PSEB Punjab Board Class 7 Science Textbook Solution Chapter 4 Heat Exercise Questions and Answers
Chapter 4 Heat
Q.1 Fill in the blanks:
1.) The hotness of an object is measured by its………
Answer:- Thermometer.
2.) The mode of heat transfer without any medium is called…….
Answer:- Radiation.
3.) Air is ………. conductor of heat
Answer:- Poor
4.) The normal temperature of human body is……….°C.
Answer:- 37
Q.2.) True or false:
1.) Metals are insulators of heat.(T/F)
Answer:- False
2.) Sea breeze arise due to condition.(T/F)
Answer:- True
3.) We get heat from the sun by radiation.(T/F)
Answer:- True
4.) Wool is good conductor of heat.(T/F)
Answer:- False
5.) Range of clinical thermometer is 35°C to 42°C.(T/F)
Answer:- True
Q.3.) Match the column:
Answers
1.) land breeze blows during – ii) night
2.) sea breeze blows during – iii) daytime.
3.) We prefer light coloured clothes in – iv) summer.
4.) We prefer dark clothes in – i) winter.
Q.4) Choose the correct option
1.) Temperature of human body is:
(a) 100°C □
(b) 0°C □
(c) 37°C □
(d) 98°C □
Answer:- c. 37°C
2.) Which is bad conductor of heat.
(a)Aluminium □
(b) Iron □
(c) Copper □
(d) Wool □
Answer:- d. Wool
3.) One liter of water at 30°C is mixed with 1 liter of water at 50°C Then temperature of mixture will be:
(a) 80°C □
(b) More than 50°C but less than 80°C □
(c) 20°C □
(d) Between 30°C and 50°C □
Answer:- b.) more than 50°C but less than 80°C
4.) A wooden spoon is dipped in a cup of ice cream. It’s other end :
(a) becomes cold by the process of conduction□
(b) becomes cold by the process of convection □
(c) become cold by the process of radiation □
(d) does not become cold □
Answer:- d.) does not become cold
5.) Land breeze is:
(a) cooler from land to sea □
(b) cool air from sea to land □
(c) hot air from land to sea □
(d) hot air from sea to land □
Answer:- c. hot air from land to sea
Q.5.) Short answer type questions
1.) State the condition which determines the direction of flow of heat.
Answer:- Temperature determines the direction of flow of heat.
2.) What is the clinical thermometer? Write it range.
Answer:- The thermometer used to measure body temperature is called a clinical thermometer. It ranges from 35°C to 42°C.
3.) What is the use of kink in a clinical thermometer?
Answer:- Kink in the Mercury thermometer helps to prevent the falling of mercury present in capillary tube into bulb. It ensures that the user takes the correct reading of temperature.
4.) What is lab thermometer? Write it’s range.
Answer:- The thermometer which is used to measure the temperature of other objects is known as a lab thermometer. It’s range is -10°C to 110°C.
5.) Why do we prefer dark clothes in winter and light ones in summer?
Answer :- Black or dark surfaces absorb more heat than lighter one therefore we feel warmer in winters by wearing dark colour clothes while in summer we prefer white or light color clothes as they absorb less heat.
6.) Why do we wear woolen clothes in winter?
Answer :- Woollen clothes keep us warm in winter because wool is a bad conductor of heat and it has air trapped inside the wool fibres.
Q.6.) Long answer type questions:
1.) What are various modes of heat transfer? Explain.
Answer:- There are three modes of heat transfer.
1) Conduction
2) Convection
3) Radiation
- Conduction of Heat
Heat conduction is a process in which heat is transferred from the hotter part to the colder part in a body without involving any actual movement of the molecules of the body. Heat transfer takes place from one molecule to another molecule as a result of the vibratory motion of the molecules. Heat transfer through the process of conduction occurs in substances which are in direct contact with each other. It generally takes place in solids.
Conduction example: When frying vegetables in a pan. Heat transfer takes place from flame to the pan and then to the vegetables.
Based on the conductivity of heat, substances can be classified as conductors and insulators. Substances that conduct heat easily are known as conductors and those that do not conduct heat are known as insulators.
- Convection of Heat
In this process, heat is transferred in the liquid and gases from a region of higher temperature to a region of lower temperature. Convection heat transfer occurs partly due to the actual movement of molecules or due to the mass transfer.
For example. Heating of milk in a pan.
- Radiation of Heat
It is the process in which heat is transferred from one body to another body without involving the molecules of the medium. Radiation heat transfer does not depend on the medium.
For example: In a microwave, the substances are heated directly without any heating medium.
2.) Explain the formation of sea breeze and land breeze in coastal areas.
Answer:- An interesting phenomenon is experienced by people living near coasts called sea breeze and land breeze relating to convection of air. During the daytime the land gets heated due to the sun.
The air over the land gets heated, expands and rises upwards. The cooler air from the sea rushes towards the coast to take its place and setting up convection currents. Therefore, the air rushing from the sea to the coast is called sea breeze. That is why people in coastal areas make windows of their houses facing the sea for fresh breeze.
During night time, exactly the reverse happens. Due to its higher absorption capacity the water cools down more slowly than the land, so cool air from the land moves towards the sea. Therefore the air rushing from the land towards the sea is called land breeze.
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