General Science Notes Chapter 9 Class 7 Maharashtra Board

Notes For All Chapters – General Science Class 7

1. Introduction

• Heat is a form of energy that makes objects hot or cold.
• It flows from a hot object to a cold object until both reach the same temperature.
• Heat is important in cooking, heating, and industrial work.

2. What is Temperature?

• Temperature measures how hot or cold an object is.
• It is measured in Celsius (°C), Fahrenheit (°F), or Kelvin (K).
• Thermometers are used to measure temperature.

Types of Thermometers:

1. Clinical Thermometer – Measures body temperature (35°C to 42°C).
2. Laboratory Thermometer – Measures temperature of liquids and objects.

3. Modes of Heat Transfer

Heat moves from one place to another in three ways:

(A) Conduction

• Heat transfers through solids by direct contact.
• Example: A metal spoon gets hot when placed in hot tea.
• Good conductors: Metals (iron, copper, aluminum).
• Poor conductors (insulators): Wood, plastic, rubber.

(B) Convection

• Heat transfers through liquids and gases by movement of particles.
• Example: Boiling water – hot water rises and cold water sinks.
• Sea Breeze and Land Breeze are caused by convection.
  • Sea Breeze (Daytime): Cool air from the sea moves toward the warm land.
  • Land Breeze (Nighttime): Cool air from land moves toward the warm sea.

(C) Radiation

• Heat transfers without a medium (through space).
• Example: The Sun’s heat reaches Earth by radiation.
• Dark colors absorb more heat, while light colors reflect heat.

4. Good and Bad Conductors of Heat

5. Applications of Heat Transfer

1. Cooking Utensils

• Made of metal (good conductors) for quick cooking.
• Handles are made of wood or plastic (insulators) to avoid burns.

2. Houses in Hot and Cold Regions

• In hot regions (like Rajasthan), houses are painted white to reflect heat.
• In cold regions, black clothes are worn to absorb heat.

3. Thermos Flask

• Double-walled with vacuum to prevent heat transfer.
• Shiny inner surface reduces radiation.

6. Why Do We Feel Hot or Cold?

• Heat flows from our body to cold objects (feels cold).
• Heat flows from hot objects to our body (feels hot).
• Example: An iron pillar feels colder than wood in winter because iron is a good conductor and absorbs heat from our hand quickly.

7. Expansion and Contraction of Substances

• Expansion: When heated, substances increase in size (molecules move apart).
• Contraction: When cooled, substances shrink in size (molecules come closer).

Examples:

• Metal bridges have gaps to allow expansion in summer.
• Telephone wires sag in summer due to expansion and become tight in winter due to contraction.
• Glass cracks if hot water is poured suddenly because it expands unevenly.

8. Applications of Heat in Daily Life

• Cooking food.
• Drying clothes in the sun (radiation).
• Using heaters in winter (convection).
• Cooling rooms with fans and ACs (convection).
• Hot air balloons rise due to convection.