Heat and Thermodynamics: Temperature, Heat Transfer, and Laws

Heat and Thermodynamics is a core Physics topic for PSC exams. Expect 2-3 questions per paper covering temperature scales, heat transfer, and everyday applications.

Temperature and Its Measurement

Temperature Scales

Scale	Freezing Point of Water	Boiling Point of Water	Divisions	Inventor
Celsius (C)	0	100	100	Anders Celsius (1742)
Fahrenheit (F)	32	212	180	Daniel Fahrenheit (1724)
Kelvin (K)	273.15	373.15	100	Lord Kelvin (1848)

Conversion Formulas

Conversion	Formula
C to F	F = (9/5)C + 32
F to C	C = (5/9)(F - 32)
C to K	K = C + 273.15
Equal reading (C = F)	-40 degrees

Types of Thermometers

Thermometer	Principle	Range	Use
Mercury	Expansion of mercury	-39 to 357 C	Clinical, laboratory
Alcohol	Expansion of alcohol	-115 to 78 C	Cold regions
Clinical	Mercury expansion	35 to 42 C (95-108 F)	Human body temperature
Pyrometer	Thermal radiation	Above 1000 C	Furnaces, kilns
Thermocouple	Seebeck effect (EMF)	-200 to 1600 C	Industrial

Normal human body temperature: 37 C (98.6 F)

Heat vs Temperature

Property	Heat	Temperature
Definition	Total kinetic energy of molecules	Average kinetic energy of molecules
SI Unit	Joule (J)	Kelvin (K)
Other units	Calorie, BTU	Celsius, Fahrenheit
Measured by	Calorimeter	Thermometer
Transfer direction	Hot to cold body	Does not transfer

1 Calorie = 4.186 Joules (Joule’s mechanical equivalent of heat)

Specific Heat Capacity

Substance	Specific Heat (J/kg K)	Significance
Water	4,186	Highest among common substances
Ice	2,090	About half of water
Aluminium	900	Used in cooking vessels
Iron	450	Heats up quickly
Copper	385	Good heat conductor
Lead	128	Very low — heats/cools fast

Formula: Q = m x c x delta-T (Heat = mass x specific heat x change in temperature)

Why Water Has High Specific Heat

• Acts as coolant in radiators and industrial processes
• Moderates coastal climate (sea breeze/land breeze)
• Used in hot water bags for warmth
• Explains why land heats faster than sea

Latent Heat

Type	Definition	Water Value
Latent heat of fusion	Heat to melt solid to liquid at same temperature	334 kJ/kg (80 cal/g for ice)
Latent heat of vaporisation	Heat to convert liquid to gas at same temperature	2,260 kJ/kg (540 cal/g for water)

Key fact: Temperature remains constant during change of state (melting/boiling).

Heat Transfer Methods

Method	Medium Required	Mechanism	Example
Conduction	Solid (mainly)	Molecule-to-molecule contact	Metal spoon in hot tea
Convection	Fluid (liquid/gas)	Bulk movement of heated fluid	Sea breeze, boiling water
Radiation	No medium needed	Electromagnetic waves	Sun’s heat reaching Earth

Conduction Details

Term	Meaning
Good conductors	Metals (silver best, then copper, aluminium)
Bad conductors (insulators)	Wood, rubber, air, glass, plastic
Applications of bad conductors	Wooden handles on utensils, woollen clothes, double-walled flasks

Convection Details

Phenomenon	Explanation
Sea breeze (day)	Land heats faster; air rises over land; cool air from sea rushes in
Land breeze (night)	Sea retains heat; air rises over sea; cool air from land moves seaward
Ventilation	Hot air rises, escapes from top; fresh cool air enters from below
Trade winds	Equatorial heating causes global convection patterns

Radiation Details

Property	Detail
Type of waves	Infrared (electromagnetic)
Speed	Speed of light (3 x 10^8 m/s)
Black body	Perfect absorber and emitter of radiation
Good absorbers	Dark, rough surfaces
Good reflectors	Light, polished surfaces
Thermos flask	Silvered walls reduce radiation; vacuum reduces conduction/convection

Laws of Thermodynamics

Law	Statement	Everyday Meaning
Zeroth Law	If A is in thermal equilibrium with B, and B with C, then A is with C	Basis of temperature measurement
First Law	Heat added = Change in internal energy + Work done (dQ = dU + dW)	Energy cannot be created or destroyed (conservation of energy)
Second Law	Heat cannot spontaneously flow from cold to hot body	Entropy of isolated system always increases
Third Law	Entropy approaches zero as temperature approaches absolute zero	Absolute zero (0 K) is unattainable

Second Law — Two Statements

Statement	By	Meaning
Kelvin-Planck	Lord Kelvin	No engine can convert ALL heat into work (100% efficiency impossible)
Clausius	Rudolf Clausius	Heat cannot flow from cold to hot without external work (refrigerator needs power)

Thermal Expansion

Type	Occurs In	Coefficient
Linear expansion	Solids (length)	Alpha
Areal/Superficial expansion	Solids (area)	Beta = 2 x Alpha
Volume/Cubical expansion	Solids/liquids (volume)	Gamma = 3 x Alpha

Practical Applications

Application	Principle
Gaps in railway tracks	Allow for expansion in summer
Bimetallic strip (thermostat)	Two metals expand differently; strip bends
Overhead wires sag in summer	Linear expansion
Bridges on rollers	One end free to expand
Riveting	Hot rivet contracts on cooling, holds plates tight

Anomalous Expansion of Water

Property	Detail
Behaviour	Water contracts from 0 C to 4 C; expands above 4 C
Maximum density	At 4 C
Significance	Lakes freeze from top; aquatic life survives below ice
Called	Anomalous (abnormal) expansion

Everyday Phenomena Explained

Phenomenon	Scientific Explanation
Sweating cools body	Evaporation causes cooling (latent heat absorbed from skin)
Pressure cooker cooks fast	Boiling point increases with pressure
Ice at 0 C more cooling than water at 0 C	Ice absorbs 80 cal/g latent heat while melting
Steam burns worse than boiling water	Steam releases 540 cal/g latent heat
Desert hot days, cold nights	Sand has low specific heat
Black clothes absorb more heat	Dark surfaces are good absorbers of radiation

PSC Repeated Questions

Question	Answer
SI unit of heat	Joule
SI unit of temperature	Kelvin
Normal body temperature	37 C / 98.6 F
Best conductor of heat	Silver
Highest specific heat (common)	Water
Heat transfer without medium	Radiation
Temperature at which C = F	-40
Absolute zero	0 K = -273.15 C
Principle of thermos flask	Reduces all three modes of heat transfer
Water has maximum density at	4 C
Latent heat of ice	80 cal/g (334 kJ/kg)
Latent heat of steam	540 cal/g (2260 kJ/kg)
Gaps in rail tracks are for	Thermal expansion
Sea breeze occurs during	Daytime