Chapter 9: Heat
1) 20 Important Words & Meanings (Meanings in Hindi)
Heatऊष्मा जो गर्मी देती है और वस्तु का ताप बढ़ाती है।
Temperatureकितना गर्म/ठंडा है इसकी माप।
Conductionठोस में कण-से-कण ऊष्मा का सीधा स्थानांतरण।
Convectionद्रव/वायु में गरम भाग ऊपर और ठंडा भाग नीचे जाने से ऊष्मा का फैलना।
Radiationबिना किसी माध्यम के तरंगों द्वारा ऊष्मा का आना-जाना।
Conductorजो ऊष्मा आसानी से पास होने दे (जैसे ताँबा)।
Insulatorजो ऊष्मा को आसानी से न जाने दे (जैसे लकड़ी, प्लास्टिक)।
Expansionगरम होने पर आकार/आयतन बढ़ना।
Contractionठंडा होने पर आकार/आयतन घटना।
Thermal Equilibriumदो वस्तुएँ समान ताप पर होकर ऊष्मा का आदान-प्रदान बंद होना।
Specific Heat Capacity1 किग्रा पदार्थ का ताप $1^{\circ}$C बढ़ाने के लिए आवश्यक ऊष्मा।
Thermometerताप मापने का यंत्र।
Celsius Scale$0^{\circ}$C पर बर्फ पिघलना और $100^{\circ}$C पर जल उबलना वाला पैमाना।
Black Bodyजो विकिरण को अधिकतर सोख ले।
Reflectionऊष्मा/प्रकाश का सतह से लौटना।
Absorptionऊष्मा का सतह द्वारा सोख लिया जाना।
Dewठंडी सतह पर भाप के जलकण बन जाना।
Sea Breezeदिन में समुद्र से जमीन की ओर ठंडी हवा चलना।
Land Breezeरात में जमीन से समुद्र की ओर हवा चलना।
Thermos (Dewar) Flaskवस्तु को लम्बे समय तक गरम/ठंडा रखने वाला पात्र।
2) Important Notes (Quick Revision)
- Heat flows from hot to cold until thermal equilibrium is reached.
- Modes of heat transfer: Conduction (needs material; best in solids), Convection (in liquids/gases by currents), Radiation (no medium needed—Sun → Earth).
- Good conductors (metals) pass heat quickly; insulators (wood, plastic, air, wool) slow the flow—hence handles, woollens, thermocol.
- Colour matters: Black/dark, rough surfaces absorb & emit heat better; shiny/white surfaces reflect more.
- Expansion on heating, contraction on cooling—allow gaps in rails/bridges; wires sag more in summer.
- Liquids & gases expand on heating → density decreases → convection currents (sea/land breeze, room heating patterns).
- Thermos flask: vacuum (no conduction/convection) + silvered walls (reflection) reduce heat loss.
- Useful formula (with MathJax): Heat absorbed/released: $Q = m\,c\,\\Delta T$ where $m$ = mass, $c$ = specific heat capacity, $\\Delta T$ = temperature change.
| Process | Needs medium? | Key idea | Example |
|---|---|---|---|
| Conduction | Yes (best in solids) | Particle-to-particle | Metal spoon in hot tea gets hot |
| Convection | Yes (fluids) | Warm fluid rises, cool sinks | Water heating; sea breeze |
| Radiation | No | Electromagnetic waves | Sun warming Earth; bonfire warmth |
3) 20 Important “One-Word Answer” Type Questions
- Which colour absorbs maximum heat?Black.
- Which mode of heat transfer needs no medium?Radiation.
- Heat flows naturally from _____ to _____.Hot to cold.
- What is the unit of temperature in SI?Kelvin (K).
- Which metal is commonly used for cooking utensils?Aluminium / Copper / Steel (good conductors).
- Which is a good insulator used in flasks?Air/Vacuum (vacuum best).
- Name the device to measure temperature.Thermometer.
- During heating, solids usually _____.Expand.
- Sea breeze blows from?Sea to land (daytime).
- Land breeze blows from?Land to sea (night).
- Which surface reflects heat most?Shiny/Polished/White.
- Convection occurs in which states?Liquids and gases.
- What is $\\Delta T$ in $Q=m c \\Delta T$?Temperature change.
- Name the flask designed by Sir James Dewar.Dewar/ Thermos flask.
- Why are ACs fixed near the ceiling?Cold air sinks; efficient circulation.
- Why are heaters near the floor?Hot air rises; warms room by convection.
- Does radiation need air?No.
- State of water at 100°C (at 1 atm).Boiling/Steam formation.
- Rails have small gaps due to?Thermal expansion.
- Which clothing is best in summer?White/light coloured cotton.
4) 20 Very Short Answer Questions (1–2 lines)
- Why does rubbing palms make them warm?Friction converts mechanical energy into heat.
- Why wrap cloth on a ladle while stirring boiling milk?Cloth is an insulator; it reduces heat conduction to hand.
- Why hold a steel glass with a handkerchief for hot milk?To reduce heat transfer and protect the hand.
- What happens to density of water on heating?It decreases; warm water rises (convection).
- How does Sun’s heat reach Earth?By radiation (no medium needed).
- Why does a black tin heat water faster in sunlight?It absorbs more radiation than a shiny/white tin.
- Why do woollen clothes keep us warm?Trapped air in wool is a good insulator.
- Why do we see water drops on outer surface of a cold glass?Condensation of atmospheric moisture (dew).
- Why are cooking pans metal but handles plastic/wood?Metal conducts heat to food; handle insulates hand.
- Why do telephone wires sag in summer?Thermal expansion increases length, so they sag.
- Why are houses in hot regions painted white?White reflects heat; keeps interiors cooler.
- What prevents conduction/convection in a thermos?Vacuum between double walls.
- What reduces radiation loss in a thermos?Silvered (shiny) inner surfaces reflect heat.
- Why are AC vents on top in buses/rooms?Cold air sinks to mix and cool uniformly.
- Why can hot tea be held in a china cup but not steel?China is poorer conductor; steel is good conductor.
- In which medium is conduction fastest?Solids (especially metals).
- What creates sea breeze?Land heats faster; warm air rises; cool sea air moves in.
- Why does ice at test-tube bottom not melt when top water boils?Hot water stays above; convection cannot carry heat downward.
- Which colour for penguin’s outer coat and why?Black—absorbs more solar heat.
- State the basic heat formula.$Q = m c \\Delta T$.
5) 20 Short Answer Questions (2–3 lines)
- Explain conduction with a pin-and-wax bar experiment.Heating one end makes nearby particles vibrate more and pass energy along; pins drop first near the flame—heat travels from hot to cold end.
- How does convection set up in water?Heated water at the bottom expands, becomes less dense, rises; cooler water descends, forming convection currents.
- How can a candle warm your hands via radiation?Infrared radiation travels through air and is absorbed by skin—no contact or convection is required at a distance.
- Why are railway tracks laid with small gaps?To allow for expansion in heat; without gaps, rails would buckle.
- Describe working of a thermos flask.Double-walled with vacuum (no conduction/convection) and silvered walls (reflect radiation); thus contents stay hot/cold longer.
- Why are dark clothes preferred in winter?They absorb more heat radiation, helping retain warmth.
- Explain land breeze at night.Land cools faster than sea; cooler, denser air from land flows towards the relatively warmer sea.
- Why does an iron pillar feel colder than wood in winter?Iron conducts heat away from your hand faster, so your skin cools more—feels colder.
- What is specific heat capacity?Heat required to raise the temperature of 1 kg of a substance by $1^{\\circ}$C; higher $c$ means slower temperature change.
- State two uses of poor conductors.Pan handles (plastic/wood), house insulation (thermocol, wool, cork).
- How does colour affect solar heating of water?Black containers absorb more solar radiation → faster heating than white/shiny ones.
- Why are heaters placed low in rooms?Heated air rises and circulates by convection, warming the room evenly.
- Give an example where convection is blocked.Boiling top layer in a slanted test tube while ice remains at bottom—hot water doesn’t sink.
- Why does dew form on grass in winter mornings?Grass cools by night radiation; nearby air cools below dew point, water vapour condenses as droplets.
- Is vacuum a conductor or insulator for heat?Perfect insulator for conduction/convection; but radiation still passes.
- Why metallic kettles boil water faster than glass ones?Metals conduct heat better, transferring energy to water quickly.
- Explain why hot air balloons rise.Heated air inside is less dense; buoyant force lifts the balloon (convection principle).
- When does $Q$ become negative in $Q=m c \\Delta T$?During cooling ($\\Delta T$ negative); the body releases heat.
- Why are pressure cookers shiny inside?Shiny surface reflects radiation, reducing energy loss and sticking.
- Why does a black-painted solar cooker work well?Black interior absorbs and retains more solar radiation, raising temperature efficiently.
6) Textbook Exercise – Perfect Answers
Q1) Fill in the blanks with the proper word.
- (a) Maximum heat is absorbed by a black coloured object.
- (b) Radiation of heat does not require a medium.
- (c) Conduction of heat takes place through a good conductor substance.
- (d) The shining surface in a thermos flask decreases the outgoing heat by reflection.
- (e) Cooking utensils are made from metals due to their property of conduction.
- (f) The earth receives heat from the sun by radiation.
Q2) What will absorb heat?
Strong absorbers (typically dark/rough): Iron griddle (tava), soil, water (good absorber of IR), steel spoon (if darkened).
Moderate/Poor absorbers (often light/shiny/transparent): Glass vessel/glass (transmits/reflects), plastic plate (if light coloured), wooden board/wooden spoon (light surfaces reflect more), wax (usually light; absorbs less).
Note: All materials absorb some heat; colour & surface finish decide “how much”.
Q3) Write answers to the following.
(a) How does a cold compress lower fever? Cold pack conducts heat away from the forehead; blood near skin cools and circulates, lowering body temperature.
(b) Why are houses in Rajasthan painted white? White reflects more solar radiation, keeping interiors cooler in hot climate.
(c) What are the modes of heat transfer? Conduction, Convection, and Radiation.
(d) Which mode causes sea & land breezes? Convection: differential heating of land and sea produces pressure differences and moving air.
(e) Why is penguins’ outer coat black? Black feathers absorb more solar heat, helping conserve warmth in cold Antarctica.
(f) Why are heaters near floor and ACs near ceiling? Hot air rises (so heater low promotes upward convection); cold air sinks (so AC high allows cool air to descend and mix).
Q4) Give scientific reasons.
(a) Ordinary glass bottle cracks with boiling water; borosil doesn’t. Ordinary glass has low thermal shock resistance; rapid expansion creates stress and cracks. Borosilicate glass has low thermal expansion—tolerates sudden heating.
(b) Telephone wires sag in summer and become straight in winter. Metals expand on heating (longer → sag in summer) and contract on cooling (shorter → taut in winter).
(c) Dew drops form on grass in winter. Nighttime radiation cools grass below dew point; water vapour condenses into droplets.
(d) Iron pillar feels colder than wooden pole in winter. Iron conducts heat away from skin faster than wood; faster heat loss makes it feel colder.
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