Heat Book Back Questions 8th Science Lesson 4

8th Science Lesson 4

4] Heat

Book Back Questions with Answer and Do You Know Box Content

Do You Know?

Electric wires used for long distance transmission of electricity will expand during day time and contract at night. That is why they will not be set very tightly. If they are set very tightly they will break when they cool at night.

Water is the only matter on the Earth that can be found naturally in all three states – Solid, Liquid and Gas.

All metals are good conductors of heat. The substances which does not conduct heat easily are called bad conductors or insulators. Wood, cork, cotton, wool, glass, rubber, etc. are insulators.

Heat transfer by radiation is visible to our eyes. When a substance is heated to 500^oC the radiation begins to become visible to the eye as a dull red glow, and it is sensed as warmth by the skin. Further heating rapidly increases the amount of radiation, and its perceived colour becomes orange, yellow and finally white.

The amount of energy in food items is measured by the unit kilo calorie. 1 kilo calorie = 4200 J (Approximately).

Water has higher heat capacity than most other substances. This accounts for the use of water as common coolant. 100 g of water can take away more heat than 100 g of oil.

Problem 1: The temperature of a metal ball is 30^oC. When an energy of 3000 J is supplied, its temperature raises by 40^oC. Calculate its heat capacity.

Solution: Heat capacity, C` = Q/∆T

Here, Q = 3000 J

∆T = 40^oC – 30^oC = 10^oC or 10K

C` = 3000/10 = 300 JK^-1

The heat capacity of the metal ball is 300 JK^-1.

Problem 2: The energy required to raise the temperature of an iron ball by 1 K is 500 JK^-1. Calculate the amount of energy required to raise its temperature by 20 K.

Solution: Heat capacity, C` = Q/∆T

Q = C` x ∆T

Here, C` = 500 JK^-1

∆T = 20 K

⸫Q = 500 x 20 = 10000 J. the amount of heat energy required is 10000 J.

Problem 3: An energy of 84000 J is required to raise the temperature of 2 kg of water from 60^oC to 70 ^oC. Calculate the specific heat capacity of water.

Solution: Specific heat capacity, C = Q/m x ∆T

Here, Q = 84000 J

m = 2 kg

∆T = 70^oC – 60^oC = 10^oC or 10 K

C = 84000/2 x 140 = 4200 J kg^-1 K^-1

The Specific heat capacity of water is 4200 J kg^-1 K^-1.

Problem 4: The specific heat capacity of a metal is 160 J kg^-1 K^-1. Calculate the amount of heat energy required to raise the temperature of 500 gram of the metal from 125^oC to 325^oC.

Solution: Specific heat capacity, C = Q/m x ∆T

Q = C x m x ∆

Here, C = 160 K kg K^-1

m = 500 g, g = 0.5 kg

∆T = 325^oC – 125^oC = 200^oC or 200 K = 160 x 0.5 x 200 = 16000 J. the amount of heat energy required is 16000 J.

The world’s first ice-calorimeter was used in the year 1782 by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat generated by various chemical changes.

The vacuum flask was invented by Scottish scientist Sir James Dewar in 1892. In his honour it is called as Dewar flask. It’s also known as Dewar bottle.

Choose the best answers:

1. Heat is a form of _________________.

(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 the above

5. The process of converting a liquid into a solid is called ______________.

(a) Sublimation

(b) Condensation

(c) Freezing

(d) Deposition

6. Conduction is the way of heat transfer which takes place in a _______________.

(a) Solid

(b) Liquid

(c) Gas

(d) All the them

Fill in the blanks:

1. A calorimeter is a device used to measure the ______________.

2. _______________ is defined as the amount of heat required to raise the temperature of 1 kg of a substance by 1^oC.

3. A thermostat is a device which maintains _______________.

4. The process of converting a substance from gaseous state to solid state is called _________________.

5. If you apply heat energy, the temperature of a system will ________________.

6. If the temperature of a liquid in a container is decreased, then the inter-atomic distance will _______________.

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.

2. The dimensions of a substance are increased if the temperature of the substance is decreased.

3. The process of converting a substance from solid state to gaseous state is called condensation.

4. Convection is the process by which the thermal energy flows in solids.

5. The amount of heat gained by a substance is equal to the product of its mass and latent heat.

6. In a thermos flask, the silvered walls reflect and radiate the heat outside.

Match the following:

1. Conduction – Liquid

2. Convection – Gas to liquid

3. Radiation – Solid to gas

4. Sublimation – Vacuum

5. Condensation – Solid

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.

(a) Both assertion and reason are true and reason is the correct explanation of the assertion.

(b) Both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) Assertion is true, but reason is false.

(d) Both assertion and reason are false.

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.

(a) Both assertion and reason are true and reason is the correct explanation of the assertion.

(b) Both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) Assertion is true, but reason is false.

(d) Both assertion and reason are false.

Answers:

Choose the best answers:

1. Thermal energy 2. All the above 3. All the above 4. Gas 5. Freezing 6. Solid

Fill in the blanks:

1. Heat capacity of water 2. Specific heat capacity 3. Temperature of an object constant

4. Deposition 5. Increase 6. Decrease

State true or false. If false, correct the statement:

1.True

2.False

Correct answer: The dimensions of a substance are increased if the temperature of the substance is increased

3.False

Correct answer: The process of converting a substance from solid state to gaseous state is called sublimation

4.False

Correct answer: Convection is the process by which the thermal energy flows in liquids and gases

5.True

6.False

Correct answer: In a thermos flask, the silvered walls reflect and radiate the heat back into the flask itself

Match the following:

1.Conduction- Solid

2.Convection- Liquid

3.Radiation- Vaccum

4.Sublimation- Solid to gas

5.Condensation- Gas to liquid

Consider the statements given below and choose the correct option:

1.Both assertion and reason are true, but reason is not the correct explanation of assertion.

2.Both assertion and reason are true and reason is the correct explanation of assertion