Heat and Thermodynamics

Three common scales: Celsius (C), Fahrenheit (F), and Kelvin (K). Conversions: C/5 = (F-32)/9, and K = C + 273.15 (often 273). Water freezes at 0 C = 32 F = 273 K and boils at 100 C = 212 F = 373 K. Kelvin is the SI unit; 0 K is absolute zero, the lowest possible temperature. Memory aid: 'C-five, F-minus-32-nine'. Quick fact: -40 degrees is the same on both Celsius and Fahrenheit scales. Normal human body temperature is 37 C = 98.6 F. Always convert to Kelvin for gas-law problems.

Heat travels by three modes. Conduction: transfer through direct contact without movement of the material (mostly in solids/metals) — e.g. a metal spoon getting hot in tea. Convection: transfer by actual movement of heated fluid (liquids and gases) — e.g. sea breeze, boiling water, room heater warming air. Radiation: transfer through electromagnetic waves needing NO medium — e.g. heat from the Sun reaching Earth, warmth from a fire. Memory aid: 'Con-tact, Cur-rents, Rays' for Conduction, Convection, Radiation. Radiation is the only mode that works in vacuum, which is why Sun's heat reaches us through empty space.

Specific heat (c) is the heat needed to raise 1 kg of a substance by 1 degree C; Q = mc(delta T). Water has an unusually high specific heat (4186 J/kg.C), so it heats and cools slowly — moderating climate near seas. Latent heat is the heat absorbed or released during a change of state at constant temperature. Latent heat of fusion of ice = 334 J/g (melting); latent heat of vaporization of water = 2260 J/g (boiling). This is why steam at 100 C causes far more severe burns than boiling water at 100 C — it releases extra latent heat on condensing. Temperature stays constant during melting and boiling.

Cover the answer, recall, then check. 12 cards on heat and temperature for RPF SI.

Q1. Difference between heat and temperature?
A1. Heat is a form of energy transferred due to a temperature difference (SI unit: joule). Temperature is the degree of hotness measuring average kinetic energy of molecules (SI unit: kelvin).

Q2. What is the SI unit of temperature and the value of 0°C in kelvin?
A2. Kelvin (K). 0°C = 273.15 K (≈ 273 K); water freezes at 273 K and boils at 373 K.

Q3. Give the formula to convert Celsius to Fahrenheit.
A3. F = (9/5)·C + 32. So 0°C = 32°F and 100°C = 212°F. (−40°C = −40°F, where both scales meet.)

Q4. Name the three modes of heat transfer.
A4. Conduction (through solids, no matter movement), convection (in fluids, with matter movement), and radiation (no medium needed, e.g. Sun's heat).

Q5. Why does water show anomalous expansion?
A5. Between 0°C and 4°C water contracts on heating; it has maximum density at 4°C. This lets fish survive under ice, as the 4°C water sinks to the bottom.

Q6. Define specific heat capacity and state why water is special.
A6. Heat needed to raise 1 kg of a substance by 1°C. Water has a very high specific heat (≈ 4200 J/kg·°C), so it heats and cools slowly — used as a coolant.

Q7. What is latent heat of fusion of ice?
A7. Heat absorbed to change 1 kg of ice at 0°C to water at 0°C without temperature change ≈ 3.34 × 10⁵ J/kg (334 J/g).

Q8. What is latent heat of vaporisation of water?
A8. Heat absorbed to change 1 kg of water at 100°C to steam at 100°C ≈ 22.6 × 10⁵ J/kg (2260 J/g). This is why steam burns are more severe than boiling-water burns.

Q9. Why does evaporation cause cooling?
A9. Evaporating molecules take latent heat from the surroundings/liquid, lowering their temperature — the reason sweating cools the body.

Q10. Which liquid is used in laboratory thermometers and why?
A10. Mercury — it is a good conductor, expands uniformly, is opaque and stays liquid over a wide range (−39°C to 357°C).

Q11. Why are cooking utensils made of metal but their handles of wood/plastic?
A11. Metals are good conductors of heat (fast cooking); wood/plastic are insulators (poor conductors), keeping handles cool to hold.

Q12. What is the sea-breeze/land-breeze phenomenon based on?
A12. Convection. By day, land heats faster so air rises and cool sea air blows in (sea breeze); at night it reverses (land breeze).

Worked Example

Problem: How much heat energy is required to raise the temperature of 2 kg of water from 25 °C to 75 °C? (Specific heat capacity of water = 4200 J/kg·°C.)

Solution:
Step 1 — Use the heat formula: Q = m × c × ΔT, where m = mass, c = specific heat, ΔT = rise in temperature.
Step 2 — Find ΔT: ΔT = 75 − 25 = 50 °C.
Step 3 — Substitute the values:
Q = 2 kg × 4200 J/kg·°C × 50 °C.
Q = 2 × 4200 × 50 = 4,20,000 J.
Step 4 — Convert to kilojoules: 4,20,000 J = 420 kJ.

Answer: The heat required is 4,20,000 J (420 kJ).