Chapter 4 - Heat Notes
1. Heat is a form of energy.
2. A reliable measure of the hotness of an object is its temperature. Temperature is measured by a device called thermometer.
3. The hotness and coldness of a body is relative.
4. The thermometer that measures our body temperature is called a clinical thermometer.
5. Clinical thermometer consists of a long, narrow, uniform glass tube and a bulb at one end. It has a kink which prevents mercury level from falling on its own. The range of this thermometer is from 35°C to 42°C.
6. The normal temperature of human body is 37°C.
7. To measure the temperature of non-living objects, a laboratory thermometer is used. The range of laboratory thermometer is from -10°C to 110°C.
8. Nowadays, a digital thermometer is used instead of a clinical thermometer. It has no mercury in it.
9. Heat always flows from higher temperature to lower temperature. The three ways in which heat can flow from one object to another are conduction, convection and radiation.
10. The process by which heat is transferred from the hotter end to the colder end of an solid object is known as conduction.
11. The materials which allow heat to pass through them easily are called conductors. Example: Iron, Copper, Zinc, etc.
12. The material which do not allow heat to pass through them easily are called poor conductors or insulators. Example: plastic, rubber, wood, etc.
13. Convection is the process of transferring heat from a hotter region to cooler region by the actual movement of particles. It generally occurs in liquid and gaseous medium.
14. Convection takes place when the air near the heat source gets hot and rises. The air from sides comes in to take its place.
15. The flow of cool air from the sea towards the land to replace the hot air on land, is called sea breeze. This phenomenon takes place during the day.
16. During the night, the currents of air flow from the cooler land towards the warmer sea. This is called land breeze.
17. The process of transmission of heat without any contact between the source and the object is called radiation.
18. All hot bodies radiate heat.
19. Dark-coloured objects absorb radiation better than the light-coloured objects. That is the reason we feel more comfortable in light-coloured clothes in the summer.
20. Woollen clothes keep us warm during winter. This is because wool is a poor conductor of heat and it has air trapped in between the fibres.
21. Precautions to be observed while reading a clinical thermometer
• Thermometer should be washed before and after use, preferably with an antiseptic solution.
• Ensure that before use the mercury level is below 35°C.
• Read the thermometer keeping the level of mercury along the line of sight.
• Handle the thermometer with care. If it hits against some hard object, it can break.
• Don’t hold the thermometer by the bulb while reading it.
22. The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by a thermometer called the maximum-minimum thermometer.
23. Precautions needed while reading a clinical thermometer
• The laboratory thermometer should be kept upright not tilted.
• The bulb should be surrounded from all sides by the substance of which the temperature is to be measured. The bulb should not touch the surface of the container.
24. Mercury is a toxic substance and is very difficult to dispose of if a thermometer breaks. These days, digital thermometers are available which do not use mercury.
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