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Question 15 Marks
Describe different sources of heat energy.
Answer
The different sources of heat energy are the sun, earth, wood and biomass, coal and petroleum.
1.The Sun:- The primary source of heat energy on the earth’s surface is the sun. Heat and light energy is produced because of the fusion of hydrogen atoms in sun.
2.Earth:- We can produce superheated steam by pumping water from deep inside of the earth in a volcanic region. This superheated steam can be used as heat energy for various purposes.
3.Wood and Biomass:- Heat energy can be produced from wood and biomass by burning them. Dried leaves, cow dung, or any other non-usable products of animals and vegetables are the constituents of biomass. Biomass can be converted into combustible gases such as methane, which on combustion will give us heat energy.
4.Coal and Petroleum:- The forests got buried under the earth’s surface, got converted into coal or petroleum due to the slow action of heat and pressure in the earth. Coal and petroleum produces heat energy on burning.
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Question 25 Marks
How do the fusion point and solidification point of ice and water compare numerically?
Answer
Melting point is the same as fusion point and solidification point is also called freezing.
Because melting point and freezing point describe the same transition of matter, in this case from liquid to solid (freezing) or equivalently, from solid to liquid (melting).
What you may not realize is that while water is freezing or melting, its temperature is not changing! It is stuck on $0 \circ C 0 \circ C$ during the entire melting or freezing process. It is easier to see this for boiling points. if you put a thermometer in water and heat it, the temperature will rise until it reaches $100 \circ C 100 \circ C$, and then it starts boiling. And while it boils, it will stay at $100 \circ C 100 \cdot C !$All the way until the water has all boiled away. Now if you could somehow trap the steam (gaseous water) and keep heating it, the steam could have a temperature higher than $100 \circ C 100 \circ C$.
So to sum this all up, when matter is transitioning from solid to liquid (melting) or liquid to solid (freezing), its temperature is fixed at the melting/freezing point, which is the same temperature.
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[5 Mark Question Answer] - PHYSICS STD 7 Questions - Vidyadip