📑 Topics on this page (Jump to Topic):
📌 1. Heat, Temperature & Thermometers
- Heat is a form of energy (unit joule; also calorie); temperature tells the degree of hotness (kelvin). Heat flows from higher to lower temperature.
- Absolute zero = 0 K = −273.15 °C — the lowest possible temperature.
- Clinical thermometer: 35–42 °C (94–108 °F). For very high temperatures (furnace, Sun) a pyrometer is used.
- Mercury is used in thermometers because it is shiny, does not wet glass, expands uniformly and stays liquid over a wide range (−39 to 357 °C).
📌 2. Thermal Expansion, Specific & Latent Heat
- Gaps between rail tracks, rollers in bridges, sagging power lines in summer — all due to thermal expansion.
- Anomalous expansion of water: between 0 and 4 °C water contracts on heating; its density is maximum at 4 °C.
- Specific heat = heat to raise 1 kg by 1 K. Water has a very high specific heat (4186 J/kg·K) — so seas warm and cool slowly, coastal climate is moderate and water is used as a coolant (radiator).
- Latent heat = heat used to change state without changing temperature. Latent heat of fusion of ice ≈ 80 cal/g; of vaporisation of steam ≈ 540 cal/g — so steam burns are worse than burns from boiling water.
| Situation | Effect on boiling / melting point |
|---|---|
| Raising pressure (pressure cooker) | Boiling point rises (~120 °C), food cooks faster |
| Low pressure on mountains | Boiling point falls, food takes longer |
| Salt/sugar (impurity) in water | Boiling point rises, freezing point falls |
| Pressure on ice | Melting point falls (regelation — skating) |
📌 3. Heat Transfer & Thermodynamics
| Mode | Medium | Examples |
|---|---|---|
| Conduction | Mainly solids, particles stay in place | Handle of a hot spoon, bottom of a pan; metals conduct, wood/air insulate |
| Convection | Liquids and gases, particles move | Sea/land breeze, boiling water, AC fitted high |
| Radiation | No medium needed, travels at speed of light | Sun's heat reaching Earth, warmth near a fire |
- Black rough surfaces absorb and emit well; white/shiny surfaces reflect — hence white clothes in summer.
- Thermos flask (Dewar): vacuum between double glass walls (stops conduction and convection), silvered surfaces (stop radiation).
- First law of thermodynamics = conservation of energy (ΔQ = ΔU + W). Second law: heat does not flow by itself from cold to hot; no engine is 100% efficient. A refrigerator uses external work to pump heat out.
- Cooling by evaporation: water in an earthen pot stays cool, drying sweat cools the body. In high humidity sweat dries slowly — sultry weather.
⚠️ Exam Traps:
- Woollen clothes do not "give" heat; air trapped in the fibres is a poor conductor and keeps body heat in.
- Opening a fridge door in a closed room does not cool the room; it warms it slightly.
- Sea breeze blows by day (sea → land), land breeze at night (land → sea).
📌 4. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Absolute zero temperature is: Practice
Correct answer: (A)
0 K = −273.15 °C.
0 K = −273.15 °C.
Q2. The instrument used to measure very high temperatures (e.g. a furnace) is: Practice
Correct answer: (B)
A pyrometer measures temperature from emitted radiation without contact.
A pyrometer measures temperature from emitted radiation without contact.
Q3. Food cooks faster in a pressure cooker because: Practice
Correct answer: (D)
Water inside boils at about 120 °C.
Water inside boils at about 120 °C.
Q4. Why are burns from steam more severe than from boiling water? Practice
Correct answer: (A)
Steam at 100 °C releases about 540 cal/g extra heat while condensing.
Steam at 100 °C releases about 540 cal/g extra heat while condensing.
Q5. Heat from the Sun reaches the Earth by: Practice
Correct answer: (C)
Space is a vacuum; radiation needs no medium.
Space is a vacuum; radiation needs no medium.
Q6. Cooking takes longer on mountains because: Practice
Correct answer: (B)
Water boils below 100 °C, so food cooks slowly.
Water boils below 100 °C, so food cooks slowly.
Q7. A small gap is left between rail tracks because of: Practice
Correct answer: (A)
Rails expand in summer; without gaps they would buckle.
Rails expand in summer; without gaps they would buckle.
Q8. When does a sea breeze blow? Practice
Correct answer: (B)
Land heats faster by day, air rises there — convection.
Land heats faster by day, air rises there — convection.
Q9. Water is used as a coolant in radiators because of its high: Practice
Correct answer: (A)
High specific heat = absorbs much heat with little temperature rise.
High specific heat = absorbs much heat with little temperature rise.
Q10. In a thermos flask, the vacuum between the walls prevents: Practice
Correct answer: (D)
No particles in a vacuum, so no conduction or convection; silvering stops radiation.
No particles in a vacuum, so no conduction or convection; silvering stops radiation.
Q11. Water in an earthen pot stays cool due to: Practice
Correct answer: (C)
Water seeping through pores evaporates and takes away latent heat.
Water seeping through pores evaporates and takes away latent heat.
Q12. Which relation between kelvin and celsius is correct? Practice
Correct answer: (B)
K = °C + 273.15 (about 273).
K = °C + 273.15 (about 273).
Q13. Adding salt to water makes its boiling point: Practice
Correct answer: (A)
A non-volatile solute raises boiling point and lowers freezing point.
A non-volatile solute raises boiling point and lowers freezing point.
Q14. Why are white clothes comfortable in summer? Practice
Correct answer: (D)
White reflects most radiation; black absorbs it.
White reflects most radiation; black absorbs it.
Q15. Woollen clothes keep us warm because: Practice
Correct answer: (C)
Air is an insulator, so body heat is retained.
Air is an insulator, so body heat is retained.
Q16. The latent heat of fusion of ice is about: Practice
Correct answer: (D)
≈ 80 cal/g (336 J/g); vaporisation ≈ 540 cal/g.
≈ 80 cal/g (336 J/g); vaporisation ≈ 540 cal/g.
Q17. The first law of thermodynamics is a statement of: Practice
Correct answer: (C)
ΔQ = ΔU + W.
ΔQ = ΔU + W.
Q18. If a running fridge is left open in a closed room, the room temperature will: Practice
Correct answer: (C)
The fridge dumps heat into the same room and its motor work also becomes heat.
The fridge dumps heat into the same room and its motor work also becomes heat.
Q19. The usual range of a clinical thermometer is: Practice
Correct answer: (D)
Human body temperature varies within this range.
Human body temperature varies within this range.
Q20. Heat transfer by conduction mainly occurs in: Practice
Correct answer: (B)
In solids, closely packed particles pass on energy by vibration.
In solids, closely packed particles pass on energy by vibration.