🔥 Physics — Heat and Its Transmission
1. Heat and Temperature
Heat is a form of energy that produces the sensation of warmth. SI unit: Joule (J) | 1 cal = 4.2 J
Temperature is the measure of hotness or coldness of a body. Heat flows from hotter to colder body.
TEMPERATURE SCALES
| Scale | Ice Point | Boiling Point |
|---|---|---|
| Celsius | 0°C | 100°C |
| Fahrenheit | 32°F | 212°F |
| Kelvin | 273 K | 373 K |
| Reaumur | 0°R | 80°R |
RELATION BETWEEN SCALES
- C/100 = (F − 32)/180 = R/80 = (K − 273)/100
- K = 273 + °C
- −40°C = −40°F (same reading)
- Triple point of water: 273.16 K
- Mercury thermometer range: −30°C to 300°C
THERMOMETERS
- Clinical thermometer: 35°C to 43°C (96°F to 110°F)
- Electronic thermometer: Uses thermistors; range −40°F to 450°F
- Mercury: Commonly used from −30°C to 300°C
- Thermometer was developed by Galileo
2. Thermal Expansion
Expansion of a body caused by heating.
EXPANSION OF SOLIDS
| Type | Formula | Unit |
|---|---|---|
| Linear (α) | α = ΔL / (L·Î”t) | °C⁻¹ |
| Superficial (β) | β = ΔA / (A·Î”t) | °C⁻¹ |
| Volume (γ) | γ = ΔV / (V·Î”t) | °C⁻¹ |
RELATION BETWEEN COEFFICIENTS
- β = 2α
- γ = 3α
- α : β : γ = 1 : 2 : 3
APPLICATIONS
- Gaps left between railway rails to prevent buckling
- Telephone wires not tightened — prevent breaking in winter
ANOMALOUS EXPANSION OF WATER
- Water contracts from 0°C to 4°C
- Volume is minimum at 4°C
- Density is maximum at 4°C
- This unusual behaviour is called anomalous expansion
EXPANSION OF LIQUIDS
- Apparent expansion: Container expansion not considered
- Real expansion: Container expansion considered
- γr = γa + γs
3. Calorimetry
FORMULAS
- Q = m·s·Î”θ (Heat = mass × specific heat × temperature change)
- Q = m·L (Heat = mass × latent heat)
SPECIFIC HEAT
- Amount of heat required to raise 1 g by 1°C
- Water: 4200 J/kg°C (high specific heat)
- Water used as coolant due to high specific heat
LATENT HEAT
- Latent heat of fusion: 80 cal/g
- Latent heat of vaporisation: 536 cal/g
- Heat absorbed/released at constant temperature during change of state
4. Thermodynamics
LAWS OF THERMODYNAMICS
- Zeroth Law: Thermal equilibrium
- First Law: ΔU = Q − W (energy conservation)
- Second Law: Heat cannot be 100% converted to work
- Carnot Engine: Maximum efficiency: η = 1 − T₂/T₁
KEY CONCEPTS
- Entropy: Measure of molecular disorder
- Heat engine: Converts heat to mechanical work
- Refrigerator: Based on Clausius statement
- Evaporation: Molecules escape slowly from liquid surface
5. Humidity
- Absolute humidity: Mass of water vapour per unit volume
- Relative humidity: Ratio of actual vapour to saturation vapour × 100
- Measured by hygrometer
- Comfortable range: ~50% at 22–25°C
6. Transmission of Heat
THREE METHODS
| Method | Medium | Key Points |
|---|---|---|
| Conduction | Solids | No particle movement; metals are good conductors |
| Convection | Fluids | Particle movement; sea breeze, land breeze |
| Radiation | No medium required | Electromagnetic waves; black bodies are good emitters |
CONDUCTION FACTS
- Metals are good conductors of heat
- Wood, cotton, wool, glass are bad conductors
- Two thin blankets give more warmth than one thick blanket (air layer acts as insulator)
- Refrigerators have double walls with thermocol between them
CONVECTION FACTS
- Cooling unit in refrigerator fitted near top (cold air moves downward)
- Radiator in motor car works on convection
- Sea breeze: Day — air over land rises, cool air from sea moves in
- Land breeze: Night — air over sea rises, cool air from land moves out
RADIATION FACTS
- White: Bad absorber, good reflector
- Black: Good absorber, bad reflector
- Clothes of light colours in summer
- Clothes of dark colours in winter
- Cloudy nights are warmer than clear nights (clouds reflect radiation)
NEWTON'S LAW OF COOLING
- Rate of heat loss ∝ temperature difference (body − surroundings)
- Hot water cools faster than fresh water in refrigerator
7. Black Body and Radiation Laws
STEFAN'S LAW
- E ∝ T⁴
- Energy radiated per second per unit area ∝ fourth power of absolute temperature
WIEN'S DISPLACEMENT LAW
- λmax ∝ 1/T
- Wavelength of maximum intensity ∝ 1/temperature
- As temperature increases: red → orange → yellow → blue/violet
KIRCHHOFF'S LAW
- Good absorbers are good emitters
- Poor absorbers are poor emitters
- Ratio of emissive power to absorptive power is same for all bodies
GREENHOUSE EFFECT
- Glass traps heat radiation
- Car parked in sun with windows closed gets warm due to greenhouse effect
- CO₂ absorbs infrared radiation → global warming
8. Kinetic Theory of Gases
GAS LAWS
- Boyle's Law: pV = constant (T constant)
- Charles' Law: V ∝ T (p constant)
- Gay-Lussac's Law: p ∝ T (V constant)
- Ideal Gas Equation: pV = nRT
KEY POINTS
- Ideal gas particles: tiny, perfectly elastic collisions
- Real gases behave ideally at high temperature and low pressure
Summary: Heat and Its Transmission at a Glance
| Topic | Key Formula | Key Point |
|---|---|---|
| Temperature Scales | C/100 = (F−32)/180 | K = 273 + °C |
| Linear Expansion | α = ΔL/(L·Î”t) | β = 2α, γ = 3α |
| Specific Heat | Q = m·s·Î”θ | Water: 4200 J/kg°C |
| Latent Heat | Q = m·L | Fusion: 80 cal/g |
| Stefan's Law | E ∝ T⁴ | Black body radiation |
| Wien's Law | λmax ∝ 1/T | Red → blue as T increases |
| Ideal Gas | pV = nRT | High T, low P |
KEY TAKEAWAYS FOR UPSC
- Water: Highest specific heat, maximum density at 4°C
- Steam: More heat than water at 100°C
- Mercury thermometer: Range −30°C to 300°C
- Black body: Absorbs all radiation, good emitter
- White: Reflects heat; Black: Absorbs heat
- Greenhouse effect: CO₂ traps infrared radiation
- Cloudy nights: Warmer than clear nights
High-Yield Interactive UPSC Quiz (Physics — Heat and Its Transmission)
Q1. At what temperature is the density of water maximum?
Correct Answer: B. The density of water is maximum at 4°C (277 K).
Q2. What is the specific heat capacity of water?
Correct Answer: B. Water has a specific heat capacity of 4200 J/kg°C, which is high compared to most substances.
Q3. The latent heat of fusion of ice is:
Correct Answer: B. The latent heat of fusion of ice is 80 cal/g.
Q4. Which method of heat transfer does NOT require a medium?
Correct Answer: C. Radiation does not require a medium for propagation.
Q5. According to Stefan's Law, energy radiated is proportional to:
Correct Answer: D. Stefan's Law states E ∝ T⁴ (energy radiated per unit area per second).
Q6. According to Wien's Displacement Law, as temperature increases, the wavelength of maximum emission:
Correct Answer: A. Wien's Law states λmax ∝ 1/T, so wavelength decreases as temperature increases.
Q7. Which colour is a good absorber of heat radiation?
Correct Answer: B. Black colour is a good absorber and bad reflector of heat radiation.
Q8. The first law of thermodynamics is based on:
Correct Answer: B. The first law of thermodynamics (ΔU = Q − W) is based on the conservation of energy.
Q9. The relation between coefficients of thermal expansion is:
Correct Answer: B. The relation is α : β : γ = 1 : 2 : 3.
Q10. Which gas is primarily responsible for the greenhouse effect?
Correct Answer: C. Carbon dioxide (CO₂) absorbs infrared radiation and is a major greenhouse gas.