Samacheer Kalvi 8th Science Solutions Chapter 4 Pdf
Tamilnadu Board Samacheer Kalvi 8th Science Solutions Chapter 4: Tamilnadu State Board Solution Class 8 Science Chapter 4 – Heat.
Samacheer Kalvi 8th Science Solutions Chapter 4: Overview
|
Board |
Samacheer Kalvi |
| Class |
8 |
|
Subject |
Science |
| Chapter |
4 |
|
Chapter Name |
Heat |
Samacheer Kalvi 8th Science Solutions Chapter 4 Pdf
(I ) Choose the best answer.
1) Heat is a form of thermalenergy.
a ) electrical energy
b) gravitational energy
c) thermal energy
d) None of these
2) If you apply some heat energy to a substance, which of the following can take place in it?
a ) Expansion
b) Increase in temperature
c) Change of state
d) All the above.
3) Which of the following substances will absorb more heat energy?
a ) Solid
b) Liquid
c) Gas
d) All the above
(4) If you apply equal amount of heat to a solid, liquid and gas individually ,which of the following will have more expansion?
a) Solid
b) Liquid
c) Gas
d) All of them
(5) The process of converting a liquid into a solid is called freezing.
a) sublimation
b) condensation
c) freezing
d) deposition
(6) Conduction is the way of heat transfer which takes place in a solid .
a) solid
b) liquid
c) gas
d) All of them
(II) Fill in the blanks.
1 ) A calorimeter is a device used to measure theheat capacity of liquids.
2) Specific heat capacity is defined as the amount of heat required to raise the temperature of 1 kg of a substance by 1° C.
3) A thermostat is a device which maintains temperature of an object constant.
4) The process of converting a substance from gaseous state to solid state is called deposition.
5) If you apply heat energy, the temperature of a system will increase.
6) If the temperature of a liquid in a container is decreased, then the interatomic distance will decrease.
(III). State True or False. If false, correct the statement.
1 ) The applied heat energy can be realised as an increase in the average kinetic energy of the molecules.
This statement is true.
2 ) The dimensions of a substance are increased if the temperature of the substance is decreased.
This statement is false.
Explanation :
If the temperature of the substance is increased, then the dimensions of a substance are also increased.
3) The process of converting a substance from solid state to gaseous state is called condensation.
This statement is false.
Explanation:
Sublimation refers to the process of transforming a substance from solid state to gaseous state.
4) Convection is the process by which the thermal energy flows in solids.
This statement is false.
Explanation:
There is flow of thermal energy in liquids and gases and this process is known as convection.
5) The amount of heat gained by a substance is equal to the product of its mass and latent heat.
This statement is true.
6) In a thermos flask, the silvered walls reflect and radiate the heat outside.
This statement is false.
Explanation:
The silvered walls in a thermos flask does not reflect and radiate the heat outside but instead ,it reflect and radiate the heat back to the liquid in the bottle.
(IV.) Match the following.
| Conduction | Liquid |
| Convection | Gas to liquid |
| Radiation | Solid to gas |
| Sublimation | Vacuum |
| Condensation | Solid |
Answer :
| Conduction | Solid |
| Convection | Liquid |
| Radiation | Vacuum |
| Sublimation | Solid to gas |
| Condensation | Gas to liquid |
(V) Consider the statements given below and choose the correct option.
1.) Assertion : Radiation is a form of heat transfer which takes place only in vacuum.
Reason : The thermal energy is transferred from one part of a substance to another part without the actual movement of the atoms or molecules.
Answer : b ) Both assertion and reason are true ,but reason is not the correct explanation of assertion.
Explanation :
Electromagnetic waves are only involved in radiation and not particles and for this reason it is possible to transfer heat over a vacuum.
2.) Assertion : A system can be converted from one state to another state.
Reason : It takes place when the temperature of the system is constant.
Answer : a ) Both assertion and reason are true and reason is the correct explanation of assertion .
(VI.) Answer briefly.
1 ) What are the applications of conduction in our daily life?
Conduction in daily life
- Foods are usually cooked in vessels, which are composed of metals. There is transmission of heat from the metal to the food when the vessel is heated.
- There is transmission of heat from the iron to the cloth, when we iron the dresses.
- As they possess low thermal conductivity, for this reason handles of cooking utensils are composed of plastic or wood.
- As snow has low thermal conductivity, for this reason the temperature inside igloo is not so cold.
2 ) What are the effects of heat?
Effects of Heat
The supply of heat energy to any substance, results in bringing about numerous changes. We mostly perceive three important changes in our daily life. They are : Expansion, Increase in temperature and Change in state.
Explanation :
Expansion
The atoms in the ball gain heat energy ,the moment the ball is heated . They try to move each other apart by exerting force and begins to vibrate. Consequently, there is an occurrence of expansion. For this reason, the ball fails to go through the ring. After some moment, the ball retains back its original size due to the reduction of heat energy to the surrounding and this makes it easier for the ball to pass through the ring . This clearly indicates that , heat energy is the main reason behind expansion in solids. This expansion is maximum in gases but it occurs in liquids also.
Rise in Temperature
Water molecules receive heat energy ,the moment the water is heated. The kinetic energy of the molecules are increased because of this heat energy. When more energy are received by the molecules, there is a rise in the temperature of water.This clearly depicts that ,heat energy is the major reason behind the rise in temperature.
Change of state
Change of state refers to the physical process in which there is movement of matter from one state to another state. Melting, evaporation, boiling, freezing and condensation are some of the relevant examples of such changes.
3) Name three types of heat transfer.
There is transmission of heat energy from one part of the substance to another part of the substance, when there is supply of heat energy to any substance. It occurs in distinct ways, based on the state of the substance.
Conduction, Convection and Radiation are mainly the three types of heat transfer.
Explanation :
Conduction
Conduction refers to the process by which there is transmission of heat between two terminal points of similar solid or through two solid substances that have varying temperatures but in connection with each other .Therefore ,the transmission of heat in solids from the domain of higher temperature to the domain of lower temperature in the absence of actual movement of molecules is known as conduction.
Convection
When water is boiled in the vessel ,water molecules which are at the bottom tends to move in a upward direction as they are exposed to heat energy . Then the topmost molecules comes in a downward direction and gets heated . Thus, convection refers to this type of heat transfer . Air in the atmosphere is also heated in the similar process. Therefore, convection refers to the process of transmission of heat from places of high temperature to places of low temperature with actual movement of molecules. Convection occurs in both liquids and gases.
Radiation
The third form of heat transfer is radiation. There is transmission of heat through solids, by the process of conduction , there is transmission of heat through liquids and gases, by the process of convection but there is transmission of heat through vacuum by the process of radiation .It is this form of heat energy, which enables the Earth to receive heat energy from the sun.The process of heat transfer from one place to another place in the form of electromagnetic waves is known as radiation.
4) What is conduction?
Conduction refers to the process by which there is transmission of heat between two terminal points of similar solid or through two solid substances that have varying temperatures but in connection with each other .Therefore ,the transmission of heat in solids from the domain of higher temperature to the domain of lower temperature in the absence of actual movement of molecules is known as conduction.
5) Write a note on convection.
When water is boiled in the vessel ,water molecules which are at the bottom tends to move in a upward direction as they are exposed to heat energy . Then topmost molecules comes in a downward direction and gets heated . Thus, convection refers to this type of heat transfer . Air in the atmosphere is also heated in the similar process. Therefore, convection refers to the process of transmission of heat from places of high temperature to places of low temperature with actual movement of molecules. Convection occurs in both liquids and gases.
6) Define specific heat capacity.
The expression of the heat capacity of a substance for unit mass is known as specific heat capacity. The quantity of heat energy necessary to increase the temperature of 1 kilogram of a substance by 1° C or 1 k , is known as the specific heat capacity of a substance. It is indicated by the symbol C. J kg -1 k -1 is the SI unit of specific heat capacity.
7 ) Define one calorie .
As we all know that, heat is a form of energy. In SI system the unit of energy is joule. For this reason, heat is also measured in joule . It is indicated by the symbol J. Calorie is one of the most commonly used unit of heat. One calorie may be defined as the quantity of heat energy necessary to increase the temperature of 1 gram of water through 1 °C . The relationship between calorie and joule is expressed as , 1 calorie is equivalent to 4.186 J.
(VII.) Answer in detail.
1 ) With the help of a neat diagram, explain the working of a calorimeter.
A calorimeter is an instrument that is mainly designed for the purpose of measuring the quantity of heat gained or lost by a substance. It contains a vessel which are composed of copper or aluminium and these metals possess a good thermal and electrical conductivity. In order to avert heat reduction to the environment , these metallic vessel is kept in an insulating jacket. It consists of two holes. A thermometer is inserted through one hole, for the purpose of measuring the temperature of the contents. Through another hole, a stirrer is inserted in order to stir the content in the vessel . The liquid inside the vessel gets heated up by transferring current through the heating element. Therefore, this device enables us to measure the heat capacity of water.
2) Write a note on thermostat.
A thermostat is an instrument that is mainly used for maintaining the temperature of a place or an object uniform. The word ‘ thermo’ means heat and ‘static’ means remaining the same and these are the two Greek words from where the word thermostat is derived. Devices that gets heated or cools down to a pre – set temperatures, thermostat are used in those systems or devices. After reaching a specific temperature ,it performs an essential role in turning an appliance or a circuit on or off. Building heater, central heater in a room, air conditioner, water heater and kitchen equipment’s are for instance, the various devices which makes use of thermostat. Thermostat possesses the capacity to function both as sensor and controller of a thermal system.
3) Explain the working of thermos flask.
A thermos flask contains two evacuated walls. On the inner part ,it is silvered. The presence of vacuum between the two walls plays a crucial role in averting heat transmission from the inner side to the outer side by conduction and convection. There is nearly no transmission of heat from the interior wall to the exterior wall, as there is only minimal quantity of air present between the walls . The connecting point of two walls, facilitates the occurrence of conduction, at the top most of the bottle and through an insulated assistance at the basal. The silver walls possess the capacity to reflect the radiated heat back to the liquid in the bottle.
(VIII.) Higher Order Thinking Questions.
1 ) Why does the bottom of a lake not freeze in severe winter though the surface is all frozen?
Here, a process of insulation takes place , which means the ice that are on the surface plays a major role in insulating decease water underneath . For this reason, lakes does not freeze entirely.A large amount of heat is to be removed for the purpose of freezing water into ice. So, it is quite difficult to withdraw this heat.
2) Which one of the following statements about thermal conductivity is correct ? Give reason.
a) Steel > Wood > Water
b) Steel > Water > Wood
c) Water >Steel > Wood
d) Water > Wood > Steel
Answer : b) Steel > Water > Wood
Explanation :
The flow of heat per unit time is known as thermal conductivity. Steel possess a greater thermal conductivity when compared to water and wood. 50.2 w/mk is the thermal conductivity of steel, 0.6 w/mk is the thermal conductivity of water and 0.12 w/mk is thermal conductivity of wood. So, this clearly indicates that the thermal conductivity of steel is much higher in comparison to water and wood.
(IX). Numerical Problems.
1 ) An iron ball requires 1000 J of heat to raise its temperature by 20 °C . Calculate the heat capacity of the ball.
The formula of Heat capacity
C ‘ = Q / ∆ T
Here, Q is equivalent to 1000 J
So, T = 20 °C – 0° C
= 20 ° C = 20 k
C = 1000 / 20 = 50 JK -1
Therefore, the heat capacity of the ball is 50 JK -1
2 ) The heat capacity of the vessel of mass 100 kg is 8000 J/°K . Find its specific heat capacity.
The formula of specific heat capacity
C= Q / m × ∆ T
So, the mass is 100 kg , which is already given
Heat capacity ( C’ ) = 8000 J/k
So, C’ = Q / ∆T = 8000 J/k
Therefore, C= Q / m× ∆T
= 8000 J /k × 100 kg -1
= 800000 J kg -1k-1
Therefore, the specific heat capacity is 800000 J kg -1 k-1
