Heat and Energy Chapter Notes | Physics Class 9 ICSE PDF Download

Heat And Temperature; Anomalous Expansion

Concept of Heat (Heat as Energy)

• Heat is a form of energy resulting from the random motion of molecules in a substance.
• It is produced by various means, such as rubbing palms (mechanical energy), passing current through a wire (electrical energy), or burning coal (chemical energy).
• Each body consists of molecules that possess kinetic energy due to their motion and potential energy due to attractive forces between them.
• The total internal energy of a body is the sum of its internal kinetic and potential energies.
• A hot body has more internal energy than an identical cold body.
• When a hot body is in contact with a cold body, energy transfers from the hot body to the cold body, making the cold body warmer and the hot body cooler.
• Heat flows from a body that feels hot to our hand (e.g., warm water) and from our hand to a body that feels cold (e.g., ice).
• Definition: Heat is the internal energy of a body’s molecules, flowing from a hotter body to a colder one.
• Example: When you touch a hot cup of tea, your hand feels warm because heat flows from the tea to your hand. Conversely, touching an ice cube feels cold because heat flows from your hand to the ice.

Concept of Temperature

• Temperature indicates the thermal state of a body, i.e., how hot or cold it is.
• It determines the direction of heat flow when two bodies at different temperatures are in contact; heat flows from the higher-temperature body to the lower-temperature one.
• Adding heat to a body increases the vibration of its particles, raising its temperature, provided its physical state or dimensions remain unchanged.

Definition: Temperature is a measure of a body’s thermal state, dictating the direction of heat flow between bodies in contact.

• Two bodies at the same temperature have no heat transfer between them, but they may not have the same amount of heat energy.
• The amount of heat in a body depends on its mass, temperature, and material, not just temperature alone.

Unit of Temperature: The SI unit is kelvin (K). Other units include degree Celsius (°C) and degree Fahrenheit (°F).

Temperature Conversion: T K = 273 + t°C (more precisely, T K = 273.15 + t°C).

• Ice point: 0°C = 32°F = 273 K; Steam point: 100°C = 212°F = 373 K.
• One degree on the Celsius scale equals 9/5 degrees on the Fahrenheit scale.
• Celsius and Fahrenheit scales are related by: C/5 = (F - 32)/9.
• Absolute zero (0 K) is -273°C, where molecular motion stops.

Difference Between Heat and Temperature:

• Heat is energy from molecular motion; temperature measures the degree of hotness or coldness.
• SI unit of heat is joule (J); SI unit of temperature is kelvin (K).
• Heat depends on mass, temperature, and material; temperature depends on the average kinetic energy of molecules.
• Heat is measured by calorimetry; temperature is measured by a thermometer.
• Two bodies with the same heat may have different temperatures, and vice versa.
• Total heat of two bodies in contact equals the sum of their individual heats; their resultant temperature lies between their initial temperatures.
• Example: If you heat two pots of water—one small and one large—with the same amount of heat, the smaller pot reaches a higher temperature because its mass is less, showing that temperature depends on mass and material, not just heat input.

Thermal Expansion

• Most substances (solids, liquids, gases) expand when heated and contract when cooled; this is called thermal expansion.
• Solids expand in length (linear expansion), area (superficial expansion), and volume (cubical expansion).
• Liquids and gases, lacking a definite shape, exhibit only cubical (volume) expansion.
• Liquids expand more than solids, and gases expand more than liquids.
• Some substances, like water (0°C to 4°C), silver iodide (80°C to 141°C), and silica (below -80°C), contract on heating and expand on cooling; this is called anomalous expansion.
• Example: A metal rod expands when heated in a fire, increasing its length, area, and volume, demonstrating thermal expansion in solids.

Anomalous Expansion of Water

• Water exhibits unusual behavior: it contracts when heated from 0°C to 4°C and expands when heated above 4°C.
• When cooled from 10°C to 4°C, water contracts; below 4°C to 0°C, it expands.
• Definition: The expansion of water when cooled from 4°C to 0°C is called anomalous expansion of water.
• Volume of 1 g of water is minimum at 4°C (1.0000 cm³).
• Density of water is maximum at 4°C (1 g cm^-3 or 1000 kg m^-3).
• When heated from 0°C to 4°C, water’s density increases; above 4°C, it decreases.
• When cooled from 10°C to 4°C, density increases; below 4°C, it decreases.
• Example: If you cool a liter of water from 10°C to 0°C, its volume decreases until 4°C, then increases slightly as it approaches 0°C, showing anomalous expansion.

Hope’s Experiment to Demonstrate the Anomalous Expansion of Water

• In 1805, T.C. Hope designed an experiment using Hope’s apparatus to demonstrate water’s anomalous expansion.
• Apparatus: A tall metallic cylinder with two openings (P near the top, Q near the bottom), fitted with thermometers T₁ (top) and T₂ (bottom), surrounded by a trough with a freezing mixture (ice and salt).
• Procedure: The cylinder is filled with water at room temperature, and temperatures are recorded at regular intervals.
• Observations:
  • Initially, both thermometers show the same room temperature.
  • T₂ (bottom) decreases first, reaching a steady 4°C, while T₁ (top) remains unchanged.
  • After T₂ stabilizes at 4°C, T₁ falls to 0°C and stabilizes, while T₂ remains at 4°C.
• Explanation:
  • Initially, water is at room temperature (e.g., 12°C).
  • The freezing mixture cools the central water, increasing its density, so it sinks, and warmer water rises, cooling T₂ to 4°C.
  • Below 4°C, water expands, reducing density, so it rises, cooling the upper water, and T₁ drops to 0°C as ice forms at the top.
  • Water at the bottom remains at 4°C (maximum density), while the top is at 0°C (ice).
• Example: In Hope’s experiment, if the room temperature is 12°C, the bottom thermometer T₂ reaches 4°C first due to denser water sinking, and later T₁ reaches 0°C as less dense water rises and freezes.

Consequences of Anomalous Expansion of Water

• Preservation of Aquatic Life:
  • In winter, when air temperature drops below 0°C, water at a pond’s surface cools to 4°C, contracts, and sinks, allowing warmer water to rise.
  • Below 4°C, water expands, stays at the surface, and freezes into ice at 0°C.
  • Ice, a poor conductor of heat, insulates the water below, keeping it at 4°C, allowing aquatic life to survive.
• Bursting of Pipes and Crop Damage:
  • In winter, water in pipes expands below 4°C, exerting pressure and causing pipes to burst.
  • Water in plant capillaries expands similarly, damaging crops.
  • Flooding fields with water can protect plants by maintaining higher temperatures.
• Example: In a frozen pond, fish survive because water at 4°C stays at the bottom, insulated by ice at 0°C on the surface, demonstrating how anomalous expansion protects aquatic life.

Energy Flow and Its Importance

Energy Flow in an Ecosystem

• An ecosystem consists of biotic components (producers, consumers, decomposers) and abiotic components (light, heat, rain, humidity, inorganic/organic substances).
• Energy is essential for all biotic activities, with the sun being the primary energy source for ecosystems.
• Of the sun’s energy reaching Earth:
  • 56–60% is absorbed by the atmosphere.
  • 10% heats water and land.
  • 8% reaches plants, with only 0.02% used in photosynthesis to produce food.
• Producers: Photosynthetic plants and bacteria that produce food using solar energy.
• Food Chain: Energy transfers from producers (plants) to primary consumers (e.g., krill), to secondary consumers (e.g., small fish), to tertiary consumers (e.g., large fish), and possibly to humans.
• Energy Flow:
  • Producers synthesize organic substances via photosynthesis, storing energy as gross primary production (e.g., 20,810 cal).
  • Producers use some energy in respiration (e.g., 11,977 cal), leaving net primary production (e.g., 8,833 cal) for growth and metabolism.
  • Primary consumers obtain a small portion of energy (e.g., 3,368 cal), with the rest (e.g., 5,465 cal) wasted in decay.
  • Primary consumers use some energy in respiration (e.g., 1,890 cal), storing the rest (e.g., 1,478 cal).
  • Secondary consumers obtain less energy (e.g., 383 cal), using some for respiration (e.g., 316 cal) and storing the rest (e.g., 67 cal).
  • Tertiary consumers obtain minimal energy (e.g., 21 cal), with most wasted in decay (e.g., 46 cal) or respiration (e.g., 15 cal).
  • Energy flow is linear, moving in one direction and ending in a degraded, unusable state.
• Example: In a pond ecosystem, algae (producers) use sunlight to produce food, which is eaten by small fish (primary consumers), then larger fish (secondary consumers), and finally humans (tertiary consumers), with energy decreasing at each step.

Application of Laws of Thermodynamics in Energy Flow

• Energy flow in ecosystems follows the laws of thermodynamics.
• First Law (Conservation of Energy): Energy can change forms but cannot be created or destroyed. For example, solar energy transforms into chemical energy in plants.
• Second Law: Energy transfer is not 100% efficient; some energy is lost as unusable heat due to friction or radiation (e.g., energy lost in respiration or decay).
• Total energy (useful + unusable) remains constant, but useful energy decreases as it becomes degraded.
• Example: When plants convert solar energy into chemical energy, some energy is lost as heat during photosynthesis, illustrating the second law’s inefficiency.

Energy Sources

Sources of Energy

• Energy is required for daily activities like cooking, lighting, running gadgets, vehicles, and industrial production.
• Most energy needs are met by heat and electricity, obtained from fuels like coal, wood, kerosene, or nuclear energy.
• Characteristics of a Good Energy Source:
  • Provides adequate, steady energy over a long period.
  • Safe, convenient, economical, and easy to store and transport.
• Classification: Energy sources are divided into renewable (non-conventional) and non-renewable (conventional) based on availability.
• Example: Solar panels provide electricity for remote areas, meeting the criteria of a safe and renewable energy source.

Renewable or the Nonconventional Sources of Energy

• Renewable sources provide energy continuously and are inexhaustible.
• Main renewable sources: Sun, wind, flowing water (hydro), biomass, tides, oceans, geothermal spots, nuclear fuel.
• Most renewable energy indirectly comes from the sun.
• Wood is renewable but takes over 15 years to regrow, and excessive use causes deforestation and environmental imbalance.
• Sun as a Source:
  • Solar energy is produced by nuclear fusion in the sun, radiating a small fraction to Earth.
  • Solar constant: ~1.34 kW m^-2 reaching Earth’s upper atmosphere.
  • Solar energy drives winds, storms, rains, and photosynthesis in plants.
• Wind as a Source:
  • Wind energy is the kinetic energy of moving air, indirectly from solar energy due to unequal heating of Earth.
  • Used historically for sailing, grinding grains, and drawing water.
• Flowing Water (Hydro) as a Source:
  • Hydro energy is the kinetic energy of flowing water, driven by the solar-powered water cycle.
  • Used for centuries in water mills and transporting logs.
• Biomass as a Source:
  • Biomass includes wastes and dead parts of plants, trees, and animals, containing carbon compounds.
  • Bioenergy (chemical energy in biomass) is used as fuel for heat or to produce biogas (65% methane).
  • Biogas plants (e.g., gobar gas plants) use slurry (animal dung and water) to generate fuel for engines or electricity.
• Tides as a Source:
  • Tidal energy is the energy from rising and falling ocean water, harnessed by dams across narrow sea openings.
  • Limited due to insufficient tidal range and few suitable sites.
• Oceans as a Source:
  • Ocean Thermal Energy (OTE): Energy from temperature differences (up to 20°C) between surface and deeper ocean water, used in OTE conversion power plants.
  • Oceanic Waves Energy: Kinetic energy from fast-moving sea waves, also solar-driven, with potential for electricity generation.
• Geothermal Spots as a Source:
  • Geothermal energy is heat from hot rocks below Earth’s surface (hot spots).
  • Steam from heated underground water rotates turbines to generate electricity (e.g., in Madhya Pradesh, India).
• Nuclear Fuel as a Source:
  • Nuclear energy comes from nuclear fission (splitting uranium nuclei) or fusion (combining light nuclei).
  • Fission of one uranium nucleus releases ~200 MeV, with a chain reaction producing large energy.
  • Controlled fission in nuclear reactors (using cadmium rods) generates electricity.
  • Fusion is not yet harnessed for electricity.
  • Energy follows Einstein’s relation: E = mc², where m is mass loss, c = 3 × 10⁸ m s^-1.
• Example: A windmill converts wind’s kinetic energy into electricity, demonstrating a renewable source that harnesses solar-driven air movement.

Non-Renewable or the Conventional Sources of Energy

• Non-renewable sources are finite, formed over millions of years, and cannot be quickly replaced.
• Main sources: Coal, petroleum, natural gas (fossil fuels).
• Coal:
  • Composed of carbon, hydrogen, oxygen, nitrogen, and sulfur compounds.
  • Found in mines (e.g., Jharkhand, West Bengal in India).
  • Widely used due to abundance.
• Petroleum:
  • A viscous liquid (crude oil) found under Earth’s crust, composed of hydrocarbons and other compounds.
  • Obtained by drilling wells (e.g., in Assam, Mumbai).
  • Refined by fractional distillation to produce fuels like petrol, diesel, and LPG (liquefied petroleum gas with butane, propane, ethane).
  • LPG, mixed with ethyl mercaptan for leak detection, is used in domestic stoves.
• Natural Gas:
  • Found under Earth’s crust, mainly methane (95%) with ethane and propane.
  • Obtained by digging wells (e.g., Tripura, Mumbai offshore).
  • Used as fuel for heat production.
• Distinction:
  • Renewable sources are inexhaustible, non-conventional, regenerable (e.g., sun, wind).
  • Non-renewable sources are exhaustible, conventional, non-regenerable (e.g., coal, petroleum).
• Example: Burning coal in a power plant generates electricity but depletes finite reserves, unlike solar energy, which is inexhaustible.

Judicious Use of Energy

• Non-conventional sources cannot fully meet energy needs, so conventional sources are also used.
• Fossil fuels are limited, and overuse may cause an energy crisis.
• Measures for Judicious Use:
  • Use fossil fuels only when no alternatives exist.
  • Avoid energy wastage.
  • Ban tree cutting and promote afforestation.
  • Encourage community energy use.
  • Promote renewable sources in rural areas for lighting and tube wells.
  • Develop technologies for greater use of renewable sources (solar, wind, hydro, bioenergy, ocean).
  • Explore controlled nuclear fusion of deuterium in sea water for endless energy.
• Example: Using solar panels for street lighting in rural areas reduces reliance on coal-based electricity, conserving non-renewable resources.

Production of Electricity from Solar Energy

Solar energy is converted to electricity using solar cells or solar power plants.

Solar Cell:

• Converts sunlight directly into electricity using semiconductors like silicon or gallium.
• Sunlight on an impurity-added semiconductor creates a potential difference, producing current (e.g., 0.4–0.5 V, 60 mA for a 4 cm² cell).
• Solar panels (arrays of cells) increase efficiency and are used in satellites, water pumps, street lighting, and small devices like watches.
• Produces DC electricity, requiring storage batteries for nighttime use.
• Advantages: No maintenance, long-lasting, no running cost, pollution-free, ideal for remote areas.
• Disadvantages: High initial cost, low efficiency, produces DC unsuitable for some household uses.

Solar Power Plant:

• Uses concentrated sunlight to heat water pipes, producing steam to drive turbines and generate electricity.
• Example: A 50 kW plant in Gurgaon, Haryana.

Example: A solar panel powering a water pump in a village uses sunlight to generate DC electricity, stored in batteries for nighttime use.

Production of Electricity from Wind Energy

• Wind energy is converted to electricity using wind generators with windmills (turbines).
• Mechanism: Wind rotates turbine blades, turning a dynamo’s armature to produce alternating EMF.
• Wind farms (multiple generators) combine power for large-scale supply (e.g., >1025 MW in Gujarat, Tamil Nadu).
• Advantages: Pollution-free, renewable.
• Limitations: Requires steady winds (>15 km h^-1), large land areas, high setup costs.
• Example: A wind farm in a coastal area generates electricity by converting wind’s kinetic energy into rotational energy for dynamos.

Production of Electricity from Water (or Hydro) Energy

• Hydro energy is converted to electricity (hydroelectric power) using turbines.
• Mechanism: Water stored in a dam at high altitude has potential energy, which becomes kinetic energy when falling on turbines, rotating a generator to produce EMF.
• Mini hydroelectric plants can be built on small rivers with ~10 m dams.
• India generates 23% of its electricity from hydro energy, with plans for 4 × 10¹¹ kWh.
• Advantages: Pollution-free, renewable, supports irrigation and flood control.
• Limitations: Limited suitable sites, ecological disruption, destruction of local flora and fauna.
• Example: A dam on a river stores water at height, releasing it to spin turbines and generate electricity for nearby towns.

Production of Electricity from Nuclear Energy

• Nuclear energy is converted to electricity via controlled nuclear fission in nuclear power plants.
• Mechanism: Fission of uranium-235 or plutonium-239 releases heat, absorbed by a coolant, which heats water to produce steam, driving turbines to generate electricity.
• India has nuclear power plants in Tarapur, Rana Pratap Sagar, Kalpakkam, and Narora, contributing ~3% of total electricity.
• Advantages: Small fuel amounts produce large energy, long-lasting fuel.
• Limitations: Harmful radiation, high environmental pollution from waste.
• Example: A nuclear reactor uses controlled fission to heat water, producing steam to generate electricity, with strict safety measures to handle radiation.

Energy Degradation

• Energy transformation often results in some energy becoming unusable (e.g., heat from friction or radiation), called dissipation.
• Definition: The gradual decrease of useful energy due to losses is called energy degradation.
• Examples:
  • In a light bulb, <25% of electrical energy becomes light; the rest is lost as heat and invisible radiation.
  • In a vehicle, fuel energy is partly used for motion, but most is lost as heat, friction, and sound.
  • Cooking on a fire radiates most heat to the atmosphere, wasting energy.
  • Electrical appliances lose energy as heat.
  • Electricity transmission loses energy as heat in wires.
  • All machines have efficiency <1, losing some input energy to degraded forms.
• Example: When you run a fan, some electrical energy turns into air movement, but much is lost as heat in the motor, illustrating energy degradation.

Green House Effect And Global Warming

Green House Effect

• Discovered by Joseph Fourier in 1824, the greenhouse effect warms Earth’s surface and lower atmosphere by absorbing long-wavelength infrared radiation emitted from the surface.
• Solar radiation includes gamma rays, X-rays, UV rays, visible light, infrared, and radio waves.
• Earth’s atmosphere allows only visible light and short-wavelength infrared to pass; gamma rays, X-rays, and UV are absorbed by the ozone layer, while long-wavelength infrared and radio waves are reflected by the ionosphere and polar ice caps.
• Earth’s surface absorbs solar radiation, heats up, and emits long-wavelength infrared, which is trapped by clouds and greenhouse gases, warming the atmosphere.
• Greenhouse Gases: Carbon dioxide, water vapor, methane, chlorofluorocarbons.
• These gases trap heat, maintaining Earth’s average temperature at ~15.5°C (60°F); without them, it would be -18°C (0°F).
• Increased greenhouse gas concentrations raise Earth’s temperature above 15.5°C.
• Human Activities Increasing CO₂:
  • Burning fuels, deforestation, transportation, industrial production (e.g., cement factories).
  • Population growth (emitting ~32 giga tonnes of CO₂ annually).
  • Imbalanced CO₂ cycle (oceans and plants cannot absorb all CO₂).
• Methane concentration has doubled due to rice cultivation, animal husbandry, gas exploration, biomass burning, and pipeline leaks.
• CO₂ contributes 60% to the enhanced greenhouse effect.
• Example: A greenhouse traps heat like Earth’s atmosphere, keeping plants warm by absorbing infrared radiation, similar to how CO₂ traps heat on Earth.

Global Warming

Definition: 
Global warming is the increase in Earth’s average surface temperature due to higher greenhouse gas concentrations. Earth’s temperature has risen by 0.74 ± 0.18°C per decade over the last 50 years, with a faster rate since 2000, potentially reaching 64°C by 2100.

Causes:

• 25% increase in CO₂ from industrial growth, fossil fuel combustion, deforestation.
• Doubled methane from agriculture, gas exploration, biomass burning, pipeline leaks.
• 5% annual increase in chlorofluorocarbons.

Impacts:

• Climate variability, forcing migration of people and animals.
• Changed blooming seasons for plants.
• Altered regional climates affecting simple organisms.
• Disrupted global ecology.
• Increased heat stroke deaths.

Future Predictions:

• 30–70% of plant and animal species may vanish by 2050–2100, disrupting ecosystems.
• Ocean warming will kill or displace marine species, favoring warm-water species.
• Glacier melting (e.g., Siberia, Greenland) and rising sea levels (3.1 mm/year).
• Shift in farming regions, reducing yields in low latitudes and increasing malnutrition.
• New diseases and increased heat-related deaths due to warmer climates.
• More frequent heat waves, heavy precipitation, and climate variability.
• Higher air conditioning costs.
• Sea level rise flooding coastal areas, damaging infrastructure.

Example: Rising sea levels due to melting Arctic ice threaten coastal cities, forcing residents to relocate, illustrating global warming’s impact.

Ways to Minimise the Impact of Global Warming

Technological Measures:

• Use renewable sources (wind, solar, tidal, geothermal) instead of fossil fuel power plants, which produce 21.3% of greenhouse gases.
• Switch to battery-operated vehicles (charged by renewable energy) to reduce 14% of emissions from transportation.
• Use bio-char stoves for cooking to burn biomass without smoke, producing charcoal residue for soil.

Economic Measures:

• Promote reforestation and sustainable land use, verified by satellite imaging, to reduce deforestation (90% caused by agriculture).
• Impose carbon tax on industries based on emissions, encouraging energy-efficient practices and teleconferencing.

Policy Measures:

• Educate children on sustainable lifestyles, emphasizing cooperation over materialism.
• Control population growth through family planning, welfare reforms, and women’s empowerment via education.

Global Efforts: The 2015 Paris Climate Conference (COP21) aims to limit global warming to below 2°C by 2100 with zero carbon emissions by 2030–2050.

Example: Switching to electric cars charged by solar energy reduces CO₂ emissions, helping mitigate global warming.