Ch 5: Heat Transfer – Physics Class 8

🌡️ Heat and Temperature

Heat	Temperature
A form of energy that flows from a hotter body to a cooler body	A measure of the degree of hotness or coldness of a body
Depends on mass, material, and temperature difference	Does not depend on mass of the body
SI unit: Joule (J)	SI unit: Kelvin (K); commonly expressed in °C
Measured by calorimeter	Measured by thermometer

📏 Temperature Scales and Conversions

Celsius (°C): Water freezes at 0 °C, boils at 100 °C.
Fahrenheit (°F): Water freezes at 32 °F, boils at 212 °F.
Kelvin (K): SI unit. Absolute zero = 0 K = −273 °C (no thermal energy).

°C = (°F − 32) × 5/9 | K = °C + 273

🌡️ Thermometers

Property	Clinical Thermometer	Laboratory Thermometer
Range	35 °C to 42 °C	−10 °C to 110 °C
Use	Measuring body temperature	Measuring temperature in experiments
Kink	Has a kink (prevents mercury from falling back)	No kink
Position of reading	Can be removed from patient before reading	Must be read while still in the substance

        Principle of Thermometer: Based on the uniform expansion of mercury with increase in temperature. Mercury is used because it is opaque (easily visible), expands uniformly, and does not wet glass.
      

🔥 Three Modes of Heat Transfer

Conduction

Transfer of heat through a material from particle to particle without the material itself moving. Occurs mainly in solids. E.g. a metal rod heated at one end becomes hot at the other.

Convection

Transfer of heat by actual movement of the heated fluid (liquid or gas). Hot fluid becomes less dense and rises; cooler fluid sinks. Occurs in liquids and gases. E.g. boiling water, sea breeze.

Radiation

Transfer of heat energy in the form of electromagnetic waves (infrared) without the need for any medium. Can travel through vacuum. E.g. heat from the Sun reaching Earth.

⚡ Good and Bad Conductors

Good Conductors	Bad Conductors (Insulators)
Metals: copper, aluminium, silver, iron	Wood, plastic, rubber, glass, air, wool, cork
Allow heat to flow through them easily	Do not allow heat to flow through them easily
Used in: cooking pans, heat sinks, radiators	Used in: handles of utensils, thermos flasks, winter clothing

🌊 Convection in Nature — Land and Sea Breezes

Sea Breeze (During the Day)

Land heats up faster than sea during the day.
Air above land becomes warm, expands, and rises.
Cooler air from the sea moves in to replace it → sea breeze (blows from sea to land).

Land Breeze (During the Night)

Land cools faster than sea at night.
Air above the sea is warmer, rises; cooler air from land replaces it → land breeze (blows from land to sea).

💡 Applications: Radiation in Daily Life

Dark/black surfaces are good absorbers and good emitters of radiation.
Light/white/polished surfaces are poor absorbers and poor emitters (good reflectors) of radiation.
Thermos (Vacuum) Flask: Double-walled with vacuum (prevents conduction/convection) and silvered walls (reduces radiation). Keeps liquids hot or cold.
Greenhouse Effect: Glass of a greenhouse allows sunlight in but traps heat radiation — keeps the interior warm. On Earth, CO₂ and other greenhouse gases act similarly.
Wearing white clothes in summer reflects radiation and keeps us cool.

🏷️ Key Terms

Heat Temperature Celsius Fahrenheit Kelvin Thermometer Clinical thermometer Conduction Convection Radiation Good conductor Insulator Sea breeze Land breeze Thermos flask Greenhouse effect