wind that blows from the sea towards the land during the day called?
Mark karo — dashboard par progress dikhega.