Heat Book Back Questions 9th Science Lesson 7

9th Science Lesson 7

7] Heat

Book Back Questions with Answer and Do You Know Box Content

Do You Know?

When a dog keeps out its tongue and breathes hard, the moisture on the tongue turns into water and it evaporates. Since, heat energy is needed to turn a liquid into a gas, het is removed from dog’s tongue. This helps to cool the body of the dog.

While firing wood, we can observe all the three ways f heat transfer. Heat in one end of the wood will be transferred to other end due to conduction. The air near the wood will become warm and replace the air above. This is convection. Our hands will be warm because heat reaches us in the form of radiation.

Problem 1: Convert the following: (i) 25^oC to Kelvin (ii) 200 K to ^oC

Solution: (i) T_K = T_oC + 273.15 = 25 + 273.15 = 298.15 K

(ii) T_oC = T_K = 273.15 = T_oC = 200 – 273.15 = -73.15^oC

Problem 2: Convert the following: (i) 35^oC to Fahrenheit (^oF) (ii) 14^oF to ^oC

Solution: (i) T_oF = T_oC x 1.8 + 32 = 25^oC x 1.8 + 32 = 77^oF

(ii) T_oC =(T_oF – 32) /1.8 = (14^oF – 32) /1.8 = -10^oC

Problem 3: Calculate the heat energy required to raise the temperature of 2 kg of water from 10^oC to 50^oC. Specific heat capacity of water 4200 JKg^-1 K^-1

Solution: Given m = 2 kg, ∆T = (50 – 10) = 40^oC. In terms of Kelvin, ∆T = (323.15 – 283.15) = 40K, C = 4200 J Kg^-1K^-1

⸫ Heat energy required, Q = m x C x ∆T = 2 x 4200 x 40 = 3,36,000 J.

Water in its various form, has different specific heat capacities. Water (Liquid state) = 4200 JKg^-1 K^-1; Ice (Solid state) = 2100 JKg^-1 K^-1; Steam (Gaseous state) = 460 JKg^-1 K^-1

Problem 4: An iron ball requires 5000 J heat energy to raise its temperature by 20 K. Calculate the heat capacity of the iron ball.

Solution: Given, Q = 5000 J, ∆T = 20 K. Heat Capacity, C = Heat energy required, Q/Rise in temperature, ∆T = 5000/20 = 250 JK^-1

Problem 5: How much heat energy is required to melt 5 kg of ice? (Specific latent heat of ice = 336 Jg^-1)

Solution: Given, m = 5 Kg = 5000 g, L = 336 Jg^-1

Heat energy required = m x L = 5000 x 336 = 1680000 J or 1.68 x 10⁶ J

Problem 6: How much boiling water at 100^oC is needed to melt 2 kg of ice so that the mixture which is all water is at 0^oC?

[Specific heat capacity of water = 4.2 JKg^-1 and Specific latent heat of ice = 336 Jg^-1].

Solution: Given, mass of ice = 2 kg = 2000 g. Let ‘m’ be the mass of boiling water required. Heat lost = Heat gained.

m x c x ∆t = m x L

m x 4.2 x (100 – 0) = 2000 x 336

m = 2000 x 336 / 4.2 x 100 = 1600 g or 1.6 kg.

Choose the best answers:

1. Calorie is the unit of

(a) Heat

(b) Work

(c) Temperature

(d) Food

2. SI unit of temperature is

(a) Fahrenheit

(b) Joule

(c) Celsius

(d) 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

4. In which mode of transfer of heat, molecules pass on heat energy to neighbouring molecules without actually moving from their positions?

(a) Radiation

(b) Conduction

(c) Convection

(d) Both B and C

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

Fill in the blanks:

1. The fastest mode of heat transfer is ________________.

2. During day time, air blows from _______________ to __________________.

3. Liquids and gases are generally ______________ conductors of heat.

4. The fixed temperature at which matter changes state from solid to liquid is called ______________.

Consider the statements given below and choose the correct option:

(a) If both assertion and reason are true and reason is the correct explanation of assertion

(b) If both assertion and reason are true but reason is not the correct explanation of assertion

(c) If assertion is true but reason is false

(d) 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.

2. Assertion: Maximum sunlight reaches earth’s surface during the noon time.

Reason: Heat from the sun reaches earth’s surface by radiation.

3. Assertion: When water is heated up to 100⁰ C, there is no raise in temperature until all water gets converted into water vapour.

Reason: Boiling point of water is 10⁰ C.

Answers:

Choose the best answers:

1. Heat 2. Kelvin 3. Rod – 1 4. Conduction 5. thermos flask

Fill in the blanks:

1. Radiation 2. Land, sea 3. Bad 4. Melting point

Consider the statements given below and choose the correct option:

1.If both assertion and reason are true and reason is the correct explanation of assertion

2.If both assertion and reason are true but reason is not the correct explanation of assertion.

3.If assertion is true but reason is false.