Class 7 Notes

Class 7 Science Notes Chapter 4 (Heat) – Science Book

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Philoid

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Science
Okay, let's focus on the key concepts from NCERT Class 7 Science, Chapter 4: Heat, structured for government exam preparation.

Chapter 4: Heat - Detailed Notes

1. Introduction: Hot and Cold

  • Heat: A form of energy that flows from a body at a higher temperature to a body at a lower temperature.
  • Sensation: We perceive objects as hot or cold through our sense of touch, but this is relative and not always reliable for accurate measurement.
  • Temperature: A reliable measure of the degree of hotness or coldness of an object.

2. Measuring Temperature

  • Thermometer: The device used to measure temperature.
  • Units: Temperature is commonly measured in degrees Celsius (°C).

3. Types of Thermometers

  • Clinical Thermometer:

    • Purpose: Used to measure human body temperature.
    • Structure: Consists of a long, narrow, uniform glass tube with a bulb at one end containing mercury. Features a kink (constriction) near the bulb.
    • Kink's Function: Prevents the mercury level from falling back immediately after removal from the mouth, allowing time for an accurate reading.
    • Temperature Range: Reads temperature from 35°C to 42°C. This range covers typical human body temperatures.
    • Normal Human Body Temperature: Approximately 37°C. Note that this is an average value and can vary slightly among individuals.
    • Reading Precautions:
      • Wash before and after use (preferably with an antiseptic solution).
      • Ensure mercury level is below 35°C before use (requires a few jerks).
      • Read keeping the level of mercury along the line of sight.
      • Handle with care; it can break easily.
      • Do not hold the thermometer by the bulb while reading.
      • Do not use for measuring the temperature of objects other than the human body.
      • Avoid placing it in direct sunlight or near a flame.

  • Laboratory Thermometer:

    • Purpose: Used to measure the temperature of objects in laboratories (e.g., water, chemicals).
    • Structure: Similar to a clinical thermometer but generally longer and does not have a kink.
    • Temperature Range: Typically ranges from -10°C to 110°C (ranges can vary).
    • Reading Precautions:
      • Should be kept upright, not tilted.
      • The bulb should be surrounded by the substance whose temperature is being measured.
      • The bulb should not touch the surface (bottom or sides) of the container.
      • Temperature must be read while the thermometer bulb is still in contact with the substance (mercury level falls quickly upon removal due to the absence of a kink).

  • Digital Thermometers:

    • Increasingly common.
    • Do not use mercury (safer, less environmental hazard).
    • Provide easy-to-read digital displays.

4. Transfer of Heat

  • Principle: Heat always flows from a region of higher temperature to a region of lower temperature.
  • Modes of Heat Transfer: There are three ways heat can be transferred:

    • Conduction:

      • Mechanism: Heat transfer primarily in solids. It occurs through direct contact, where heat energy is passed from one particle to the adjacent particle without the actual movement of the particles from their positions.
      • Conductors: Materials that allow heat to pass through them easily. Examples: Metals (Iron, Aluminium, Copper, Silver).
      • Insulators (Poor Conductors): Materials that do not allow heat to pass through them easily. Examples: Wood, Plastic, Glass, Rubber, Air, Water.
      • Applications: Cooking utensils are made of metals (conductors) but have handles made of plastic or wood (insulators).

    • Convection:

      • Mechanism: Heat transfer in liquids and gases (fluids). It involves the actual movement of heated particles. When a fluid is heated, the particles near the heat source become hot, expand, become less dense (lighter), and rise. Cooler, denser particles from above sink to take their place, get heated, and rise. This continuous circulation sets up convection currents.
      • Examples:
        • Boiling water.
        • Sea Breeze: During the day, land heats up faster than the sea. The air above the land becomes hot and rises. Cooler air from the sea blows towards the land to take its place. This is sea breeze.
        • Land Breeze: During the night, land cools down faster than the sea. The air above the sea is warmer and rises. Cooler air from the land blows towards the sea to take its place. This is land breeze.
        • Ventilators in rooms are placed high because warm air (containing CO2) rises and escapes through them.

    • Radiation:

      • Mechanism: Heat transfer that does not require any medium. Heat travels as electromagnetic waves.
      • Examples:
        • Heat from the Sun reaching the Earth.
        • Heat felt from a fireplace or room heater.
        • A hot utensil kept away from the flame cools down by transferring heat to the surroundings via radiation.
      • Properties:
        • All hot bodies radiate heat.
        • When this heat falls on an object, a part is reflected, a part is absorbed, and a part may be transmitted.
        • The temperature of the object increases due to the absorbed part of the heat.
        • Dark-coloured surfaces absorb more heat radiation than light-coloured surfaces.
        • Dark-coloured surfaces radiate heat better than light-coloured surfaces.

5. Clothes We Wear in Summer and Winter

  • Summer:
    • We prefer light-coloured clothes.
    • Reason: Light colours reflect most of the heat radiation that falls on them, keeping us cool.
    • Cotton clothes are preferred as they allow air circulation and absorb sweat.
  • Winter:
    • We prefer dark-coloured clothes.
    • Reason: Dark colours absorb more heat radiation, keeping us warm.
    • We wear woollen clothes.
    • Reason: Wool is a poor conductor (insulator) of heat. Additionally, air gets trapped in the fibres of wool. Since air is also a poor conductor of heat, this trapped air layer prevents the loss of body heat to the colder surroundings. Wearing multiple layers of clothing traps more air and provides better insulation than one thick layer.

Key Takeaways for Exams:

  • Definitions of Heat and Temperature.
  • Difference between Clinical and Laboratory thermometers (Range, Kink, Usage).
  • Normal human body temperature (37°C).
  • Precautions for using thermometers.
  • Three modes of heat transfer: Conduction (Solids), Convection (Fluids), Radiation (No medium).
  • Definitions and examples of Conductors and Insulators.
  • Mechanism and examples of Convection (Sea Breeze, Land Breeze).
  • Properties of Radiation (No medium needed, absorption/reflection by surfaces).
  • Scientific reasons for wearing light-coloured clothes in summer and dark-coloured/woollen clothes in winter (related to radiation and insulation).
  • The role of trapped air as an insulator (in woollen clothes, blankets).

This covers the essential points from Chapter 4 'Heat' relevant for competitive government exam preparation, focusing on concepts, definitions, differences, and applications as presented in the NCERT Class 7 textbook.

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