PA05 — Heat & Thermodynamics

✈ Physics – PA05 · AFCAT General Awareness AFCAT Level

Heat and thermodynamics explain how energy flows, how temperature scales relate, and why gas behaves differently under pressure and temperature. AFCAT questions here are mostly recall — temperature at which Celsius equals Fahrenheit (−40°), radiation in vacuum, and the one gas law most likely to appear.

📌 AFCAT Focus: Temperature conversion formulae (especially −40° where C = F), which mode of heat transfer works in vacuum (radiation only), the latent heat concept (why boiling water stays at 100°C despite continued heating), and Boyle's Law (P×V = constant at constant temperature).

1. Temperature Scales & Conversion

Fig. 1 — Three Temperature Scales: Key Reference Points Side by Side

⚠ AFCAT Trap — Temperature Equality: At what temperature do Celsius and Fahrenheit give the same reading? The answer is −40°. Set F = C in the formula: C = (9/5)C + 32 → −(4/5)C = 32 → C = −40. This is a direct question that appears repeatedly. Memorise: C = F = −40.

2. Modes of Heat Transfer

Fig. 2 — Three Modes of Heat Transfer: Which Works in Vacuum?

3. Specific Heat & Latent Heat

Heat Formulae:

● Heat for temperature change: Q = mcΔT
    m = mass; c = specific heat capacity; ΔT = temperature change
    Water: c = 4200 J/kg·K (highest among common substances — why oceans regulate climate)

● Heat for phase change: Q = mL (temperature stays constant during phase change)
    L = latent heat; Fusion (ice→water): L = 3.36×10⁵ J/kg
    Vaporisation (water→steam): L = 2.26×10⁶ J/kg

Why does boiling water stay at 100°C even with continued heating?
All the heat goes into breaking molecular bonds (phase change) — none raises temperature. This is latent heat.

4. Gas Laws

▶ Boyle's Law (Constant Temperature)

P ∝ 1/V → P₁V₁ = P₂V₂
Pressure doubles → Volume halves
Temperature must be constant (isothermal)
Example: Squeezing a balloon reduces volume, increases pressure

▶ Charles's Law (Constant Pressure)

V ∝ T → V₁/T₁ = V₂/T₂ (T in Kelvin!)
Temperature doubles → Volume doubles
Always use Kelvin (K = °C + 273)
Ideal gas equation: PV = nRT

📝 AFCAT PYQs — Heat & Thermodynamics

Q1. At what temperature are Celsius and Fahrenheit scales equal? AFCAT PYQ

(a) 0°(b) 100°(c) −40°(d) −100°

✔ Answer: (c) −40°

Solve: C = (9/5)C + 32 → C − (9/5)C = 32 → (−4/5)C = 32 → C = −40. So at −40°, both Celsius and Fahrenheit give the same reading. This is one of the most frequently repeated AFCAT Physics questions. No calculation is needed in the exam — just memorise −40°.

Q2. Which mode of heat transfer can occur through vacuum? AFCAT PYQ

(a) Conduction(b) Convection(c) Radiation(d) Both conduction and convection

✔ Answer: (c) Radiation

Radiation travels as electromagnetic waves and does NOT need any medium — it passes perfectly through vacuum. This is how the Sun's heat reaches Earth (through 150 million km of vacuum). Conduction requires direct physical contact; convection requires fluid movement — both impossible in vacuum.

Q3. A gas is compressed at constant temperature. If its volume becomes half, its pressure: AFCAT PYQ

(a) Halves(b) Doubles(c) Stays same(d) Becomes four times

✔ Answer: (b) Doubles

Boyle's Law: P₁V₁ = P₂V₂ (at constant temperature). If V₂ = V₁/2, then P₂ = P₁V₁/V₂ = P₁V₁/(V₁/2) = 2P₁. Pressure doubles when volume halves. This direct application of Boyle's Law is frequently asked in AFCAT.

🧠 Quick Memory Chart — PA05 Heat

🌡 Temperature

°F = (9/5)°C + 32
K = °C + 273
C = F at −40°
Boiling: 100°C = 212°F = 373K
Absolute zero: 0K = −273°C

🔥 Heat Transfer

Conduction: solids, contact
Convection: fluids only
Radiation: works in vacuum
Black = best absorber & emitter
Thermos: vacuum + silvering

☁ Gas Laws

Boyle's: P₁V₁ = P₂V₂ (const T)
Charles's: V/T = const (const P)
V doubles → P halves
Always use Kelvin for Charles's
Latent heat: phase change at const T

📝 Practice Exercise

E1. The boiling point of water on the Kelvin scale is:

(a) 100 K(b) 212 K(c) 373 K(d) 273 K

E2. Which surface is the best radiator of heat?

(a) White shiny surface(b) Rough black surface(c) Smooth silver surface(d) Yellow surface

E3. Water has the highest specific heat capacity. This means:

(a) It heats up very quickly(b) It absorbs a lot of heat for small temperature rise(c) It is the best conductor(d) It freezes quickly

Answers: E1 → (c) 373 K [K = 100 + 273] | E2 → (b) Rough black surface [Black body = best absorber AND emitter; shiny/white surfaces reflect, not absorb] | E3 → (b) Absorbs large heat for small temperature rise [Q = mcΔT; large c → large Q needed for same ΔT — why coastal climates are moderate]

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