Sources and Effects of Heat Energy
Unit 9 Grade 8 Science
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1: Write some uses of heat in our daily life?
Ans: We need heat for ripening crops and fruits. We need heat to keep the Earth’s environment warm, melt ice, and in the preparation of industrial products.
2: What are the sources of heat?
Ans: Sun, metabolism, fossil fuels (coal, oil, petrol, etc), electricity, etc are the main sources of heat.
3: What are the components of material objects?
Ans: All kinds of material objects are made up of tiny particles such as atoms and molecules.
4: What is thermal expansion?
Ans: When an object is heated, the object expands. And the expansion of material objects due to heating is called thermal expansion.
5: What is a thermal contraction?
Ans: When an object is cooled, the object contracts. This contraction of material objects on cooling is called thermal contraction.
6: What is the thermal expansion of solids?
Ans: When solids are heated, the vibratory motion of their particles become faster and they began to push each other further apart, which result’s thermal expansion of solids.
7: What is a contraction of solids?
Ans: When solids are cooled, particles slow down and solids contract.
8: What is the effect of heat on gas?
Ans: Like solids and liquids, gases also expand on heating.
9: What is riveting?
Ans: The process in which two metal plates are joined together by the means of rivets is called riveting.
10: How do we fix a metal tyre over the wheel?
Ans: First of all, we heat the metal tyre. as we heat the metal tyre, it expands and its diameter increases. Then hot tyre can easily be fitted into the wheel. On cooling, the metal tyre contracts and fits over the wheel tightly.
Ans: A thermostat is a device that is used to control the temperature in electrical appliances.
12: Write some uses of thermostats.
Ans:Electric irons, refrigerators, air conditioners, ovens and stoves etc use a thermostat. It is also used in fire alarms.
13: What is the function of electric iron?
Ans: In an electric iron, when electric current flows through its heating element, it becomes hot. The bimetallic strip connected with the heating element also begins to heat up. On getting hot, the bimetallic strip bends and is disconnected from the heating element. Thus the circuit opens and the switch turns to OFF the electric iron. On cooling, the bimetallic strip straights. The circuit is again closed and the iron is switched ON.
14: What is the function of a fire alarm?
Ans: In case the fire breaks out, the bimetallic strip used in the fire alarm gets hot and bends to touch the contact point of the battery. In this way, the circuit is completed, and the bell begins to ring to warn of the fire.
15: What is a thermometer?
Ans: A thermometer is a device that is used for measuring temperature.