Class 7 Science Heat Chapter Notes

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These Class 7 Science chapter notes on Heat cover temperature and its measurement on the Celsius, Fahrenheit and Kelvin scales, the clinical thermometer and how to read it, the three modes of heat transfer (conduction, convection and radiation), land and sea breeze, conductors and insulators, thermodynamic processes, the effects of heat, with the chapter exercises and their answers.

1.1 Introduction

We wear woollen clothes during winter when it is cold outside. Woollen clothes keep us warm. We prefer to wear light-coloured cotton clothes when it is hot, as these give us a feeling of coolness. Objects around us can be classified into hot or cold objects depending upon their temperatures. For example, tea is hot whereas ice is cold.

Measuring Temperature

The degree of hotness of an object is measured by its temperature. A thermometer is a device which measures the temperature of an object. The thermometer that measures our body temperature is called a clinical thermometer.

[Figure 1.1: Clinical thermometer. It has a long, narrow, uniform glass tube with a bulb at one end containing mercury, and a small shining thread of mercury visible outside the bulb.]

A clinical thermometer consists of a long, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen.

Scales of thermometers are broadly of three types:
(i) Celsius scale (°C): The lower fixed point is 0°C and the upper fixed point is 100°C.
(ii) Fahrenheit scale (°F): The lower fixed point is 32°F and the upper fixed point is 212°F.
(iii) Kelvin scale (K): The lower fixed point is 273.15 K and the upper fixed point is 373.15 K.

1.2 Relation among the scales

(i) \( \frac{C}{5} = \frac{F - 32}{9} \)
(ii) K = C + 273.15

Notes:
(i) The lower and upper limits are two fixed points of any temperature measuring scale.
(ii) A clinical thermometer reads temperature from 35°C to 42°C.

1.3 Reading a thermometer

Wash the thermometer, preferably with an antiseptic solution. Hold it firmly and give it a few jerks. The jerks bring the level of mercury down. Ensure that it falls below 35°C. Now place the bulb of the thermometer under your tongue. After one minute, take the thermometer out and note the reading. This is your body temperature. The temperature should always be stated with its unit, °C.

[Figure 1.2: Correct method of reading a clinical thermometer, with the bulb placed under the tongue.]

The normal temperature of the human body is 37°C or 98.4°F. The temperature of every person may not be exactly 37°C; it could be slightly higher or slightly lower. What we call normal temperature is the average body temperature of a large number of healthy persons.

Precautions while reading a clinical thermometer
● The thermometer should be washed before and after use, preferably with an antiseptic solution.
● Ensure that before use the mercury level is below 35°C.
● Read the thermometer keeping the level of mercury along the line of sight.
● Handle the thermometer with care. If it hits against some hard object, it can break.
● Do not hold the thermometer by the bulb while reading it.

Caution: Do not use a clinical thermometer for measuring the temperature of any object other than the human body. Also avoid keeping the thermometer in the sun or near a flame, as it may break.

1.4 Transfer of heat

There are three modes of transfer of heat. They are conduction, convection and radiation.

Conduction: The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction. In solids, heat is generally transferred by the process of conduction.

[Figure 1.3: Flow of heat through a metal strip. One end of a metal strip is heated by a candle flame, and wax pins fixed along the strip at equal distances fall off one by one as the heat reaches them.]

Depending on the type of heat conduction, materials are of two types:
(i) Good conductors: Materials which allow the flow of heat through them, for example metals such as iron and copper.
(ii) Bad conductors: Materials which do not allow the flow of heat through them, for example wood and plastic.

Convection: Convection is the mode of heat transfer by actual motion of matter. When water is heated, the water near the flame gets hot and rises up. The cold water from the sides moves down towards the source of heat. This water also gets hot, rises, and water from the sides moves down. This process continues till the whole water gets heated.

[Figure 1.4: Convection of heat in water. A round-bottomed flask of water is placed on a stand over a burner.]

Radiation: Radiation is the mode of heat transfer without any medium. The heat from the sun comes to us by the process of radiation. All hot bodies radiate heat. When this heat falls on an object, a part of it is reflected, a part is absorbed and a part may be transmitted. The temperature of the object increases due to the absorbed part of the heat.

1.5 Land and sea breeze

During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze.

The water cools down more slowly than the land. So, at night, the cool air from the land moves towards the sea. This is called the land breeze.

[Figure 1.5: Land breeze and sea breeze. The day time panel shows arrows moving from the sea towards the land, which is the sea breeze. The night time panel shows arrows moving from the land towards the sea, which is the land breeze. Circular arrows marked Hot and Cool show the cycle of air movement.]

1.6 Conductors and insulators

Conductors: Materials which allow heat to pass through them easily are called conductors of heat, for example aluminium, iron and copper.

Insulators: Materials which do not allow heat to pass through them are called insulators of heat, for example plastic and wood.

1.7 Thermodynamic processes

(i) Isothermal process: A process in which the temperature of the system remains constant.
(ii) Adiabatic process: A process in which no heat enters or leaves the system, that is, the heat of the system remains constant.
(iii) Isobaric process: A process in which the pressure of the system remains constant.
(iv) Isochoric process: A process in which the volume remains constant.

1.8 Effects of heat

(i) Heat causes a change in the temperature of a material.
(ii) Heat can lead to the expansion of materials. This expansion can be in the length, area or volume of the material.
● Linear expansion: Expansion in the length of the material by heating.
● Superficial expansion: Expansion in the area of the material by heating.
● Cubical expansion: Expansion in the volume of the material by heating.
(iii) Heat can change the state of a material:
Liquid → gas (vaporisation), on heating
Solid → liquid (fusion or melting), on heating
Solid → gas (sublimation), on heating

Note: The process of converting a solid directly into a gas by heating, without going through the intermediate liquid state, is called sublimation.

● The process of conversion of a gas directly into a solid without going through the intermediate liquid state is called deposition (also called frosting).
● Heat always flows from a higher temperature to a lower temperature, under normal circumstances.
● Inside a refrigerator, heat is made to flow from a low temperature to a higher temperature by doing external work.

EXERCISES

Objective Questions

1. The celsius scale reading is 40°C. The Fahrenheit scale reading will be
(a) 100°F
(b) 90°F
(c) 104°F
(d) 50°F

Answer: (c) 104°F

1. Use the relation \( \frac{C}{5} = \frac{F - 32}{9} \).
2. Substitute C = 40: \( \frac{40}{5} = \frac{F - 32}{9} \), so F - 32 = 72.
3. Hence F = 104°F, which is option (c). (The option is printed as "(b) 104°F" in the source; its correct label is (c).)

Teacher's Note:
a) Multiply the Celsius reading by 9/5 and add 32.
b) 40°C is a common trap; 90°F and 100°F are not conversions of 40°C.

2. No medium is required for transfer of heat by the process of
(a) Conduction
(b) Convection
(c) Radiation
(d) Relegation

Answer: (c) Radiation

1. Conduction needs a material medium, such as a metal rod.
2. Convection needs a fluid in which the hot part moves.
3. Radiation travels through empty space, so it needs no medium.

Teacher's Note:
a) Heat from the sun reaches the earth through vacuum by radiation.
b) Conduction and convection both need matter, which is why a vacuum flask keeps tea hot for longer.

3. Which of the following is good conductor of heat?
(a) Wood
(b) Iron
(c) Plastic
(d) Paper

Answer: (b) Iron

1. Wood, plastic and paper do not allow heat to flow through them easily.
2. Iron is a metal, and metals are good conductors of heat.
3. Hence, the good conductor is iron.

Teacher's Note:
a) Metals such as iron, copper and aluminium are good conductors.
b) Wood, plastic and paper are insulators.

4. When a body is heated. Its
(a) length increases
(b) breadth increases
(c) volume increases
(d) All of the above

Answer: (d) All of the above

1. On heating, a body expands in length, area and volume.
2. Hence, its length, breadth and volume all increase.

Teacher's Note:
a) Linear, superficial and cubical expansion all occur when a body is heated.
b) Solids, liquids and gases all expand on heating, though by different amounts.

5. The range of clinical thermometer is
(a) from 30°C to 40°C
(b) From 35°C to 42°C
(c) from 20°C to 25°C
(d) from 25°C to 30°C

Answer: (b) From 35°C to 42°C

1. A clinical thermometer is used to measure human body temperature.
2. Its scale runs from 35°C to 42°C.

Teacher's Note:
a) Normal body temperature, 37°C, lies within this range.
b) Shake the mercury below 35°C before use.

6. The range of Laboratory thermometer is
(a) from −10°C to 110°C
(b) from 10°C to 100°C
(c) from −10°C to 100°C
(d) from 0°C to 100°C

Answer: (a) from −10°C to 110°C

1. A laboratory thermometer measures a wider range than the clinical thermometer.
2. Its usual range is from −10°C to 110°C.

Teacher's Note:
a) The lower value is below zero, which allows measuring freezing temperatures.
b) A clinical thermometer has a much narrower range, so it cannot be used in a laboratory.

7. One litre of water at 25°C is mixed with one litre of water at 40°C. The temperature of the mixture will be
(a) 25°C
(b) 40°C
(c) more than 40°C but less than 60°C
(d) between 25°C and 40°C

Answer: (d) between 25°C and 40°C

1. The cold water at 25°C takes in heat from the warm water at 40°C.
2. The warm water loses heat, so the final temperature lies between the two starting temperatures.

Teacher's Note:
a) The mixture cannot be warmer than the hotter water, so option (c) is wrong.
b) Equal volumes give a mixture at about the average, 32.5°C, if no heat is lost to the surroundings.

8. If the refrigerator is kept open in a room, the temperature of the room will
(a) decrease
(b) increase
(c) remains unchanged
(d) either (a) or (c)

Answer: (b) increase

1. A refrigerator removes heat from its inside and releases it to the room, along with the heat generated by its work.
2. With the door open, the room receives this heat, so the room temperature increases.

Teacher's Note:
a) An open refrigerator cools only a small space inside itself and warms the whole room.
b) Keeping the door shut is the simplest way to save electricity.

9. A metal sheet with a circular hole is heated. The hole
(a) gets larger
(b) gets smaller
(c) remains of the same size
(d) gets deformed

Answer: (a) gets larger

1. On heating, the metal sheet expands in all directions.
2. The hole expands with the metal, so it becomes larger.

Teacher's Note:
a) Holes in metal sheets expand like the surrounding metal, not shrink.
b) This is why a metal lid can loosen when the jar is warmed.

10. An iron ball at 50°C is dropped in a mug containing water at 40°C. The heat will
(a) flow from iron ball to water
(b) flow from water to iron ball
(c) not flow from iron ball to water or from water to iron ball
(d) increases the temperature of both

Answer: (a) flow from iron ball to water

1. Heat always flows from a higher temperature to a lower temperature.
2. The iron ball is at 50°C and the water is at 40°C, so heat flows from the ball to the water until both reach the same temperature.

Teacher's Note:
a) Heat flow stops only when the two temperatures become equal.
b) The water gains heat, so its temperature rises.

Subjective Questions

1. Define temperature. Name a device used to measure temperature.

Answer:
1. Temperature is the degree of hotness or coldness of an object.
2. A thermometer is the device used to measure temperature.

Teacher's Note:
a) Temperature tells how hot an object is, not how much heat it contains.
b) The unit is written with the reading, for example 37°C.

2. Explain the construction of a clinical thermometer with a neat diagram.

Answer:
1. A clinical thermometer is made of a long, narrow, uniform glass tube.
2. The tube has a bulb at one end, which contains mercury.
3. A small, shining thread of mercury is seen in the tube above the bulb.
4. The tube is marked with a scale from 35°C to 42°C.
[Figure: diagram to be added. Clinical thermometer with a long glass tube, a bulb of mercury at one end, a thin mercury thread in the tube and the scale from 35°C to 42°C marked along it.]

Teacher's Note:
a) The narrow bore makes a small change in temperature show as a visible movement of the thread.
b) Mercury is used because it expands evenly and clearly.

3. Convert the following temperature measurement in Fahrenheit scale?
(a) 30°C (b) −40° (c) 40°C (d) 100°C

Answer:
1. Use F = \( \frac{9}{5} \) C + 32.
2. (a) 30°C: F = \( \frac{9}{5} \) × 30 + 32 = 54 + 32 = 86°F.
3. (b) −40° (taken as −40°C): F = \( \frac{9}{5} \) × (−40) + 32 = −72 + 32 = −40°F.
4. (c) 40°C: F = \( \frac{9}{5} \) × 40 + 32 = 72 + 32 = 104°F.
5. (d) 100°C: F = \( \frac{9}{5} \) × 100 + 32 = 180 + 32 = 212°F.

Teacher's Note:
a) Multiply by 9/5 first, then add 32.
b) The value −40 is the one temperature that reads the same on both scales.
c) 100°C converts to the boiling point of water, 212°F.

4. Convert the following temperature measurement in celsius scale?
(a) 98°C (b) 36°F (c) 42°F

Answer:
1. Use C = \( \frac{5}{9} \) (F − 32).
2. (a) 98°C is already a Celsius reading, so it stays 98°C.
3. (b) 36°F: C = \( \frac{5}{9} \) × (36 − 32) = \( \frac{5}{9} \) × 4 = 2.2°C.
4. (c) 42°F: C = \( \frac{5}{9} \) × (42 − 32) = \( \frac{5}{9} \) × 10 = 5.6°C.

Teacher's Note:
a) Part (a) is printed as 98°C, which is already Celsius. If 98°F was meant, it converts to 36.7°C, close to normal body temperature.
b) Subtract 32 before multiplying by 5/9.
c) A value of 36°F is far below normal body temperature, so check the unit before converting.

5. Explain land and sea breeze with a neat diagram.

Answer:
1. During the day, the land heats up faster than the sea.
2. The air over the land becomes hot, rises, and cooler air from the sea moves in towards the land. This moving air is the sea breeze.
3. At night, the water cools more slowly than the land, so the air over the sea is warmer and rises.
4. The cooler air from the land moves towards the sea. This moving air is the land breeze.
[Figure: diagram to be added. Day time: arrows from the sea towards the land, marked sea breeze. Night time: arrows from the land towards the sea, marked land breeze.]

Teacher's Note:
a) Land and sea breezes both happen because land and water heat and cool at different rates.
b) The breeze is always named after the direction it comes from.

6. Define conduction. Explain an activity where heat transfer takes place due to conduction.

Answer:
1. Conduction is the process by which heat is transferred from the hotter end to the colder end of an object, without the object moving.
2. Activity: Fix a number of wax balls or pins at equal distances along a metal rod with a little wax. Heat one end of the rod with a candle flame.
3. The heat travels along the rod, the wax melts, and the balls or pins fall off one by one, starting from the end nearest the flame.

Teacher's Note:
a) Heat moves through solids by conduction, with particles passing energy to their neighbours.
b) Metals conduct heat well, while wood and plastic do not.

7. Define convection. Explain an activity where heat transfer takes place due to convection.

Answer:
1. Convection is the mode of heat transfer by the actual motion of matter.
2. Activity: Fill a beaker with water, drop a few crystals of potassium permanganate at the bottom, and heat the beaker gently from below.
3. The heated water near the flame becomes lighter and rises, and the cooler water from the top moves down. The coloured stream shows this circulation, so heat spreads through the whole water.

Teacher's Note:
a) Convection happens only in fluids, that is, liquids and gases.
b) The movement of the water is what carries the heat upwards.

8. Define radiation. Explain an activity where heat transfer takes place due to radiation.

Answer:
1. Radiation is the mode of heat transfer that needs no medium.
2. Activity: Hold your palm near a hot electric bulb or a fire, but not touching it, and feel the warmth on your hand.
3. The heat reaches the hand through the air by radiation. Heat from the sun also reaches the earth through empty space by radiation.

Teacher's Note:
a) A dark, rough surface absorbs more radiant heat than a shiny, light surface.
b) Radiation is the only mode that works through vacuum.

9. Discuss why wearing layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.

Answer:
1. Each layer of clothing traps a thin layer of air between the layers.
2. Air is a poor conductor of heat, so the trapped air slows down the loss of body heat.
3. A single thick piece of cloth traps less air in total, so more heat escapes than with several layers.

Teacher's Note:
a) Still air is one of the best insulators available.
b) Woollen clothes are warm because they hold a lot of trapped air.

10. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.

Answer:
1. White surfaces reflect most of the heat radiation from the sun.
2. Less heat is absorbed by the walls, so less heat passes into the rooms.
3. Hence the house stays cooler in hot climates.

Teacher's Note:
a) Dark surfaces absorb more heat and become hotter.
b) Light-coloured cotton clothes work on the same principle.

11. If the refrigerator is kept open in a room, what will happen ? Explain.

Answer:
1. The refrigerator takes heat out of its inside and releases it into the room, and it also releases the heat produced by its own working.
2. Since the door is open, the cooling only affects a small space inside the refrigerator, and the heat released into the room is not removed.
3. Hence the temperature of the room increases.

Teacher's Note:
a) Keeping the door closed helps the refrigerator cool efficiently.
b) Heat flows out of the refrigerator only when external work is done by the compressor.

1.2 Measuring Temperature

Relation among different scales :

In solids, generally, the heat is transfererred by the process of conduction.

Depending on type heat conduction, the materials are of 2 types :