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PHYSICS [3 Mark Question Answer]

Heat Transfer · ICSE STD 8 (ICSE - English Medium). 3 questions.

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[3 Mark Question Answer]

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3 questions · timed · auto-graded

Question 13 Marks
Explain, with the help of a diagram, the working of an automatic fire alarm.
Answer
An automatic fire alarm uses an iron$-$brass bimetallic strp.

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The end $A$ of the strip is tightly fixed on a wooden board and the end $B$ is free to move up and down. An adjustable contact screw $C$ made of brass is also provided. This screw is held $1\  mm$ above the end $B$ of the strip. In case of a fire, the bimetallic strip gets heated and bends. The end $B$ rises up as brass expands more than iron. When the end $B$ touches the contact screw $C$, the electrical circuit is completed and the electric bell starts ringing.
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Question 33 Marks
Explain the process of evaporation.
Answer
The change of a substance from its liquid state to its gaseous state at any temperature below its boiling point is called evaporation. In a liquid, the molecules are in constant random motion. The speed of these molecules keep changing due to their collisions with other molecules. If molecules near the surface of the liquid gain sufficient energy, they escape and become vapour.
This is the process of evaporation. When the temperature of the liquid increases, the average kinetic energy of the molecules increases. This means that there will be a larger proportion of the molecules that have sufficient energy to escape from the surface. Hence, the rate of evaporation increases with increase in temperature.
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[3 Mark Question Answer] - PHYSICS STD 8 Questions - Vidyadip