Samacheer Kalvi 9th Science Solutions Chapter 7 Pdf
Tamilnadu Board Samacheer Kalvi 9th Science Solutions Chapter 7: Tamilnadu State Board Solution Class 9 Science Chapter 7 – HEAT.
Samacheer Kalvi 9th Science Solutions Chapter 7: Overview
| Board | Samacheer Kalvi |
| Class | 9 |
| Subject | Science |
| Chapter | 7 |
| Chapter Name | HEAT |
Samacheer Kalvi 9th Science Solutions Chapter 7 Pdf
Chapter 7
Heat
I) Choose the correct answer.
1) Calorie is the unit of
a) Heat
b) Work
c) Temperature
d) Food
Ans: a) Heat
Since, the Calorie is the unit of heat and 1calorie = 4.184J
2) SI unit of temperature is
a) Fahrenheit
b) Joule
c) Celsius
d) Kelvin
Ans: d) Kelvin
Because, the SI unit of temperature is kelvin.
3) Two cylindrical rods of same length have the area of cross section in the ratio 2:1. If both the rods are made up of same material, which of them conduct heat faster?
a) Both rods
b) Rod-2
c) Rod-1
d) None of them
Ans: c) Rod-1
Because, the heat transfer or conduction of heat is directly proportional to the area of cross section of the material. And hence, here the rod 1 has area twice that of rod 2 and hence, rod1 conducts faster.
4) In which mode of transfer of heat, molecules pass on heat energy to neighboring molecules without actually moving from their positions?
a) Radiation
b) Conduction
c) Convection
d) Both B and C
Ans: b) Conduction
Because, conduction is the process of transfer of heat from high temperature region to low temperature region without the actual migration of the molecules of the substance.
5) A device in which the loss of heat due to conduction, convection and radiation is minimized is
a) solar cell
b) Solar cooker
c) Thermometer
d) Thermos flask
Ans: d) Thermos flask
Because, in thermos flask the heat loss due to conduction, convection and radiation is minimized due to which the temperature of the liquid inside it remains constant.
Fill in the blanks.
1) The fastest mode of heat transfer is—-.
Ans: The fastest mode of heat transfer isradiation.
Because, it transfers heat in vacuum also in the form of electromagnetic waves.
2) During day time, air blows from—- to—-.
Ans: During day time, air blows from seato land.
Because, in day time the land is warmer as compared to sea. So air blows from high pressure to low pressure.
3) Liquids and gases are generally—— conductors of heat.
Ans: Liquids and gases are generally badconductors of heat
Because, the molecules in liquid and gases are widely separated or the intermolecular force of attraction is less in liquid and gases. Due to which they are bad conductors of heat.
4) The fixed temperature at which matter changes state from solid to liquid is called—.
Ans: The fixed temperature at which matter changes state from solid to liquid is called melting point.
Because, the temperature at which solid is converted into liquid is called as melting point of that solid.
III. Assertion and reason type questions. Mark the correct choice as:
If both assertion and reason are true and reason is the correct explanation of assertion.
If both assertion and reason are true but reason is not the correct explanation of assertion.
If assertion is true but reason is false.
If assertion is false but reason is true.
1) Assertion: Food can be cooked faster in vessels with copper bottom
Reason: Copper is the best conductor of heat.
Ans: a. if both assertion and reason are true and reason is the correct explanation of assertion.
2) Assertion: Maximum sunlight reaches earth’s surface during the noon time
Reason: Heat from the sun reaches earth’s surface by radiation.
Ans: b. if both assertion and reason are true but reason is not the correct explanation of assertion.
Because, at noon sun is at overhead, due to which the heat or sunlight reaches to earth is maximum at noon.
3) Assertion: When water is heated up to 100oC, there is no raise in temperature until all water gets converted into water vapor.
Reason: Boiling point of water is 10°C.
Ans: c. if assertion is true but reason is false.
Because, the boiling point of water is 1000C
Answer briefly.
1) Define conduction.
Ans: There are three modes of transfer of heat that are conduction, convection and radiation. The phenomenon of transfer of heat through the medium without actual migration of the molecules or atoms of the medium is called as conduction. For example: when we have given heat to the one end of iron rod suppose, then that end becomes hot and this heat is transferred to other end. Due to which we feel another end also warmer. In this case, the heat is transferred by the mode of conduction. The fastest mode of heat transfer is the radiation, because it transfers heat through the vacuum in the form of thermal radiations called as electromagnetic waves. Solids are good conductors than liquids and gases.
2) Ice is kept in a double-walled container. Why?
Ans: The ice is kept in a double walled container because, if ice is in contact with the surrounding then heat will be transferred from high temperature to low temperature i.e. from surrounding to ice. And hence, ice get melted. To avoid this, the ice is kept in a double walled container. Due to which there will be a vacuum created in between two walls which does not allow the heat from surroundings to the ice and hence the ice remains as it is.
3) How does the water kept in an earthen pot remain cool?
Ans: The earthen pot has small pores on it. Water from the pot get evaporated due to the high temperature of surroundings through the pores of the pot by capillary action. Because of this, the temperature of pot decreases and thereby it cools the water inside it. Hence, water kept in an earthen pot remains cool in summer also.
4) Differentiate convection and radiation.
Ans: There are three modes of transfer of heat that are conduction, convection and radiation. The phenomenon of transfer of heat by the actual migration of molecules of fluid from higher temperature to lower temperature is called as convection. Radiation is the mode of transfer of heat from one place to another in the form of electromagnetic waves. Heat energy from sun reaches to earth in the form of radiations. Radiation is the fastest mode of transfer of heat. In convection, transfer of heat takes place within the fluid while in radiation, heat transfer takes place through electromagnetic waves without transfer of any particles.
5) Why do people prefer wearing white clothes during summer?
Ans: Because, the white colour reflects sunlight more and absorbs in less amount. In summer days, there is a too temperature and if we wear the white cloths then most of the heat get reflected by maintaining our body cool. Hence, people prefer wearing white cloths during summer. On the other hand, the black colour absorbs more heat and reflect in less amount. Due to which we prefer to wear black cloths in winter or in cold days.
6) What is specific heat capacity?
Ans: Specific heat capacity of a substance is defined as the amount of heat required to increase the temperature of 1 kg of the substance by 10C or 1 K. The SI unit of specific heat capacity is Jkg-1 K-1. Water has the highest specific heat capacity and its value is4200 J/kg°K. Hence, water is used as coolant in car radiators and factories to decrease the temperature of engines and machines thereby protecting them from damage.
7) Define thermal capacity.
Ans: Heat capacity or thermal capacity is defined as the amount of heat energy required to increase the temperature of the body by 10C. It is represented by C. Heat capacity or thermal capacity = quantity of heat required / increase in temperature
C = Q /T
Where, Q is the amount of heat required
T is temperature increased
The SI unit thermal capacity is J/K. It is also expressed in cal/oC or J/oC.
8) Define specific latent heat capacity.
Ans: The specific latent heat is defined as the amount of heat energy absorbed or evolved by unit mass of substance during the change of its state without changing in its temperature. It is denoted by L.
The SI unit of a specific latent heat is J/kg.
If Q is the amount of heat absorbed or evolved by mass m during change in its state at a constant temperature then specific latent heat capacity is given by L = Q/m.
Answer in detail.
1) Explain convection in daily life.
Ans: Convection is the process of heat transfer from region of higher temperature to the region of lower temperature through the migration or movement of fluid itself. There are many examples of convection in daily life. Hot air balloon is the example of convection in daily life. In hot air balloon the air molecules at the bottom are heated due to the source of heat and this warm air rises. Due to which the cold air is pulled down, which is again heated. The warm air is less dense than the cold air. When total air inside the hot air balloon becomes warmed it will go in upward direction. The best daily life example is chimneys in kitchen and in industries. Chimneys are used to rise up the hot gases and smoke which are lighter in atmosphere.
2) What are the changes of state in water? Explain.
Ans: The process of changing of substance from one physical state to other physical state at a definite temperature is called as change of state. There are mainly three states of matter solid, liquid and gas. The state of matter changes due the change temperature or pressure and due to the heat absorbed or removed. At normal temperature water is in the form of liquid, if we have given heat to it get evaporated and converted into gas. If further we cool that vapors then it is again converted into liquid form. The water converts into vapors at 1000C. If we lowered the temperature up to 00C then it is converted into ice. Again by supplying heat, ice is converted into water. The following are the change in state of water.
Melting: The process in which solid is converted into liquid by absorbing heat is called as melting of the solid. For example: ice is converted into water by absorbing heat energy.
Solid —————————> liquid
Freezing: The process in which liquid is converted into solid by evolving the heat energy is called as freezing of the liquid. For example: water is converted into ice at 00C.
Liquid ——————————> solid
Boiling: The process in which liquid is converted into vapor by absorbing heat energy is called as boiling of the liquid. For example: water is converted into vapors at 1000C.
Liquid ————————————–> vapors
Condensation: The process in which vapor is converted into liquid by cooling it is called as condensation of the vapor. For example: the water vapor is cooled and converted into liquid water.
Vapor———————————-> liquid
Sublimation: The process in which solid is directly converted into gaseous form is called as sublimation. For example: ice is directly converted into vapor form, iodine is directly converted into vapor form etc.
Solid——————————————> vapor
3) How can you experimentally prove water is a bad conductor of heat? How is it possible to heat water easily while cooking?
Ans: The water is the good conductor of electricity but bad conductor heat which is explained using real life example as bellow: We have taken a test tube and filled it’s one fourth portion with water. Now we have put the ice cube inside it which goes to the bottom of the tube. Now we start heating the test tube by holding it in inclined position. It is observed that, after sometime water get heated and start to boil. The boiled water molecule goes upward and cooled molecules comes down. Because warm water has less density then cold water. But, it is observed that the water from lateral sides of tube is also boiling. But the bottom part is not boiled till, due to which ice remains as it is. Thus, this experiment proves that water is a bad conductor of heat. While cooking water is heated easily because, the utensils are covered with lid due to which vapors formed again condensed and there is no convection of heat. Due to which temperature of the cooking pot increases and water get boiled easily to cook the food.
Numerical Problems.
1) What is the heat in joules required to raise the temperature of 25 grams of water from 0 °C to 100 °C? What is the heat in Calories? (Specific heat of water = 4.18 J/g °C) (Ans. 10450 J)
Ans: Given that, m = 25g, initial temp. = 00C and final temp. = 1000C
Change in temp. = ∆T = 1000C – 00C = 373K – 273K = 100K
Specific heat of water C = 4.18 J/g °C
Thus, heat gained by the substance when its temperature changes is given by
Q = m C ∆T = 25 * 4.18* 100 = 10450J
Thus, the heat required to raise the temperature by 1000C is 10450J.
In Calories the heat required is 10450/4.18calorie = 2500 Calories
2) What could be the final temperature of a mixture of 100 g of water at 90 °C and 600g of water at 20°C.? (Ans. 30°C)
Ans: Let T be the final temperature of mixture of water. The 100g of water which is at higher temperature i.e. 900C after mixing with 600g of water tries to loose heat in order to attain constant final temperature T. Hence, change in heat is given by ∆Q = m C ∆T
∆Q= 100*C* (90 – T)
And when the 600g of water which is at lower temperature i.e. 200C after mixing with water of 100g at 900C tries to receive heat to attain constant final temperature T. Hence, change in heat is given by ∆Q = m C ∆T
∆Q = 600*C* (T – 20)
Hence, 100*C* (90 – T) = 600*C* (T – 20)
90 – T = 6* (T – 20)
90 – T = 6T – 120
90 + 120 = 6T + T
210 = 7T
T = 300C
Thus, the final temperature of the mixture could be 300C.
3) How much heat energy is required to change 2 kg of ice at 0°C into water at 20°C? (Specific latent heat of fusion of water = 3, 34,000 J/kg, Specific heat capacity of water = 4200 JKg–1K–1). (Ans. 8, 36,000 J)
Ans: Here, first we have to convert 2kg of ice at 00C into 2kg of water at 00C. And then this 2kg of water at 00C is we have to convert it into 2kg of water at 200C.
Thus, total heat required = heat required to convert 2kg of ice at 00C into 2kg of water at 00C +heat required to convert 2kg of water at 00C into 2kg of water at 200C
Q = m * Specific latent heat of fusion of water + m * specific heat capacity of water*∆T
Q = 2*334000 + 2*4200* (20 – 0)
Q = 668000 + 168000
Q = 836000 J
Thus, heat required is 836000 J.
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