Overview
This lecture explains how we measure heat and temperature, the methods of heat transfer, and why we choose specific types of clothing for different seasons.
Hot and Cold: Sensing and Measuring
- Our sense of touch is unreliable for judging how hot or cold an object is.
- Temperature is a reliable measure of hotness, measured using a thermometer.
- Common objects can be classified as hot or cold, but perception can be deceiving.
Measuring Temperature
- Clinical thermometers measure human body temperature, ranging from 35°C to 42°C.
- The celsius scale is standard for measuring temperature in India.
- Normal human body temperature is about 37°C, but small variations exist.
- Always state temperature with its unit (°C).
- Thermometers must be handled carefully and cleaned before and after use.
Laboratory Thermometer
- Laboratory thermometers measure temperatures from -10°C to 110°C for objects and substances other than the human body.
- The clinical thermometer has a kink to prevent mercury from falling, making it unsuitable for other uses.
- Never use a clinical thermometer to measure hot substances or non-human objects.
Transfer of Heat
- Heat always flows from a hotter object to a colder object until temperatures equalize.
- Conduction: Heat transfers through solids; metals are good conductors, plastics and wood are insulators.
- Convection: Heat transfers in liquids and gases as hot parts rise, cold parts sink, setting up a current.
- Radiation: Heat transfer without a medium; how heat from the sun reaches Earth.
Clothing and Heat
- Dark-colored clothes absorb more heat; hence worn in winter.
- Light-colored clothes reflect heat; more comfortable in summer.
- Woollen clothes trap air, a poor conductor, keeping us warm in winter.
Everyday Applications
- Sea breeze and land breeze are caused by convection currents due to differences in heating/cooling of land and sea.
- Using hollow bricks or trapped air layers in walls helps insulate buildings.
Key Terms & Definitions
- Temperature — Measure of hotness of an object.
- Thermometer — Device for measuring temperature.
- Clinical thermometer — Used for human body temperature (35°C–42°C).
- Laboratory thermometer — Used for measuring temperature of substances other than the human body.
- Conduction — Heat transfer through solids.
- Convection — Heat transfer through liquids and gases by movement of molecules.
- Radiation — Heat transfer without a medium.
- Conductor — Material that allows heat to pass easily.
- Insulator — Material that does not allow heat to pass easily.
- Sea breeze/Land breeze — Winds caused by temperature differences between land and sea.
Action Items / Next Steps
- Complete exercises at the end of the chapter.
- Observe and record body temperatures of friends using a clinical thermometer.
- Perform activities with different materials to identify conductors and insulators.
- Discuss with a doctor the process of measuring body temperature and reasons for specific methods.
- Research normal body temperatures in domestic animals.