Heat and Energy Class 9 ICSE — Physics Chapter 6

Heat and Energy is Chapter 6 of the ICSE Class 9 Physics book. The chapter defines heat as a form of energy and explains how temperature is measured. It covers thermal expansion, including the anomalous expansion of water, then moves to energy flow in ecosystems and sources of energy. The heat and energy class 9 ICSE chapter ends with the greenhouse effect and global warming.

ICSE Class 9 Physics Chapter 6 Heat and Energy PDF

What is inside Chapter 6

22 pages · 19 sections · 1 worked examples · 58 exercise questions

• EXERCISE 6(A): 18 questions
• Multiple choice type :: 4 questions
• EXERCISE 6(B): 12 questions
• Multiple choice type :: 4 questions
• Multiple choice type: 2 questions
• EXERCISE 6(D): 15 questions
• Multiple choice type :: 3 questions

6.1 CONCEPT OF HEAT (HEAT AS ENERGY)

Heat appears in common actions around us. Rubbing palms together produces heat from mechanical work. Passing current through a wire produces heat from electrical energy. Burning coal produces heat from chemical energy. In every case, heat is produced by doing work or by supplying some other form of energy. Heat is therefore a form of energy.

Every body is made of molecules in random motion. These molecules carry internal energy. The heat energy of a body is the internal energy of its molecules. A hot body holds more internal energy than an identical cold body. When a hot body touches a cold one, energy flows from hot to cold. This flowing energy is called heat energy.

The S.I. unit of heat is the joule (J). The C.G.S. unit is the erg.

1\ \mathrm{J} = 10^{7}\ \mathrm{erg}

Other units are the calorie (cal) and kilocalorie (kcal).

1\ \text{kilocalorie} = 1000\ \text{calories}

1\ \text{cal} = 4.186\ \text{J}\ \text{(nearly } 4.2\ \text{J)}

Worked pattern: convert 2500 cal to joules and kilocalories. Multiply 2500 by 4.186 to get 10,465 J. Divide 2500 by 1000 to get 2.5 kcal.

6.2 CONCEPT OF TEMPERATURE

Temperature determines the direction of heat flow. When a hot body touches a cold body, heat flows from the higher temperature to the lower temperature. If no heat flows between two touching bodies, they are at the same temperature. But equal temperature does not mean equal heat content. The amount of heat in a body depends on its mass, temperature and material.

The S.I. unit of temperature is the kelvin (K). Celsius (°C) and Fahrenheit (°F) are common units. The Kelvin and Celsius scales relate as:

T\ \text{K} = 273 + t\ ^\circ\text{C}

So adding 273 to a Celsius reading gives Kelvin. The Celsius and Fahrenheit scales relate as:

\frac{C}{5} = \frac{F - 32}{9}

The ice point is 0°C, 32°F and 273 K. The steam point is 100°C, 212°F and 373 K. Absolute zero is 0 K = -273°C, where molecular motion ceases. One degree on the Celsius scale equals 9/5 degree on the Fahrenheit scale.

Worked pattern: convert 38°C to Kelvin and Fahrenheit. Kelvin: 273 + 38 = 311 K. Fahrenheit: 38/5 = (F − 32)/9, so F = 100.4°F.

Heat and temperature differ in several ways, as this chapter’s table shows:

These temperature conversions appear throughout the ICSE Class 9 Physics book, so master them early.

6.3 THERMAL EXPANSION

Almost all substances expand on heating and contract on cooling. This behaviour is called thermal expansion. A solid has a definite shape, so it expands in all directions. The increase in length is linear expansion. The increase in area is superficial expansion. The increase in volume is cubical expansion.

Liquids and gases have no definite shape. They show only cubical (volume) expansion. Liquids expand more than solids. Gases expand much more than liquids. The reason is that gas molecules are far apart with weak forces, so heat increases their motion more easily.

Some substances contract on heating over a certain range. Water contracts from 0°C to 4°C. Silver iodide contracts from 80°C to 141°C. Silica contracts below -80°C. This behaviour is called anomalous expansion.

Worked pattern: heat equal volumes of iron, water and air from 20°C to 40°C. The air expands most, water less, iron least. This follows the order stated in the text: gases more than liquids, liquids more than solids.

6.4 ANOMALOUS EXPANSION OF WATER

Water at 0°C contracts when heated up to 4°C. Above 4°C it expands again. If water above 4°C is cooled, it contracts until it reaches 4°C. Cooling it further from 4°C to 0°C makes it expand. This is the anomalous expansion of water.

For 1 g of water, the volume at 4°C is 1.0000 cm³. This is the minimum volume. The density at 4°C is 1 g cm⁻³, or 1000 kg m⁻³. This is the maximum density.

The temperature-volume graph falls from 0°C to 4°C, then rises from 4°C to 10°C. The temperature-density graph rises from 0°C to 4°C, then falls from 4°C to 10°C. The two graphs are mirror images of each other.

Worked pattern: 1 g of water at 4°C occupies 1.0000 cm³. Heat it to 10°C and its volume grows past that value. Cool it to 0°C and its volume also grows past that value. Only at 4°C is the volume at its smallest.

6.5 HOPE’S EXPERIMENT TO DEMONSTRATE THE ANOMALOUS EXPANSION OF WATER

T.C. Hope devised this experiment in 1805. The apparatus is a tall metallic cylinder. Thermometer T₁ sits near the top, thermometer T₂ near the bottom. A trough around the middle holds a freezing mixture of ice and salt. The cylinder is filled with pure water at room temperature.

Initially both thermometers show the same reading. As the freezing mixture cools the central water, T₂ falls first. The cooled water contracts, becomes denser, and sinks. Warm water rises to take its place. T₂ falls until it steadies at 4°C. T₁ stays nearly unchanged.

Below 4°C, the central water expands and becomes less dense. It rises. Water from the top descends. Now T₁ falls rapidly to 0°C, and ice forms at the top. Finally T₁ reads 0°C and T₂ reads 4°C.

Common mistakes to avoid:

• Heat and temperature are not the same. Heat is energy; temperature decides the direction of flow.
• Water does not always expand on heating. It contracts from 0°C to 4°C.
• In Hope’s experiment, the lower thermometer falls first, not the upper one.

6.6 CONSEQUENCES OF ANOMALOUS EXPANSION OF WATER

Aquatic life survives winter because of anomalous expansion. In a pond, surface water cools to 4°C and sinks. The whole pond reaches 4°C. Further cooling makes the top water expand and stay at the surface. When the atmosphere falls below 0°C, the surface freezes into ice. The water below the ice stays near 4°C. Ice is a poor conductor, so it insulates the water beneath. Fish and other creatures survive.

Water pipes burst because of the same effect. When the temperature falls below 4°C, water in pipes expands and exerts large pressure. The pipes crack. Plant capillaries burst the same way. That is why fields are flooded before cold nights.

Exercise 6(A) questions 16 to 18 test these consequences. Question 16 asks for the temperatures just under the ice and at the bottom of a pond. Question 17 asks for a labelled diagram of the layers.

6.7 ENERGY FLOW IN AN ECOSYSTEM

An ecosystem has biotic components (producers, consumers, decomposers) and abiotic components (light, heat, rain, humidity, inorganic and organic substances). The sun is the source of all ecosystem energy.

Solar energy is used in different ways. Nearly 56-60% is absorbed by the atmosphere. About 10% heats water and land. Only 8% falls on plants. Plants use just 0.02% of absorbed energy in photosynthesis. Photosynthetic plants and bacteria are the producers.

The flow numbers follow a fixed path. Gross primary production is 20,810 cal. Respiration uses 11,977 cal. Net primary production is 8,833 cal. Primary consumers get 3,368 cal. Secondary consumers get 383 cal. Tertiary consumers get 21 cal.

The flow is linear, moving in one fixed direction. It is not cyclic. At the end, energy reaches a degraded or unuseful state.

6.8 APPLICATION OF LAWS OF THERMODYNAMICS IN ENERGY FLOW

The first law of thermodynamics states that energy transforms from one form to another. It is neither created nor destroyed. The total energy stays constant through the ecosystem.

The second law states that part of the energy always converts to an unuseful form. This is mainly heat from friction and radiation. Energy used in respiration and decay never reaches the next consumer. So energy transfer in an ecosystem is never 100% efficient.

Worked pattern: net primary production is 8,833 cal. Primary consumers receive only 3,368 cal. The difference, 5,465 cal, is lost to decay. This loss follows the second law of thermodynamics.

This connects forward to section 6.17 on energy degradation. Each trophic level loses energy to unuseful forms.

6.9 SOURCES OF ENERGY

We need energy for cooking, lighting, running gadgets, moving vehicles, and running factories. Most requirements are met as heat and electricity. Heat comes from burning coal, wood, kerosene or gas. Electricity comes from coal, hydro-energy or nuclear energy.

A good energy source must provide adequate useful energy at a steady rate over a long period. It should be safe and convenient, economical, and easy to store and transport.

Sources are classified into two groups. Renewable or non-conventional sources can be used again and again. Non-renewable or conventional sources get exhausted with use.

6.10 RENEWABLE OR THE NON-CONVENTIONAL SOURCES OF ENERGY

Renewable sources provide energy continuously. The main renewable sources are: sun, wind, flowing water (hydro), biomass and biofuels, tides, oceans, geothermal spots, and nuclear fuel.

The sun produces energy by nuclear fusion inside its interior. The average solar energy reaching the upper atmosphere per second on 1 m² is the solar constant, about 1.34 kW m⁻². Wind is the kinetic energy of moving air. Flowing water holds hydro energy. Biomass yields biogas, which is about 65% methane.

Tides rise and fall twice a day. Oceans provide thermal energy from temperature differences up to 20°C, and wave energy. Geothermal hot spots heat underground water into steam. Nuclear fission releases about 200 MeV per uranium nucleus.

Einstein’s relation connects mass loss to energy:

E = mc^2

where c = 3 × 10⁸ m s⁻¹. If 1 g of mass converts to energy, E = 0.001 × (3 × 10⁸)² = 9 × 10¹³ J.

Fusion needs about 10⁷ K. It has not yet been harnessed. Wood is renewable, but trees take over 15 years to regrow. Cutting trees causes global warming, so wood use must be avoided.

6.11 NON-RENEWABLE OR THE CONVENTIONAL SOURCES OF ENERGY

Fossil fuels formed over millions of years from buried plant and animal remains. Coal, petroleum and natural gas are the three non-renewable sources. Once exhausted, they cannot be regenerated quickly.

Coal is found in Jharkhand, West Bengal, Orissa and Chhattisgarh. Petroleum, also called crude oil, is found in Assam and Mumbai. It is refined by fractional distillation. Natural gas is up to 95% methane, found in Tripura, Jaisalmer, offshore Mumbai and the Krishna-Godavari delta.

LPG is petroleum gas liquefied under pressure. Ethyl mercaptan (C₂H₅SH) is added so leaks can be detected by smell.

Renewable and non-renewable sources differ in availability and regeneration:

6.12 JUDICIOUS USE OF ENERGY

Fossil fuels are limited. Constant use creates an energy crisis. The chapter lists seven measures for judicious use:

1. Use fossil fuels only where no alternative exists.
2. Avoid wastage of energy.
3. Ban tree cutting; plant more trees.
4. Use energy in community groups.
5. Use renewable sources in rural areas for lights and tube wells.
6. Develop techniques to use renewables as much as possible.
7. Work toward controlled nuclear fusion of deuterium from sea water.

These measures slow the depletion of fossil fuels. They also push energy use toward sources that will not run out.

6.13 PRODUCTION OF ELECTRICITY FROM SOLAR ENERGY

A solar cell converts solar energy directly into electricity. It is made from semiconductors like silicon and gallium. A single cell of area 4 cm² produces about 0.4 to 0.5 V and 60 mA. Modern solar cells reach about 30% efficiency. A solar panel is many cells arranged over a large area.

A battery is charged during the day for night use. Solar panels power satellites, water pumps, lighting and television in remote areas. A 50 kW solar power plant operates at Gurgaon in Haryana.

Advantages: no maintenance, long life, nil running cost, suitable for remote places, no pollution.

Disadvantages: high initial cost, low efficiency, d.c. output that many household devices cannot use directly.

This chapter sits within the wider ICSE Class 9 books collection for physics and other subjects.

6.14 PRODUCTION OF ELECTRICITY FROM WIND ENERGY

A wind generator has a dynamo at the top of a pillar. The armature connects to the blades of a windmill. Wind strikes the blades, and kinetic energy becomes rotational kinetic energy. The rotating armature moves in the magnetic field of strong pole pieces. An alternating e.m.f. is produced between the terminals.

A single generator gives small power. Many generators arranged over a big area form a wind farm. India generates more than 1025 MW in coastal Gujarat and Tamil Nadu. The planned target is 20,000 MW.

Wind energy causes no pollution and is renewable. But wind speed must be at least 15 km h⁻¹. Wind farms need a large, expensive area of land.

6.15 PRODUCTION OF ELECTRICITY FROM WATER (or hydro) ENERGY

Water stored in a dam has potential energy. When water falls, potential energy becomes kinetic energy. The falling water turns the turbine. The turbine rotates the armature in a magnetic field, producing alternating e.m.f.

Hydro electricity is about 23% of India’s total electricity. A mini plant can run on a dam about 10 m high in hilly areas.

Advantages: no pollution, renewable, helps irrigation and flood control.

Limitations: flowing water is not available everywhere, dams destroy plants and animals, downstream ecology is disturbed.

6.16 PRODUCTION OF ELECTRICITY FROM NUCLEAR ENERGY

A nuclear power plant uses controlled fission of uranium-235 or plutonium-239. Cadmium rods absorb neutrons to control the chain reaction. The coolant carries heat to a heat exchanger. Water in the exchanger boils into steam. The steam drives a turbine and generator.

India has four nuclear power plants: Tarapur in Maharashtra, Rana Pratap Sagar near Kota in Rajasthan, Kalpakkam in Tamil Nadu, and Narora in Uttar Pradesh. Nuclear power is about 3% of India’s electricity.

A small amount of fuel gives huge energy over a long period. But harmful ionising radiations require high protection standards. Nuclear waste causes environmental pollution.

The four electricity production methods differ in their energy chains and India data:

6.17 ENERGY DEGRADATION

In every energy transformation, part of the energy changes into an undesirable form. This is dissipation of energy. The non-useful remainder is the degraded form of energy. As energy is used, the degraded amount grows while the usable amount falls. Total energy stays constant.

Examples from the chapter: a bulb converts less than 25% of electrical energy into light. A vehicle wastes most fuel energy on friction and sound. Cooking radiates most heat away. Transmission lines waste energy as heat. All machines have efficiency less than 1.

Worked pattern: a bulb draws 100 J of electrical energy. Less than 25 J becomes light. The rest becomes heat in the filament and invisible radiation. Only the light is useful; the heat is the degraded form.

6.18 GREEN HOUSE EFFECT

Joseph Fourier discovered the greenhouse effect in 1824. It is the warming of the planet’s surface by absorption of long wavelength infrared radiation. The atmosphere is transparent to visible light and short wavelength infrared. It is opaque to long wavelength infrared.

The greenhouse gases are carbon dioxide, water vapour, methane and chlorofluorocarbons. Memory aid: Can We Manage Climate (CO₂, Water vapour, Methane, CFCs).

Natural greenhouse gases warm the earth by about 33°C. The average surface temperature is 15.5°C. Without them it would be -18°C.

Carbon dioxide has risen 25% from industrial growth and deforestation. Methane has doubled from agriculture and gas leakage. CFCs rise at 5% per year. Impacts include species loss of 30% by 2050 and 70% by 2100, sea level rise of 3.1 mm per year, ocean warming, glacier melting and farming shifts.

6.20 WAYS TO MINIMISE THE IMPACT OF GLOBAL WARMING

Three groups of measures are needed: technological, economic and policy. They must be taken together.

Technological: fossil fuel power plants emit 21.3% of greenhouse gases. Replace them with renewables. Transportation vehicles contribute 14% of emissions. Switch to battery-powered vehicles. Use bio-char stoves for cooking.

Economic: reforestation and sustainable land use. Industries pay carbon tax based on emissions, employee hours and turnover.

Policy: educate children toward sustainable lifestyles. Control population through family planning, welfare reforms and women’s empowerment.

The 2015 Paris COP21 conference committed to limit warming below 2.0°C. Zero carbon emission is targeted between 2030 and 2050. Students can verify the syllabus on the official CISCE website.

What to practise from this chapter

Match each exercise block to the sections it tests:

• Exercise 6(A): 18 questions on heat, temperature and anomalous expansion. Questions 12, 13 and 14 are graph-drawing questions. Question 17 asks for the pond layer diagram.
• 4 MCQs after Exercise 6(A) on heat units, temperature and density of water.
• Exercise 6(B): 12 questions on energy flow and thermodynamics.
• 4 MCQs after Exercise 6(B) on food chains and ecosystem energy.
• 2 MCQs after section 6.17 on energy sources and degradation.
• Exercise 6(D): 15 questions on the greenhouse effect and global warming.
• 3 MCQs after Exercise 6(D) on greenhouse gases and temperature effects.

Frequently asked questions

Why is the density of water maximum at 4°C and not at 0°C?

Between 0°C and 4°C, water contracts when heated. So its volume is smallest at 4°C. Density is mass divided by volume, so density is highest there. At 4°C, 1 g of water occupies exactly 1.0000 cm³, giving a density of 1 g cm⁻³.

In Hope’s experiment, why does the lower thermometer reach 4°C first while the upper thermometer falls later to 0°C?

The freezing mixture cools the central water first. Cooled water contracts, becomes denser, and sinks to the bottom. So the lower thermometer reaches 4°C first. Below 4°C the water expands and rises, so the upper water cools only in the second phase and reaches 0°C later.

Why do water pipes burst in winter in colder countries?

Below 4°C, water in pipes expands instead of contracting. This expansion exerts large pressure on the pipe walls. The pipes burst under this pressure.

What is the difference between heat and temperature?

Heat is a form of energy carried by the random motion of molecules. Temperature tells the degree of hotness and decides the direction of heat flow. Equal temperature does not mean equal heat content, because heat also depends on mass and material.

Why does ice form at the top of a pond and not at the bottom?

When surface water cools to 4°C, it sinks because it is denser. Below 4°C, water expands and becomes less dense, so it stays at the surface. The surface then freezes into ice, while the water below stays near 4°C.

Reference: ICSE Class 9 Physics textbook material, chapter 6: Heat and Energy — CISCE-aligned.

More from this book

• Chapter 1: Measurements and Experimentation
• Chapter 2: Motion in One Dimension
• Chapter 3: Laws of Motion
• Chapter 4: Pressure in Fluids and Atmospheric Pressure
• Chapter 5: Upthrust in Fluids Archimedes Principle and Floatation
• Chapter 7: Reflection of Light
• Chapter 8: Propagation of Sound Waves
• Chapter 9: Current Electricity
• Chapter 10: Magnetism