What is thermal energy and heat complete question answers 2023 | SCIENCE CLASS 7 , CHAPTER 4 Solutions

III.  1. What is thermal energy?

Ans: The sum of energy possessed by all the molecules in motion is the thermal energy of the substance.

2. What does the temperature of a substance indicate?

Ans: Temperature of a substance indicates whether it is hot or cold.

3. Define heat?

Ans: Heat is the thermal energy which flows.

4. Name two good conductors of heat.

Ans: Copper and  Aluminium 

5. What are fluids?

Ans: Fluids  are the combination of gases and liquids. 

6. What is the lower point of an ordinary laboratory thermometer?

Ans: 0° C

IV.  1. How is temperature different from thermal energy?

Ans:  Temperature is the level of thermal energy present in the matter whereas thermal energy is the total energy possessed by all moving molecules of a substance. 

2. What is heat insulator?

Ans: Materials that do not conduct heat like plastics , wood , glass etc are heat insulators. 

3. How does transfer of heat takes place ?

Ans: Heat is transferred through conduction , convection and radiation. 

4. Why is mercury used in thermometer? 

Ans: As mercury is a good conductor of heat it gains heat and expands when contact with the body ,thus indicating the persons body temperature. This is the reason why mercury are used in a thermometer. 

5. How does the fur of seals and polar bears help them in the arctic region? 

Ans: Seals and polar bears have thick layers of fur . Now as air is a good heat insulator , the air trapped by the furs forms an excellent heat insulating layer ,keeping the animals warm even when temperature drops below -10°C. 

 V. 1. Define conduction ,convection and radiation.

 Ans:

•  Conduction –  This is a process of heat transfer which occurs when two objects at different temperatures are at contact with each other.                                                                                                    Eg-  When a  cold  metal rod is placed in contact with flame heat flows from flame to metal rod.
• Convection-  It is the mode of heat transfer through fluids. Here heat is transferred from a region of higher temperature to lower temperature.                                                                                      Eg-  Convection currents gives rise to breeze  in coastal regions.
• Radiation- The process of heat transfer that occurs in which objects are not in contact.                           Eg-  Heat of fire from a distance , heat of sun  even  when indoors etc . 

2.  Why does cold water kept in an open container become warm on a hot summer afternoon?

Ans:  This occurs due to transfer of heat. Cold water kept in an open container  acquires heat from the warm surrounding  becoming warm as the air around it. This continues utnil cold water and the air around it has the same temperature. 

3. Why is the bulb of thermometer made of thin glass? 

Ans: The wall of the bulb of thermometer is made of thin glass so that transfer of heat between mercury and its surrounding  can take place easily. 

4. How does a thermos flask maintain the temperature of liquids kept in it?

Ans: The thermos flask is made up of a  double walled glass vessel with vacuum between the walls. Now , glass being a poor conductor of heat  it   reduces the transfer of heat from inside the flask as well as outside of it. Again heat that could be  transferred through radiation is reduced by silvering the walls of the vacuum flask which reflects the heat rays trying to enter the flask from outside or leave the flask from inside. 

5. How do woollen garments keep us warm?

Ans: Wool being a poor conductor of heat prevents the warmth of the body to escape and also cool air to reach in.                                                                                                                                                         Woollen fibre gets  curled and forms air pockets which traps the air and gets warmed by the heat from the body . In this way , woollen garments keep us warm. 

6. How do convection currents give rise to breeze in the coastal region? 

Ans: Convection currents give rise to breeze in the coastal region during the day as well a night.                            During the day, the land heats up faster than the sea . So the air above the land becomes warmer and rise upwards  and in its place cooler air from above the sea rushes towards the land resulting in sea breeze.                                                                                                                                         But the opposite happens at night. The land cools faster and so the air above the sea gets warmer than the air over the land . The cooler air from the land than replaces  the warmer air above the  sea resulting in a land breeze. 

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