Ch 07: Temperature, Heat & Thermodynamics

🌡️ Chapter At a Glance

Temperature Scales

-273.15°C

Absolute Zero

4.186 J

Mechanical Equiv. of Heat (per cal)

Laws of Thermodynamics

Temperature — measure of hotness/coldness; SI unit = Kelvin; instrument = Thermometer
Heat — form of energy; flows from hot to cold body; SI unit = Joule
Thermodynamics — study of heat and mechanical energy relations; governed by 4 laws (Zeroth to Third)
Key concepts: Specific heat · Latent heat · Thermal expansion · State change · Heat transfer (Conduction, Convection, Radiation)

📊 Temperature Scales — Quick Comparison

Scale	Freezing Point	Boiling Point	No. of Divisions
Celsius	0°C	100°C	100
Fahrenheit	32°F	212°F	180
Reaumur	0°R	80°R	80
Kelvin	273 K	373 K	100
Rankin	492 Ra	672 Ra	180

🌡️ Temperature — Basics

Measure of hotness and coldness of a substance
Measuring instrument: Thermometer
SI Unit: Kelvin (K)
Cause of energy (heat) transfer between bodies
Heat always flows from Hot → Cold

🔗 Relation Between Temperature Scales

(C − 0)/100 = (F − 32)/180 = (R − 0)/80 = (K − 273)/100 = (Ra − 492)/180Universal Temperature Conversion Formula

📘 Solved Example

Express 60°F in Kelvin:
(F−32)/180 = (K−273)/100
(60−32)/180 = (K−273)/100
28/180 = (K−273)/100 → K = 288.5 K

⭐ Important Temperature Facts

Fact	Value
Absolute Zero Temperature	−273.15°C
Same value on Celsius & Fahrenheit	−40°C = −40°F
Same value on Fahrenheit & Kelvin	574.25°F = 574.25 K
Same value on Fahrenheit & Reaumur	−25.6°F = −25.6°R
Human Body Temperature	310.5 K = 37°C = 98.4°F
Triple Point of Water	273.16 K

⚠️ Exam Trap — Equal Values

−40°C = −40°F — only point where Celsius and Fahrenheit are equal
Human body temp: 37°C = 98.4°F = 310.5 K — all three must be memorized
Absolute zero = −273.15°C = 0 K — lowest possible temperature

🔬 Types of Thermometers & Temperature Ranges

Thermometer Type	Temperature Range	Special Use
Gas (Nitrogen)	500°C to 1500°C	Very high temperatures
Platinum	−200°C to 1000°C	Wide range, precision
Thermocouple	−200°C to 1600°C	Widest range among all
Total Radiation	Above 800°C	Very high temp (no contact)
Alcohol	Below −40°C (freezing pt −115°C)	Very low temperatures
Clinical	35°C to 42°C	Human body temperature

⚠️ Exam Trap — Thermometer Selection

Clinical thermometer range = 35°C to 42°C (only for body temperature)
Thermocouple has the widest range: −200°C to 1600°C
Alcohol thermometer is used for very low temperatures (NOT mercury — mercury freezes at −39°C)
Total Radiation thermometer = no physical contact needed (pyrometer)

🔥 Heat — Basics

Heat is a form of energy
Transfer occurs due to temperature difference
Flows from hotter body → colder body
Mechanical equivalent of heat: J = W/Q = 4.186 J/cal

SI Unit

Joule (J)

CGS Unit

Calorie (cal)

British Unit

BTU (1 BTU = 252 cal)

💧 Specific Heat (s)

📘 Definition

Heat required to raise the temperature of unit mass of a substance by 1°C.

s = Q / (mΔT)s = specific heat | Q = heat | m = mass | ΔT = temp change

SI Unit

J/kg°C

CGS Unit

cal/g°C

Water's s

4186 J/kg°C

📘 Water has Highest Specific Heat (1 cal/g°C)

Hot water bottle used to foment patients
Farmers fill fields with water to protect crops from frost
Water used in radiators to cool vehicle engines

📘 Solved Example

Heat to raise 2 kg metal from 30°C to 100°C (s = 300 J/kg°C):
Q = m × s × ΔT = 2 × 300 × 70 = 4.2 × 10⁴ J

⚖️ Latent Heat (L)

📘 Definition

Heat required for a change of state (no temperature change during state change).

L = Q / mUnit: J/kg (SI) | cal/g (CGS)

Type	Change	Value
Latent Heat of Fusion	Solid → Liquid	80 cal/g
Latent Heat of Vaporisation	Liquid → Gas	539 cal/g

⚠️ Exam Trap — Latent Heat

During state change, temperature does NOT change — only latent heat is absorbed/released
Latent heat of vaporisation (539 cal/g) >> fusion (80 cal/g) — steam burns are worse than hot water burns!

⚖️ Principle of Calorimetry

Heat Lost by Hot Object = Heat Gained by Cold ObjectConservation of thermal energy in an isolated system

Used in calorimeter experiments
Assumes no heat loss to surroundings (isolated system)
Basis for finding unknown specific heats

📏 Thermal Expansion

📘 Definition

Expansion of substances on absorbing heat. All substances (except water 0–4°C) expand on heating.

🧱 Expansion in Solids

Coefficient	Symbol	Formula	Expansion
Linear Expansion	α	ΔL / LΔt	In Length
Area (Superficial) Expansion	β	ΔA / AΔt	In Area
Volumetric Expansion	γ	ΔV / VΔt	In Volume

α : β : γ = 1 : 2 : 3Relation between coefficients of expansion | Unit: per °C

📘 Application

Gap left between railway tracks to allow for summer expansion — prevents buckling.

⚠️ Exam Trap — α:β:γ = 1:2:3

β = 2α and γ = 3α. If α is given, you can find the others. This ratio is asked very frequently!

💧 Anomalous Behaviour of Water

Water contracts (volume ↓) from 0°C to 4°C
Water expands (volume ↑) above 4°C
Volume is minimum at 4°C
Density is maximum at 4°C

⚠️ Exam Trap

In cold regions, fishes survive even when pond surface freezes — because water at 4°C (bottom) is densest and remains liquid.

💨 Expansion in Gases

Gases have the highest volume expansion
Coefficient of volumetric expansion for ideal gas:
γp = γv = 1/273.15 per °C
All ideal gases have the same coefficient of expansion

📘 Key Point

Gas expands much more than solids or liquids for the same temperature rise. Gases > Liquids > Solids in expansion.

💧 Changes of State

Process	Transition	Heat
Melting	Solid → Liquid	Absorbed
Freezing	Liquid → Solid	Released
Boiling / Evaporation	Liquid → Gas	Absorbed
Condensation / Liquefaction	Gas → Liquid	Released
Sublimation	Solid → Gas	Absorbed
Deposition	Gas → Solid	Released

📘 Examples of Sublimation

Dry ice (CO₂), Camphor (Kapoor), Naphthalene balls, Iodine — all sublime directly from solid to gas.

🔑 Important Definitions

Melting Point — constant temperature at which solid changes to liquid
Boiling Point — constant temperature at which liquid changes to vapour
Freezing Point — constant temperature at which liquid changes to solid
Sublimation Point — constant temperature at which solid changes directly to gas

⚡ Effects of Pressure & Impurity

Action	Effect on Melting Point	Effect on Boiling Point
Increase Pressure	Decreases (for ice)	Increases
Decrease Pressure	Increases	Decreases
Add Impurity	Decreases	Increases

⚠️ Exam Trap — Pressure Effects

Pressure ↑ → Boiling point ↑ → Pressure cooker works on this principle
Pressure ↓ (at altitude) → Boiling point ↓ → Food cooks slower on mountains
Pressure ↑ → Melting point of ice decreases (ice skating works on this!)
Adding salt to ice/water → lowers freezing point (used in ice cream making)

📘 Triple Point of Water

All three states (solid, liquid, gas) coexist in equilibrium at 273.16 K. This is a unique fixed point used to define the Kelvin scale.

♨️ Methods of Heat Transfer

🔩 Conduction

Heat transfer by direct contact
Occurs in solids (and mercury)
Slow process
No movement of particles

Examples:
Cooking on gas stove · Hot handle of a pan · Soldering iron

☀️ Radiation

Transfer via electromagnetic waves
No medium required (works in vacuum)
Source: Sun
Fastest method of heat transfer

Examples:
Sunlight · UV rays · Wi-Fi signals · Remote control · Infrared heaters

🌊 Convection

Transfer by movement of particles
Occurs in gases and liquids
Requires a medium
Bulk motion of fluid

Examples:
Sea breeze · Land breeze · Hot water circulation · Radiators · Desert temp. variation

⚠️ Exam Trap — Heat Transfer

Radiation is the only method that works in vacuum (e.g., heat from Sun reaches Earth)
Conduction = solids; Convection = fluids (liquids & gases); Radiation = no medium needed
Sea breeze & land breeze = Convection (NOT radiation)
Thermos flask prevents all 3 types: silvered walls (radiation) + vacuum (conduction/convection)

⚙️ Thermodynamics — Basics

Study of heat and mechanical energy relations
Thermodynamic System → a system with definite pressure, volume, temperature
Made up of molecules and atoms

PV = μRTIdeal Gas Equation | P=pressure, V=volume, μ=moles, R=gas constant, T=temperature

📜 Laws of Thermodynamics

Zeroth Law

Thermal Equilibrium

Proposed by: Fowler (1931)
If A is in thermal equilibrium with C, and B is in thermal equilibrium with C → then A and B are also in thermal equilibrium with each other
Mathematically: If T_A = T_C and T_B = T_C → then T_A = T_B
Basis of thermometry (concept of temperature measurement)

First Law

Conservation of Energy

Heat given to a system (ΔQ) = Increase in internal energy (ΔU) + Work done by system (ΔW)
Based on: Law of Conservation of Energy

ΔQ = ΔU + ΔWFirst Law of Thermodynamics

Second Law

Limits of Heat Conversion

Complete conversion of heat into mechanical energy is impossible
Kelvin-Planck Statement: Complete conversion of heat into work is impossible
Clausius Statement: Heat cannot flow spontaneously from cold to hot body without an external energy source

⚠️ Exam Trap — Thermodynamic Laws

Zeroth Law = Thermal equilibrium concept; proposed by Fowler (1931) — named "zeroth" as it was added later but is most fundamental
First Law = Energy conservation: ΔQ = ΔU + ΔW
Second Law = No 100% efficient heat engine is possible
Refrigerator/AC moves heat from cold to hot — needs external work (proves Clausius statement)
Ideal gas equation: PV = μRT (not PV = nRT confusion — μ = number of moles)

🎯 High-Frequency BPSC/BSSC Exam Points

SI unit of temperature = Kelvin; Heat = Joule
Heat flows from hot → cold
Absolute zero = −273.15°C = 0 K
Human body temp = 37°C = 98.4°F = 310.5 K
−40°C = −40°F (only equal point of C and F scales)
Specific heat of water is highest: 4186 J/kg°C = 1 cal/g°C
Latent heat of fusion = 80 cal/g; vaporisation = 539 cal/g
α : β : γ = 1 : 2 : 3 for thermal expansion coefficients
Water density is maximum at 4°C; anomalous expansion 0–4°C
Triple point of water = 273.16 K
Pressure ↑ → Boiling point ↑, Melting point of ice ↓
Impurity added → Boiling point ↑, Melting point ↓
Conduction → solids; Convection → fluids; Radiation → no medium (vacuum)
Zeroth Law proposed by Fowler (1931)
First Law: ΔQ = ΔU + ΔW (energy conservation)
Second Law: Complete conversion of heat to work is impossible
Mechanical equivalent of heat: 1 cal = 4.186 J
Thermocouple thermometer has widest range: −200°C to 1600°C
Clinical thermometer range: 35°C to 42°C

📋 Formula Quick Reference

(C/100) = (F−32)/180 = (K−273)/100Temperature Scale Conversion

Q = msΔTHeat (Specific Heat)

L = Q/mLatent Heat

α : β : γ = 1 : 2 : 3Thermal Expansion Coefficients

ΔQ = ΔU + ΔWFirst Law of Thermodynamics

PV = μRTIdeal Gas Equation

⚠️ Most Common Exam Traps

During state change, temperature does NOT change
Latent heat of steam (539) >> ice (80) — steam burns are worse
Radiation works in vacuum — Sun's heat reaches Earth via radiation
Sea breeze & land breeze = Convection
Water is densest at 4°C, not at 0°C
Ice floats because its density is LESS than water
Zeroth Law = Fowler; NOT Newton or Kelvin
Pressure cooker: pressure ↑ → boiling point ↑ → faster cooking
BTU = 252 cal (British thermal unit — not 1000 cal!)

🌡️ Chapter 07: Temperature, Heat & Thermodynamics