Chapter 3:
Heat

Class 7 Science | CBSE Curriculum

This chapter explains temperature, how to measure it using clinical and laboratory thermometers, and the three modes of heat transfer: conduction, convection, and radiation. Also covers conductors, insulators, and the science behind seasonal clothing.

Score:
0
Time:
05:00

Quick Revision: Heat

  • Temperature: A reliable measure of the hotness of an object. Measured with a thermometer.
  • Clinical Thermometer: Used to measure body temperature. Range: 35°C to 42°C. Has a kink to prevent mercury from falling back. Normal human body temperature: 37°C.
  • Laboratory Thermometer: Used for other purposes (e.g., measuring water temperature). Range: -10°C to 110°C. No kink, so must be read while in the liquid.
  • Heat Transfer: Heat always flows from a hotter object to a colder object.
  • Conduction: Transfer of heat in solids by direct contact. Materials that allow heat to pass through easily are conductors (e.g., metals). Materials that do not are insulators (e.g., wood, plastic).
  • Convection: Transfer of heat in liquids and gases by the movement of molecules. Hot fluid rises, cold fluid sinks (e.g., sea breeze, land breeze).
  • Radiation: Transfer of heat without any medium. It can travel through vacuum (e.g., heat from the sun). All hot bodies radiate heat.
  • Clothing and Heat: Dark-coloured clothes absorb more heat (good for winter). Light-coloured clothes reflect more heat (good for summer).
  • Woollen Clothes: Wool is a poor conductor and traps air, which is also an insulator. This prevents body heat from escaping to the surroundings.
  • Sea Breeze: During the day, land heats up faster than sea. Warm air over land rises, and cool air from sea rushes in.
  • Land Breeze: During the night, land cools down faster than sea. Warm air over sea rises, and cool air from land moves towards the sea.

Chapter Summary: Heat

This chapter introduces the concept of heat and temperature. We learn that our sense of touch is not a reliable indicator of hotness, so we use thermometers for accurate measurement. A clinical thermometer, with its kink, is designed to measure body temperature (range 35°C to 42°C), while a laboratory thermometer has a wider range (-10°C to 110°C) for other experiments. The normal human body temperature is 37°C.

The chapter explains that heat always flows from a hotter object to a colder object through three distinct modes: conduction (in solids, via direct contact), convection (in fluids, via the movement of molecules), and radiation (which requires no medium, like heat from the sun). We explore conductors (materials that allow heat to pass, like metals) and insulators (poor conductors like wood and plastic). Practical applications are discussed, such as why dark clothes are preferred in winter (they absorb more heat) and light clothes in summer (they reflect heat). Wool keeps us warm because it traps air, an insulator. The concepts of sea and land breezes are explained as examples of convection currents in nature.