# Chapter 7- Heat.

20 Nov 2020 4:22 pm

CHAPTER 7, heat, 9th std, science , physics, tamilnadu board,

### I. Choose the correct answer:

Question 1.

Calorie is the unit of

(a) heat

(b) work

(c) temperature

(d) food

Answer:

(a) heat

Question 2.

SI unit of temperature is

(a) fahrenheit

(b) joule

(c) Celsius

(d) kelvin

Answer:

(d) kelvin

Question 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

Answer:

(c) Rod-1

Question 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

Answer:

(b) Conduction

Question 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

Answer:

1.  (a) Heat
2.  (d) kelvin
3.  (c) Rod-1
4.  (b) Conduction
5.  (d) Thermos flask

### II. 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 ………………

Answer:

1. radiation
2. land, sea
3. bad
4. melting point

### III. Assertion and Reason type questions.

Mark the correct choice as:

(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.

Question 1.

Assertion(A): Food can be cooked faster in copper bottom vessels.

Reason (R): Copper is the best conductor of heat.

Answer:

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

Question 2.

Assertion(A): Maximum sunlight reaches earth’s surface during the afternoon time.

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

Answer:

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

Question 3.

Assertion(A): When water is heated up to 100°C, there is no raise in temperature until all water gets converted into water vapour.

Reason(R): Boiling point of water is 10°C.

Answer:

(c) If assertion is true but reason is false.

### IV. Answer briefly.

Question 1.

Define conduction.

Answer:

The process of transfer of heat in solids from a region of higher temperature to a region of lower temperature without the actual movement of molecules is called conduction.

Question 2.

Ice is kept in a double-walled container. Why?

Answer:

Ice is kept in a double-walled container so as to prevent melting of ice from the heat absorbed present in the immediate surroundings. The vacuum present in between the two walls prevents the transfer of heat from the first to the second wall and hence the ice remains in the solid form for a longer time period.

Question 3.

How does the water kept in an earthen pot remain cool?

Answer:

An earthen pot consists of small pores from which the water inside the pot constantly seeps out and gets evaporated due to the presence of high temperature around it. The evaporation process requires heat which is acquired from the surface of the pot, hence making the water and the pot cooler.

Question 4.

Differentiate convection and radiation.

Answer:

Question 5.

Why do people prefer wearing white clothes during summer?

Answer:

People prefer white or light coloured clothes during summer as they are good reflectors of heat and hence, they keep us cool.

Question 6.

What is specific heat capacity?

Answer:

• The specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1 kg of the substance by 1°C or 1 K.
• Q = mC∆T, where Q is the quantity of heat required to raise the temperature and m is the mass of the body and AT is the change in temperature of the body.
• The SI unit of specific heat capacity is Jkg– 1 K– 1. The most commonly used units of specific heat capacity are J/kg°C and J/g°C.

Question 7.

Define thermal capacity.

Answer:

• Heat capacity or thermal capacity is defined as the amount of heat energy required to raise the temperature of a body by 1°C. It is denoted by ‘C’.
• C = Q/t, where C’ is the heat capacity, ‘Q’ is the quantity of heat required and ‘f’ is rise in temperature.
• SI unit of heat capacity is J/K. It is also expressed in cal/°C, kcal/°C or J/°C.

Question 8.

Define specific latent heat capacity.

Answer:

• Specific latent heat is the amount of heat energy absorbed or liberated by unit mass of a substance during change of state without causing any change in temperature.
• Specific latent heat is given as L = Q/m, where ‘Q’ is the amount of heat energy absorbed or liberated and ‘m’ is mass of a substance during its change of phase at a constant temperature.
• The SI unit of specific latent heat is J/kg.

### V. Answer in detail.

Question 1.

Explain convection in daily life.

Convection in daily life:

(i) Hot air balloons: Air molecules at the bottom of the balloon get heated by a heat source and rise. As the warm air rises, cold air is pushed downward and it is also heated. When the hot air is trapped inside the balloon, it rises.

(ii) Breeze: During day time, the air in contact with the land becomes hot and rises. Now the cool air over the surface of the sea replaces it. It is called sea breeze. During night time, air above the sea is warmer. As the warmer air over the surface of the sea rises, cooler air above the land moves towards the sea.

(iii) Winds: Air flows from area of high pressure to area of low pressure. The warm air molecules over hot surface rise and create low pressure. So, cooler air with high pressure flows towards low pressure area. This causes wind flow.

(iv) Chimneys: Tall chimneys are kept in kitchen and industrial furnaces. As the hot gases and smoke are lighter, they rise up in the atmosphere.

Question 2.

What are the changes of state in water? Explain.

Answer:

Any matter around us can be in three forms: solid, liquid and gas, called as states of matter. Depending upon the temperature, pressure and transfer of heat, matter is converted from one state to another and is known as change of state in matter. There are different such processes in the change of state in matter.

For example;

• Water molecules are in liquid state at normal temperature. When water is heated to 100°C, it becomes steam or vapour which is a gaseous state of matter. The process by which a
liquid is converted to vapour by absorbing heat is called boiling or vaporization.
• The temperature at which a liquid changes its state to gas is called boiling point.
• On reducing the temperature of the steam it becomes water again. The process by which a vapour is converted to liquid by releasing heat is called condensation. On reducing the temperature of water further to 0°C, it becomes ice which is a solid state of water.
• The process by which a liquid is converted to solid by releasing heat is called freezing. The
temperature by which a liquid changes its state to solid is called freezing point. Ice on
heating, becomes water again by absorbing heat, a process known as melting.
• Dry ice changes directly to gaseous state without becoming liquid. This process is called
sublimation.

Thus, water changes its state when there is a change in temperature.

Question 3.

How can you experimentally prove that water is a bad conductor of heat? How is it possible to heat water easily while cooking.

Experiment to prove that water is a bad conductor of heat:

Answer:

Take a glass tube and drop an ice cube wrapped in wire gauze in it.

Now fill 3/4th of this tube with water and place it above the burner as shown in the figure.

You can observe that the water boils at the edge and the ice present in the bottom of the tube has not melted indicating that heat has not reached the bottom where the ice cube is present. This proves that water is a bad conductor of heat.

It is easy to heat water easily or quickly while cooking. This is because, while cooking the vessel or pan is usually covered with a lid.

This leads to three things;

1. Radiation from the hot water is reflected back into the pan rather than being emitted
2. Free convection is effectively eliminated, and
3. Evaporative cooling’ is also eliminated.
This in turn allows the water to be heated more easily.

### VI. Problems:

Question 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).

Solution:

Given m = 25 g, ∆T = (100 – 0) = 100°C

Or in terms of Kelvin (373.15 – 273.15) = 100K,

C = 4.18J/g°C

Heat energy required, Q = m × C × ∆T = 25 × 4.18 × 100 = 10450 J

Question 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.

Solution:

To find final temperature: ∆Q = mc

lOOg of water originally at 90°C will loose an amount of heat,

∆Q = mc ∆T

∆Q = 100 × c × (90 – T)

The same amount of heat will be absorbed by 600g of water originally at 20°C to raise its temperature to T.

∆Q = 600 × c × (T – 30)

600C (T – 20°) = 100C (90° – T)

6T – 120° = 90° – T

6T + T = 120° + 90°

7T = 210° ⇒ T = 210/7

T = 30°C

Question 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 = 3,34,000J/kg, Specific heat capacity of water = 4200Jkg– 1 K– 1 ).

Solution:

Total heat = Heat required to convert 2Kg of ice into water at 0°C + Heat required to convert 2Kg of water at 0°C to 2Kg of water at 20°C

Heat = m (hfw) + mc∆T

Here, m(mass of ice) = 2Kg

hfw (specific latent heat of water) = 3,34,000J/Kg

C (specific heat capacity of water) = 4200JKg– 1K– 1

AT (Temperature difference) = 20°C

Therefore, Heat required = (2 × 334000) + (2 × 4200) (20 – 0)

= 668000 + 8400 (20)

= 668000+ 168000

Heat required = 8,36,000 J

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