Selina Concise Solutions for ICSE Class 9 Physics Chapter 6 Heat And Energy

Question 1S. Define heat and state its S.I. unit.
Answer: Heat is the energy of random motion of molecules constituting the body. Its S.I. unit is ‘joule’.
In simple words: Heat is the energy created by the tiny particles inside an object moving around randomly. We measure this energy in units called Joules.

📝 Teacher's Note: Remind students that heat is a form of energy that is transferred, while internal energy is what a body "has". Use the analogy of a crowded room where people moving faster create more "heat".

🎯 Exam Tip: Always specify "random motion of molecules" to distinguish heat from ordered mechanical energy. Don't forget to capitalize Joule when writing the full name of the unit.

Question 2S. In which direction does heat flow when two bodies are in contact?
Answer: Heat will flow from a hot body (body at a higher temperature) to a cold body (body at a lower temperature).
In simple words: Heat always moves from the warmer object to the cooler one, just like water flows from a higher level to a lower level.

📝 Teacher's Note: Use the analogy of water flow between two tanks of different heights to explain that temperature difference, not total heat content, determines the direction of flow.

🎯 Exam Tip: Clearly state "higher temperature to lower temperature" rather than just "hot to cold" for a more scientific response.

Question 3S. State the S.I. unit of heat and its relationship with calorie.
Answer: S.I. unit of heat is ‘joule’. 1 joule = 0.24 cal
In simple words: While Joule is the standard scientific unit, we also use calories (like in food). One Joule is roughly a quarter of a calorie.

📝 Teacher's Note: It is also useful to remember the reverse: 1 calorie ≈ 4.186 Joules. Students often confuse which unit is larger.

🎯 Exam Tip: Numerical problems often require converting between calories and joules; memorizing the 4.186 (or roughly 4.2) factor is essential.

Question 4S. Define temperature and state its S.I. unit.
Answer: Temperature is the parameter which tells the thermal state of a body (i.e. the degree of hotness or coldness). The S.I. unit of temperature is ‘kelvin’.
In simple words: Temperature is a measure of how hot or cold something is. Scientists use the Kelvin scale instead of Celsius or Fahrenheit.

📝 Teacher's Note: Emphasize that Kelvin is the absolute scale. Mention that 0 K is the point where all molecular motion stops.

🎯 Exam Tip: Note that "degree Kelvin" is incorrect; the unit is simply "kelvin" or "K". Do not use a degree symbol with K.

Question 5S. Why does a piece of ice appear cold when touched?
Answer: On touching a piece of ice, heat flows from our hand (hot body) to the ice (cold body), and hence, it appears cold.
In simple words: When you touch ice, it "steals" heat from your warm hand. Your brain senses this sudden loss of heat as a feeling of coldness.

📝 Teacher's Note: This is a common misconception; we don't "feel cold" moving into us, we feel "heat leaving us".

🎯 Exam Tip: Use the phrase "heat flows from hand to ice" to explain the sensation scientifically.

Question 6S. Differentiate between heat and temperature.
Answer: Heat is a form of energy obtained due to the random motion of molecules in a substance but temperature is a quantity which decided the direction of flow of heat when two bodies at different temperature are placed in contact. Two quantities having the same amount of heat may differ in temperature.
In simple words: Heat is the total energy "fuel" in an object, while temperature is the "level" or "intensity" of that energy. A giant swimming pool might have more total heat than a cup of boiling tea, but the tea has a higher temperature.

📝 Teacher's Note: Compare heat to the volume of water in a bucket and temperature to the height/pressure of that water. This helps students visualize why a small hot object can transfer heat to a large cold one.

🎯 Exam Tip: Focus on the "direction of flow" aspect for temperature to score full marks in comparative questions.

Question 7S. What is thermal expansion?
Answer: The expansion of a substance on heating is called thermal expansion.
In simple words: Most materials grow slightly larger or longer when they get hot because their molecules start vibrating more and pushing each other further apart.

📝 Teacher's Note: Use the example of metal railroad tracks having gaps between them to allow for this expansion in summer.

🎯 Exam Tip: Define it as a "change in dimensions" (length, area, or volume) due to heat.

Question 8S. Give examples of substances that expand on heating.
Answer: Brass and iron expand on heating.
In simple words: Metals like brass and iron get bigger when they are heated up.

📝 Teacher's Note: This is the basis for bimetallic strips used in thermostats and fire alarms.

🎯 Exam Tip: Mentioning specific metals shows you understand that different materials expand at different rates.

Question 9S. Give examples of substances that contract on heating.
Answer: Water contracts on heating from 0°C to 4°C. Silver iodide contracts on heating from 80°C to 141°C.
In simple words: Usually, things expand when heated, but some special materials like water (between 0 and 4 degrees) actually shrink or get smaller.

📝 Teacher's Note: This behavior in water is called "anomalous expansion" and is a critical concept for biology and environmental science.

🎯 Exam Tip: Always specify the temperature range (0°C to 4°C) for water to get full credit.

Question 10S. Define anomalous expansion of water.
Answer: The expansion of water when it is cooled from 4°C to 0°C is known as the anomalous expansion of water.
In simple words: Water does the opposite of what most liquids do: it gets bigger when it cools down near the freezing point. This is why ice floats and pipes burst in winter.

📝 Teacher's Note: Contrast this with "thermal contraction" that happens in most liquids as they cool.

🎯 Exam Tip: Use the word "anomalous" which means "abnormal" or "unusual".

Question 11S. At what temperature is the density of water maximum?
Answer: Density of water is maximum at 4°C. Its value is 1000 \( \text{kgm}^{-3} \).
In simple words: Water is most "packed together" or heaviest at 4°C. A cubic meter of water at this temperature weighs exactly 1000 kilograms.

📝 Teacher's Note: This is why the bottom of deep frozen lakes is always 4°C, providing a safe haven for fish.

🎯 Exam Tip: Memorize both the temperature (4°C) and the density value (1000 \( \text{kg m}^{-3} \)) as they are standard constants.

Question 12S. Describe the change in volume of water when heated from 0°C to 10°C.
Answer: When a given mass of water is heated from 0°C to 4°C, it contracts, i.e. its volume decreases. On heating from 4°C to 10°C, it expands, i.e. its volume increases.
In simple words: If you warm up freezing water, it first shrinks until it hits 4 degrees, and then it starts growing/expanding like normal.

📝 Teacher's Note: This "V" shaped behavior is unique. Drawing the volume vs. temperature graph is the best way to explain this to students.

🎯 Exam Tip: Clearly separate the description into two phases: 0-4°C and 4-10°C.

Question 13S. Represent the variation of volume of water with temperature graphically.
Answer: The graph shows that volume decreases from 0°C to 4°C and then increases beyond 4°C.
In simple words: This "U" shaped curve shows that water is at its smallest size when it's exactly at 4 degrees.

📝 Teacher's Note: Make sure students notice that the Y-axis (Volume) does not start at zero; it usually starts around 0.9999 to show the small changes clearly.

🎯 Exam Tip: Label the minimum point of the curve at exactly 4°C on the X-axis.

Question 14S. Represent the variation of density of water with temperature graphically.
Answer: The graph shows that density increases from 0°C to 4°C and then decreases beyond 4°C.
In simple words: This "hump" shaped curve shows that water is at its heaviest (maximum density) when it is at 4 degrees.

📝 Teacher's Note: This is the inverse of the volume graph. Since Density = Mass / Volume, the peak density occurs where volume is minimum.

🎯 Exam Tip: The peak of this curve must be at 4°C. Remember that density of water at 4°C is \( 1 \text{ g cm}^{-3} \) or \( 1000 \text{ kg m}^{-3} \).

Question 15S. Describe Hope’s experiment to demonstrate that water has maximum density at 4°C.
Answer: Hope’s experiment to demonstrate that water has maximum density at 4°C:
Hope’s apparatus consists of a tall metallic cylinder provided with two side openings P and Q, P near the top and Q near the bottom, fitted with thermometers T₁ and T₂ in them. The central part of the cylinder is surrounded with a cylindrical trough containing a freezing mixture of ice and salt. The cylinder is fitted with pure water at room temperature.
Observations: (i) Initially, both thermometers T₁ and T₂ are at the same temperature. (ii) First, the temperature recorded by the lower thermometer T₂ starts decreasing and finally it becomes steady at 4°C, while the temperature recorded in the upper thermometer T₁ remains almost unchanged during this time. (iii) Then, the temperature recorded by the lower thermometer T₂ remains constant at 4°C and upper thermometer T₁ records a continuous fall in temperature up to 0°C and then it becomes steady.
Conclusion: Thus, finally, the temperature recorded by the upper thermometer is 0°C and that by lower thermometer is 4°C. This proves that water has maximum density at 4°C.
In simple words: By putting a "freezing ring" in the middle of a water tube, we see cold water sink to the bottom until it hits 4 degrees. Below that, the water starts rising instead of sinking, proving it gets lighter as it gets colder than 4 degrees.

📝 Teacher's Note: Use a diagram to show how convection currents work in this experiment. Explain that water at 4°C is heavy and stays at the bottom, while colder water (3, 2, 1, 0°C) is lighter and floats to the top.

🎯 Exam Tip: Be sure to mention the temperatures shown by BOTH thermometers: T₁ (top) ends at 0°C and T₂ (bottom) ends at 4°C.

Question 16S. State the temperature of water in a frozen pond: (i) just in contact with ice, (ii) at the bottom.
Answer: (i) Water just in contact with ice is at 0°C. (ii) Water at the bottom of the pond is at 4°C.
In simple words: Even when a lake is frozen over, the bottom stays at a comfortable 4 degrees, while the water touching the ice slab is at zero degrees.

📝 Teacher's Note: This is a real-life application of anomalous expansion. Ice acts as an insulator, and the 4°C water at the bottom allows fish to survive.

🎯 Exam Tip: This is a very common 2-mark question. Always remember: Bottom = 4°C, Surface contact = 0°C.

Question 17S. Show the temperature distribution in a pond during winter with a diagram.
Answer:
In simple words: This diagram shows how the water in a pond gets warmer as you go deeper, eventually reaching 4°C at the very bottom.

📝 Teacher's Note: Explain that since 4°C water is the densest, it stays at the bottom. As the surface cools below 4°C, that water becomes lighter and stays on top until it freezes.

🎯 Exam Tip: Label the layers clearly. Start from 0°C just below the ice and end at 4°C at the bottom.

Question 18S. Give reasons for the following:
(a) Water pipes burst on cold winter nights.
(b) Water in a tank starts freezing from the top and not from the bottom.
(c) Anomalous expansion of water helps preserve aquatic life.
(d) A hollow glass sphere sinks when water is heated above 4°C.
(e) A glass bottle completely filled with water bursts in a freezer.
Answer:
(a) On winter nights, as the temperature starts falling below 4°C, water in the pipe lines expands and exerts a great pressure on the pipes, causing them to burst.
(b) In winters, when temperature falls, the surface of water in the tank contracts, becomes denser and sinks to the bottom. A circulation is thus set up until the entire water in the tank reaches its maximum density at 4°C. If the temperature falls further, then the top layer expands and remains on the top till it freezes. Thus, water in a tank starts freezing from the top and not from the bottom.
(c) The anomalous expansion of water helps preserve aquatic life during very cold weather. When temperature falls, the top layer of water in a pond contracts becomes denser and sinks to the bottom. A circulation is thus set up until water in the pond reaches its maximum density at 4°C. If the temperature falls further, then the top layer expands and remains on the top till it freezes. Thus, even though the upper layers are frozen, the water near the bottom is at 4°C and the fishes can survive in it easily.
(d) On heating water above 4°C, the density of water decreases. As a result, the upthrust acting due to water on hollow glass sphere also decreases, which causes it to sink.
(e) Inside the freezer, when the temperature of water falls below 4°C, the water in the bottle starts expanding. If the bottle is completely filled and tightly closed, there is no space for water to expand, and hence, the bottle may burst.
In simple words: (a) Expanding ice is strong enough to break metal pipes. (b) Coldest water is lighter than 4°C water, so it stays on top to freeze. (c) Deep water stays at 4°C so fish don't freeze. (d) Hot water is "thinner" and can't hold up the sphere. (e) Filling a bottle to the brim is dangerous because ice needs extra room to grow.

📝 Teacher's Note: These "Give Reason" questions are all applications of anomalous expansion. Help students see that they all share the same underlying logic: water expands as it cools from 4°C to 0°C.

🎯 Exam Tip: For these questions, always mention "expansion below 4°C" and "maximum density at 4°C" to ensure you hit the key marking points.

Question 1M. Calorie is the unit of:
(a) Power
(b) Force
(c) Heat
(d) Temperature
Answer: (c) Heat
In simple words: A calorie is a way to measure heat energy, just like a liter is a way to measure water.

📝 Teacher's Note: Calorie was originally defined as the amount of heat needed to raise the temperature of 1g of water by 1°C.

🎯 Exam Tip: Calorie is a non-SI unit. If asked for SI unit, the answer is Joule.

Question 2M. What is the value of 1 Joule in calories?
(a) 4.186 cal
(b) 1 cal
(c) 0.24 cal
(d) 100 cal
Answer: (c) 0.24 cal
In simple words: One Joule is roughly a quarter (0.24) of one calorie.

📝 Teacher's Note: Derived from 1 calorie = 4.186 J. So, 1 J = 1/4.186 ≈ 0.24 cal.

🎯 Exam Tip: Remember: Calorie > Joule. A "big" calorie equals 4.2 "small" joules.

Question 3M. The SI unit of temperature is:
(a) Celsius
(b) Fahrenheit
(c) Kelvin
(d) Joule
Answer: (c) kelvin (K)
In simple words: Although we use Celsius in daily life, the official scientific unit is Kelvin.

📝 Teacher's Note: In calculations, students must often convert Celsius to Kelvin by adding 273.

🎯 Exam Tip: Don't use the degree symbol (°) for Kelvin. It's just 'K'.

Question 4M. What happens to water when it is cooled from 4°C to 0°C?
(a) It contracts
(b) It expands
(c) Its density increases
(d) No change occurs
Answer: (b) It expands
In simple words: Cooling water usually makes it shrink, but once it gets colder than 4 degrees, it weirdly starts to get bigger.

📝 Teacher's Note: This is the essence of anomalous expansion.

🎯 Exam Tip: This specific range (4 to 0) is the only time cooling causes expansion in water.

Question 5M. At what temperature is the volume of water lowest?
(a) 0°C
(b) 4°C
(c) 100°C
(d) 10°C
Answer: (b) 4°C
In simple words: Water is most "compact" or takes up the least space when it is at exactly 4 degrees.

📝 Teacher's Note: Lowest volume means highest density. These two facts are linked.

🎯 Exam Tip: Max Density and Min Volume both occur at the same magic number: 4°C.

Exercise 6(B)

Question 1S. Define an ecosystem.
Answer: A unit composed of biotic components (i.e. producers, consumers and decomposers) and abiotic components (i.e. light, heat, rain, and humidity, inorganic and organic substances) is called an ecosystem.
In simple words: An ecosystem is a neighborhood of nature where living things like plants and animals live together with non-living things like sunlight, water, and soil.

📝 Teacher's Note: Use a small pond or a school garden as a tangible example of a local ecosystem.

🎯 Exam Tip: Make sure to include both "biotic" (living) and "abiotic" (non-living) parts in your definition.

Question 2S. What is the source of energy for all ecosystems?
Answer: The source of energy for all ecosystems is the Sun.
In simple words: Every living thing on Earth gets its power from the Sun's light, either directly or by eating something else that did.

📝 Teacher's Note: Solar energy is the "engine" that drives all biological processes on Earth.

🎯 Exam Tip: The Sun is the *ultimate* source. Don't confuse it with intermediate sources like glucose.

Question 3S. Why are green plants important in an ecosystem?
Answer: Green plants absorb most of the energy falling on them and by the process of photosynthesis they produce food for the consumers. Plants, being primary producers are of great importance in the ecosystem. They also maintain the balance of oxygen and carbon dioxide on earth.
In simple words: Plants are nature's "factories"—they turn sunlight into food that everyone else eats. They also act as the Earth's lungs by making oxygen for us to breathe.

📝 Teacher's Note: Plants are the only bridge between abiotic energy (light) and biotic energy (food).

🎯 Exam Tip: Mention both "food production" and "gas balance" to give a complete answer.

Question 4S. Differentiate between producers and consumers.
Answer: Producers like plants and some bacteria are capable of producing its own food using the energy of sun but consumers are not capable of producing their own food. They depend on producers for food.
In simple words: Producers make their own "meals" from light; consumers have to go out and "buy" (eat) their meals from the producers.

📝 Teacher's Note: Use the terms Autotrophs (Producers) and Heterotrophs (Consumers).

🎯 Exam Tip: The primary difference is the "method of obtaining nutrition".

Question 5S. What is the role of a decomposer in an ecosystem?
Answer: The role of a decomposer is to break down dead organisms and then feed on them. The nutrients created by the dead organisms are returned to the soil to be later used by the producers. Once these deceased organisms are returned to the soil, they are used as food by bacteria and fungi by transforming the complex organic materials into simpler nutrients. The simpler products can then be used by producers to restart the cycle. These decomposers play an important role in every ecosystem.
In simple words: Decomposers are the cleanup crew. They turn dead leaves and animals back into soil vitamins so that new plants can grow.

📝 Teacher's Note: Without decomposers, the Earth would be covered in waste and the soil would run out of nutrients.

🎯 Exam Tip: Focus on the "recycling of nutrients" as the key function of decomposers.

Question 6S. Define a food chain.
Answer: A food chain shows the feeding relationship between different living things in a particular environment or habitat. Often, a plant will begin a food chain because it can make its own food using energy from the Sun. In addition, a food chain represents a series of events in which food and energy are transferred from one organism in an ecosystem to another. Food chains show how energy is passed from the sun to producers, from producers to consumers, and from consumers to decomposers.
In simple words: A food chain is a map that shows "who eats whom"—tracking how energy travels from the grass to the rabbit and then to the fox.

📝 Teacher's Note: Arrows in a food chain represent the flow of *energy*, not who eats who (the arrow points to the eater).

🎯 Exam Tip: Start your food chain with a "Producer" and follow the flow to "Decomposers".

Question 7S. Draw a simple food chain.
Answer:
In simple words: This shows energy flowing from the Sun to grass, then to a bug, and then to a bird.

📝 Teacher's Note: The arrows show the direction of energy transfer. The Heron gets energy *from* the cricket.

🎯 Exam Tip: Label each level (Producer, Primary Consumer, etc.) to show complete understanding.

Question 8S. How is energy maintained and passed through trophic levels?
Answer: Ecosystems maintain themselves by cycling energy and nutrients obtained from external sources. At the first trophic level, primary producers (plants, algae, and some bacteria) use solar energy to produce organic plant material through photosynthesis. Herbivores - animals that feed solely on plants - make up the second trophic level. Predators that eat herbivores comprise the third trophic level; if larger predators are present, they represent still higher trophic levels. Decomposers, which include bacteria, fungi etc., break down wastes and dead organisms and return nutrients to the soil. On average about 10 percent of net energy production at one trophic level is passed on to the next level. Processes that reduce the energy transferred include respiration, growth, and reproduction.
In simple words: Nature follows a "10% rule"—only a small bit of energy moves up to the next level when an animal eats. Most energy is used up by the animal just to stay alive, grow, and move.

📝 Teacher's Note: Explain that this energy loss is why we see many more plants than lions in a forest. Energy runs out as you go up.

🎯 Exam Tip: Be sure to mention the "10% law" by name; it is a very common keyword in marking schemes.

Question 9S. Which laws of physics govern the flow of energy in an ecosystem?
Answer: The laws of thermodynamics govern the energy flow in the ecosystem. According to the first law of thermodynamic, the energy can be transformed from one form to the other form, but it can neither be created nor destroyed. According to the second law of thermodynamics, when energy is put to work, a part of it is always converted in un-useful form such as heat mainly due to friction and radiation.
In simple words: Physics says energy never disappears, but it does get "messy." Every time an animal eats, some energy turns into heat that escapes into the air and can't be used again.

📝 Teacher's Note: This connects biology to physics. Energy enters as light, becomes chemical energy in food, and eventually leaves the earth as heat (infrared radiation).

🎯 Exam Tip: When asked how energy "escapes," mention it is lost as "non-useful heat energy."

Question 10S. Why is energy flow in an ecosystem called "linear"?
Answer: The energy flow in ecosystem is linear i.e., it moves in a fixed direction. The solar energy is absorbed by plants and a part of it is converted into food. These plants (or primary producers) are then eaten by the primary consumers, which are consumed by secondary consumers and the secondary by tertiary consumers. This cycle is unidirectional. The dead and decomposed are fed by decomposers, which return the nutrients to the soil. At the end, the energy reaches the degraded state. It does not return to the sun to make the process cyclic, thus energy flow is linear.
In simple words: Unlike water or soil which goes in a circle, energy is a one-way trip. Once it's used and turned into heat, it can't go back to the Sun to become sunlight again.

📝 Teacher's Note: Distinguish between "Cycle of Matter" (which is cyclic) and "Flow of Energy" (which is linear). This is a critical distinction in ecology.

🎯 Exam Tip: Use the term "unidirectional" to describe the one-way path of energy.

Question 11S. Represent the flow of energy through trophic levels with a diagram.
Answer:
In simple words: This pyramid shows that most energy is at the bottom (plants) and it gets smaller as you go up to top predators.

📝 Teacher's Note: The width of each bar represents the amount of energy available at that level. The shrinking width demonstrates the 10% law.

🎯 Exam Tip: Draw the energy pyramid with distinct steps and label the energy losses to the environment.

Question 1M. Plants produce food through the process of:
(a) Respiration
(b) Photosynthesis
(c) Transpiration
(d) Digestion
Answer: (b) Photosynthesis
In simple words: Photo-synthesis is the superpower plants have to bake their own food using light.

📝 Teacher's Note: "Photo" means light and "synthesis" means to put together.

🎯 Exam Tip: Remember that photosynthesis converts light energy into chemical energy stored in glucose.

Question 2M. The primary source of energy in a food chain is:
(a) Moon
(b) Soil
(c) Sun
(d) Water
Answer: (c) Sun
In simple words: The Sun is the "battery" that powers the entire planet's life cycles.

📝 Teacher's Note: Even deep-sea creatures ultimately depend on organic matter falling from the sun-lit surface.

🎯 Exam Tip: The Sun is always the starting point for energy discussions in ecology.

Question 3M. Organisms that make their own food are called:
(a) Consumers
(b) Decomposers
(c) Producers
(d) Predators
Answer: (c) Producers
In simple words: If you make it, you're a producer. Plants produce, we consume.

📝 Teacher's Note: Use the economic analogy of a factory (producer) and a shopper (consumer) to clarify this.

🎯 Exam Tip: Producers are also known as "Autotrophs".

Question 4M. A lion that eats a zebra is a:
(a) Producer
(b) Decomposer
(c) Consumer
(d) Abiotic component
Answer: (c) Consumer
In simple words: Since the lion has to eat another animal to get energy, it is consuming food, not producing it.

📝 Teacher's Note: Animals are always consumers because they cannot perform photosynthesis.

🎯 Exam Tip: Lions are specifically "secondary" or "tertiary" consumers because they eat other animals.

Exercise 6(C)

Question 1S. What are the two characteristics of a good source of energy?
Answer: 1. A source of energy should be safe and convenient to use. 2. A source of energy should be economical and easy to store and transport.
In simple words: A good battery or fuel should be easy to carry, cheap to buy, and shouldn't explode or hurt you easily.

📝 Teacher's Note: Discuss why wood was replaced by coal, and coal by gas, using these two points as a framework.

🎯 Exam Tip: Use the keywords "safe," "convenient," and "economical" to score full marks.

Question 2S. Name the two main groups of energy sources.
Answer: The two groups in which various sources of energy are classified are renewable or non-conventional sources of energy and non-renewable or conventional sources of energy. These sources are classified on the basis of their availability and utility.
In simple words: Renewable energy is like a fountain that never runs out (like wind), while non-renewable energy is like a limited tank that once used up, is gone forever (like oil).

📝 Teacher's Note: "Conventional" usually refers to what we've used for a long time (fossil fuels), while "Non-conventional" refers to newer alternatives (solar/wind).

🎯 Exam Tip: Be able to categorize sources into these two groups—this is a very common question type.

Question 3S. Differentiate between renewable and non-renewable sources of energy.
Answer: Renewable: The natural sources providing us energy continuously are called renewable sources of energy. Non-renewable: The sources of energy which have accumulated in nature over a very long period of time and cannot be quickly replaced when exhausted are called non-renewable sources of energy.

In simple words: Renewable is "forever" energy; non-renewable is "one-time use" energy.

📝 Teacher's Note: Explain that "Renewable" doesn't mean infinite, it means the recharge rate matches or exceeds our usage rate.

🎯 Exam Tip: Always provide at least two examples for each to secure full marks.

Question 4S. Give examples of renewable and non-renewable energy.
Answer: Renewable: Wood, Water and Wind. Non-renewable: Coal, Diesel and Oil.
In simple words: Wind will always blow, but we only have a limited amount of coal buried in the ground.

📝 Teacher's Note: Note that wood is technically renewable if we plant new trees, but fossil fuels like oil take millions of years to form.

🎯 Exam Tip: Group your examples clearly to show you know which is which.

Question 5S. Why is wood considered a problematic fuel?
Answer: Wood is obtained from trees. Hence, trees need to be cut down for wood to be used as a fuel. Also, burning wood releases a lot of smoke which pollutes the atmosphere.
In simple words: Using wood for fire kills trees and fills the air with itchy smoke, which is bad for both the forests and our health.

📝 Teacher's Note: Discuss the link between using wood fuel and global issues like deforestation and air pollution.

🎯 Exam Tip: Mention both "deforestation" and "environmental pollution" as your two main points.

Question 6S. Categorize the following: Sun, Wind, Coal, Petroleum, Natural gas, Tides, Nuclear fuel.
Answer: Renewable: 1. Sun, 2. Wind, 3. Flowing water, 4. Tides, 5. Nuclear fuel. Non-renewable: 1. Coal, 2. Petroleum, 3. Natural gas.
In simple words: Things like sunlight and ocean waves keep coming back; things like coal and gas eventually run out.

📝 Teacher's Note: There is a debate about nuclear fuel being renewable, but for this level, it is often grouped with advanced energy sources. Actually, the source OCR groups it with renewable/non-conventional.

🎯 Exam Tip: If the question asks for "conventional" vs "non-conventional", group fossil fuels as conventional and the rest as non-conventional.

Question 7S. Describe (1) Tidal energy and (2) Ocean energy.
Answer: 1. Tidal energy: The energy possessed by rising and falling water in tides is known as tidal energy. Dams are constructed across a narrow opening to the sea to harness tidal energy and produce electricity. However, it is not a major source of energy as the rise and fall of seawater during tides is not enough to generate electricity on a large scale.
2. Ocean energy: Water in the oceans possesses energy in two forms: Ocean thermal energy (due to temperature difference between surface and deep water) and Oceanic waves energy (kinetic energy of fast waves).

1. Ocean thermal energy- The energy available due to the difference in temperature of water at the surface and at deeper levels of ocean is called the ocean thermal energy. This energy is harnessed for producing electricity by a device called ocean thermal energy conversion power plant (OCTEC power plant).
2. Oceanic waves energy- The kinetic energy possessed by fast moving oceanic (or sea) waves is called oceanic waves energy. Though models have been made to generate electricity from oceanic waves, but so far it has not been put to practical use.
3. Geo thermal energy: The heat energy possessed by the rocks inside the Earth is called geothermal energy. The hot rocks present at the hot spots deep inside the Earth, heat the underground water and turn it into steam. This steam is compressed at high pressure between the rocks. Holes are drilled deep into the Earth up to the hot spots to extract the steam through pipes, which is utilized to rotate the turbines connected to the armature of an electric generator to produce electricity.
In simple words: We can get power from the oceans by using the rising tides, the crashing waves, or even the fact that deep water is much colder than surface water.

📝 Teacher's Note: Ocean energy is vast but very difficult to capture effectively because salt water ruins metal machinery very quickly.

🎯 Exam Tip: Mention that tidal energy is "site-specific"—it only works in very specific coastal areas.

Question 8S. Name the main source of energy on Earth.
Answer: Sun is the main source of energy on Earth.
In simple words: The Sun is the original powerhouse of our planet. It provides the heat and light that allow plants to grow and drive weather patterns, making it the primary source for almost all energy on Earth.

📝 Teacher's Note: When explaining alternative energy, emphasize that these sources (tidal, wave, geothermal) are renewable and more environmentally friendly than fossil fuels, though they can be harder to capture on a large scale.

🎯 Exam Tip: "Sun is the ultimate source of energy" is a very common one-mark question. For geothermal energy, make sure to use the key phrase "internal heat of the Earth" or "hot spots."

Question 9S. What is solar energy and how does a solar power plant work?
Answer: The energy obtained from Sun is called solar energy. A solar power plant is a device in which heat energy of sun is used to generate electricity. It consists of a large number of concave reflectors, at the focus of which there are black painted water pipes. The reflectors concentrate the heat energy of the sun rays on the pipes due to which water inside the pipes starts boiling and produces steam. The steam thus produced is used to rotate a steam turbine which drives a generator producing electricity.
In simple words: A solar plant uses mirrors to focus sunlight on water pipes, boiling the water to make steam. That steam then spins a wheel (turbine) to make electricity.

📝 Teacher's Note: This is "concentrated solar power". Contrast this with "solar photovoltaic" which turns light directly into electricity without steam.

🎯 Exam Tip: Mention "concave reflectors" and "steam turbine" as the key mechanical components.

Question 10S. What is a solar cell? List two uses and one disadvantage.
Answer: A solar cell is an electrical device that converts light energy directly into electricity with the help of photovoltaic effect. Solar cells are usually made from semiconductors like silicon and gallium. Two uses: 1. They do not require maintenance and last over a long period of time at zero running cost. 2. They are very useful for remote, inaccessible and isolated places where electric power lines cannot be laid. One disadvantage: 1. The initial cost of a solar panel is sufficiently high.
In simple words: A solar cell turns light into power instantly using special materials. They are great for space satellites or desert homes because they need no wires, but they are expensive to buy at first.

📝 Teacher's Note: The "Photovoltaic effect" is the scientific term for light creating a current in a semiconductor. Use a solar-powered calculator as a simple classroom demo.

🎯 Exam Tip: Remember that solar cells produce D.C. (Direct Current) electricity.

Question 11S. List advantages and disadvantages of solar panels.
Answer:
Advantages: 1. No pollution. 2. Zero running cost. 3. Low maintenance. 4. Great for remote areas.
Disadvantages: 1. High starting cost. 2. Low efficiency. 3. Produces d.c. electricity (needs conversion for most homes).
In simple words: Solar is clean and free once you buy it, but buying the panels is very expensive, and they don't catch all the sun's power.

📝 Teacher's Note: Efficiency is about 15-20% for common panels. Most energy is lost as heat.

🎯 Exam Tip: Mentioning that it "produces d.c. electricity" is a high-level technical point.

Question 12S.
Answer: The kinetic energy of the moving large masses of air is called the wind energy. Wind
energy is used in a wind generator to produce electricity by making use of wind mill to
drive a wind generator.
At present in India, more than 1025 MW electric power is generated using wind energy.

Question 13S. State the advantages and limitations of wind energy.
Answer:
Advantages: 1. No pollution. 2. Everlasting source.
Limitations: 1. Expensive to build. 2. Needs a large area of land.
In simple words: Windmills are clean and will never run out of wind, but they take up a lot of space and cost a lot of money to build.

📝 Teacher's Note: Also mention that wind is intermittent—it doesn't blow all the time!

🎯 Exam Tip: "Pollution-free" is the primary advantage for most renewable sources.

Question 14S. Explain the principle of a hydroelectric power plant.
Answer: The kinetic energy possessed by flowing water is called the water or hydro energy. Principle of a hydroelectric power plant is that the water flowing in high altitude rivers is collected in a high dam (or reservoir). The water from the dam is then allowed to fall on a water turbine which is located near the bottom of the dam. The shaft of the turbine is connected to the armature of an electric generator or dynamo.
In simple words: We catch river water behind a wall (dam). When we let the water fall, it spins a big underwater fan that makes electricity.

📝 Teacher's Note: Conversion path: Potential Energy → Kinetic Energy → Mechanical Energy → Electrical Energy.

🎯 Exam Tip: Be sure to mention the "turbine" and the "generator/dynamo."

Question 15S. List advantages and disadvantages of hydroelectricity.
Answer:
Advantages: 1. No environmental pollution. 2. Renewable source.
Disadvantages: 1. Destroys local ecosystems and plants/animals. 2. Disturbs ecological balance downstream.
In simple words: Hydro power is clean for the air, but the big dams drown forests and mess up the river for the animals living there.

📝 Teacher's Note: Relate this to real-world debates about building large dams like the Tehri or Narmada dams.

🎯 Exam Tip: Focus on "ecological disturbance" for the disadvantages.

Question 16S. Define nuclear energy and state the principle of a nuclear reactor.
Answer: When a heavy nucleus is bombarded with slow neutrons, it splits into two nearly equal light nuclei with a release of tremendous amount of energy (nuclear fission).
Principle: Heat from controlled nuclear fission boils water into steam. This steam rotates a turbine connected to a generator in a magnetic field to produce electricity.
In simple words: We split tiny atoms to get a massive burst of heat. We use that heat to make steam, just like a tea kettle, to spin a turbine and make power.

📝 Teacher's Note: Fission is for heavy atoms like Uranium; Fusion is for light atoms like Hydrogen (in stars).

🎯 Exam Tip: The keyword is "controlled chain reaction." Without control, it becomes an explosion.

Question 17S.
Answer: At present only about 3% of the total electrical power generated in India is obtained from
the nuclear power plants.
Tarapur in Maharahtra and Narora in Uttar Pradesh are the places where electricity is
produced using nuclear energy.

Question 18S. State advantages and disadvantages of nuclear energy.
Answer:
Advantages: 1. Tiny fuel amount makes massive energy. 2. Lasts for several years once loaded.
Disadvantages: 1. Not clean (harmful radiation). 2. Waste causes pollution.
In simple words: Nuclear power is super strong and lasts a long time, but it creates dangerous invisible rays and waste that is hard to get rid of safely.

📝 Teacher's Note: Nuclear power doesn't make smoke or CO₂, but its "radiation" is a different kind of pollution.

🎯 Exam Tip: "Tremendous amount of energy" is a key phrase for nuclear advantages.

Question 19S. Name the energy transformations in: (i) Solar cell, (ii) Windmill, (iii) Hydro dam, (iv) Nuclear plant.
Answer:
(i) Light energy → electrical energy
(ii) Mechanical energy → electrical energy
(iii) Mechanical energy → electrical energy
(iv) Nuclear energy (or heat energy) → electrical energy
In simple words: Every power source is basically a machine that changes one type of energy (like wind or heat) into electricity.

📝 Teacher's Note: Help students trace the path. For wind and hydro, it's motion (mechanical) making electricity.

🎯 Exam Tip: Always identify the "Input" and "Output" forms clearly.

Question 20S.
Answer:  Four ways for the judicious use of energy are:
1. The fossil fuels such as coal, petroleum, natural gas should be used only for the
limited purposes when there is no other alternative source of energy available.
2. The wastage of energy should be avoided.
3. Efforts must be made to make use of energy for community or group purposes.
4. The cutting of trees must be banned and more and more new trees must be roped
to grow.

Question 21S. What is degradation of energy? Give examples.
Answer: The gradual decrease of useful energy due to friction etc. is called the degradation of energy.
Examples: 1. Cooking fire heat radiating into the air.
2. Electrical appliances wasting energy as heat.
In simple words: Degradation is when energy "gets tired" and turns into useless heat that just floats away into the room.

📝 Teacher's Note: This is a result of the Second Law of Thermodynamics. Energy isn't lost, but it is no longer "useful" for doing work.

🎯 Exam Tip: "Non-useful form" is the keyword for the definition of degradation.

Question 1M. The ultimate source of energy is:
(a) Wind
(b) Coal
(c) Sun
(d) Water
Answer: (c) Sun
In simple words: Almost all the energy on our planet, from food to gasoline, originally came from the Sun's light.

📝 Teacher's Note: The sun is the primary powerhouse for almost all life processes and weather on Earth.

🎯 Exam Tip: This is a standard 1-mark objective question.

Question 2M. Sun is a:
(a) Non-renewable source
(b) Renewable source
(c) Exhaustible source
(d) Polluting source
Answer: (b) Renewable source
In simple words: The Sun will keep shining for billions of years, so we can never use it up.

📝 Teacher's Note: Contrast this with fossil fuels which are non-renewable because they take millions of years to form.

🎯 Exam Tip: "Renewable" and "Everlasting" are often used interchangeably in this context.

Exercise 6(D)

Question 1S. Define the Greenhouse Effect.
Answer: Greenhouse effect is the process of warming of planet’s surface and its lower atmosphere by absorbtion of infrared radiations of longer wavelength emitted out from the surface of planet.
In simple words: It's like the Earth is wearing a blanket of gases. The blanket lets sunlight in but keeps the Earth's own heat from escaping into space, keeping us warm.

📝 Teacher's Note: Use the "Car in the sun" analogy. The windows let light in but trap the heat inside, making the interior very hot.

🎯 Exam Tip: Be sure to mention "infrared radiations" and "longer wavelength" for a complete technical definition.

Question 2S. Name three greenhouse gases.
Answer: Carbon-di-oxide, water vapour and methane are greenhouse gases.
In simple words: These are the "blanket" gases that trap heat in our air.

📝 Teacher's Note: Carbon dioxide is the main one produced by humans burning coal and gasoline.

🎯 Exam Tip: Carbon dioxide (\( \text{CO}_2 \)) is the most common answer in 1-mark questions.

Question 3S. Which radiations pass through the atmosphere and which are absorbed?
Answer: Visible light rays and short infrared radiation pass through the atmosphere of earth. Infrared radiations of long wavelength are absorbed by the greenhouse gases.
In simple words: Bright light and fast heat from the sun get through our air easily, but the "lazy" slow heat pushing off the ground gets trapped by the air.

📝 Teacher's Note: This wavelength difference is the secret to how the whole process works. Use the filter analogy.

🎯 Exam Tip: Short wavelength = passing through; Long wavelength = absorbed.

Question 4S. Which radiations are absorbed by greenhouse gases?
Answer: Infrared radiations of long wavelength are absorbed by the green house gases.
In simple words: Once sunlight hits the ground and warms it up, the ground gives off "slow" heat. The greenhouse gases catch this slow heat and hold onto it.

📝 Teacher's Note: This absorption is what causes the warming. Without these gases catching the long-wavelength heat, it would simply vanish into the cold of outer space.

🎯 Exam Tip: Always specify "long-wavelength" infrared radiations as the specific type of energy trapped by these gases.

Question 5S. Why has the concentration of carbon dioxide in the atmosphere increased?
Answer: The concentration of carbon-di-oxide content’s of earth’s atmosphere has increased due to industrial growth, combustion of fossil fuels and clearing of forests.
In simple words: Humans have built more factories and burned more coal and oil. We have also cut down many trees that normally help "clean" the CO2 out of the air.

📝 Teacher's Note: Explain that trees act as "carbon sinks" by absorbing CO2 during photosynthesis. Removing them is a double blow: we stop the absorption and often burn the wood, releasing even more CO2.

🎯 Exam Tip: The three main keywords are "industrialization," "fossil fuels," and "deforestation."

Question 6S. What would be the average temperature on Earth in the absence of greenhouse gases?
Answer: In absence of green house gases, the average temperature on earth would be \( -18^\circ\text{C} \).
In simple words: Without our air's natural heating system, the entire world would be a frozen ball of ice, much colder than a deep freezer.

📝 Teacher's Note: This shows that the greenhouse effect is naturally a good thing; it's the increase caused by humans that is dangerous.

🎯 Exam Tip: Remember the value \( -18^\circ\text{C} \) as it is a frequent one-mark objective question.

Question 7S. What is the average warming effect of greenhouse gases on Earth's surface?
Answer: The greenhouses gases have an average warming effect on Earth’s surface of about \( 15.5^\circ\text{C} \) (or \( 60^\circ\text{F} \)).
In simple words: The natural "blanket" of gases keeps the Earth's temperature at a comfortable average of about 15.5 degrees instead of being frozen.

📝 Teacher's Note: Use this to contrast with the previous answer. The greenhouse effect adds over 30 degrees of warmth to our planet's average temperature.

🎯 Exam Tip: Be careful not to confuse the "warming effect" with the actual average temperature; here they are given as the same value (\( 15.5^\circ\text{C} \)).

Question 8S. Define global warming.
Answer: Global warming means the increase in average effective temperature near the earth’s surface due to an increase in the amount of green house gases in its atmosphere.
In simple words: Global warming is when the Earth's "blanket" of gases gets too thick, making the whole world get a "fever" and heat up over time.

📝 Teacher's Note: Differentiate between the "Greenhouse Effect" (the process) and "Global Warming" (the result of the process becoming too intense).

🎯 Exam Tip: Mention that the cause of global warming is the "increase in the amount of greenhouse gases."

Question 9S. How has human activity contributed to the rise in atmospheric temperature?
Answer: With activities industrialization, deforestation, excess burning of fossil fuel, the concentration of green house gases has increased on earth’s atmosphere. This increase in the amount of greenhouse gases present in atmosphere has caused the rise in atmospheric temperature.
In simple words: By polluting the air and cutting down forests, humans have made the Earth's heat-trapping blanket much thicker. This extra thickness is making the air hotter.

📝 Teacher's Note: This is a synthesis of cause and effect. It links human industrial progress directly to environmental changes.

🎯 Exam Tip: Focus on the logical flow: Human activity \( \implies \) More greenhouse gases \( \implies \) More heat trapped \( \implies \) Rise in temperature.

Question 10S. Which human activities lead to an increased greenhouse effect?
Answer: The increase in green house gases due to activities like industrialization, deforestation, natural gas exploration, burning of biomass, natural gas exploration, more use of gadgets like refrigerators has caused the increase of green house effect.
In simple words: Things like big factories, burning waste, mining for gas, and using old-fashioned fridges all add to the problem.

📝 Teacher's Note: Mention that older refrigerators and air conditioners used CFCs, which are incredibly potent greenhouse gases, even in small amounts.

🎯 Exam Tip: If asked to list activities, provide at least 3-4 distinct ones like deforestation and burning fossil fuels.

Question 11S. What are the consequences of global warming at the Earth's poles?
Answer: At the poles, due to increase in temperature, the snow and ice will melt which will cause flood in coastal countries. The icebergs of dark land and oceans will melt, so the dark land and oceans will become uncovered and will absorb more heat radiations coming from sun, increasing the green house effect further.
In simple words: When the North and South poles melt, they turn into extra water that floods cities. Also, as white ice disappears, dark ground is revealed which catches even more sun heat, making things even hotter.

📝 Teacher's Note: This is an example of a "Positive Feedback Loop." Melting ice leads to more heat absorption, which leads to even faster melting.

🎯 Exam Tip: Mention the "uncovering of dark land/oceans" to explain why the warming accelerates at the poles.

Question 12S. How does global warming lead to flooding?
Answer: Due to global warming, the snow and ice around the poles will melt and cause flood in coastal countries.
In simple words: It is like an ice cube melting in a full glass; the extra liquid has nowhere to go but to overflow onto the land.

📝 Teacher's Note: Emphasize that "coastal countries" are at the highest risk because they are right at sea level.

🎯 Exam Tip: Link "polar melting" directly to "rising sea levels" and "flooding."

Question 13S. What is the impact of melting glaciers on big cities?
Answer: Due to melting of polar ice and glaciers, there will be rise in sea level on coastal wet lands. It would raise worldwise sea level, thereby, many big cities in the coastal areas will be covered by sea water.
In simple words: Many of the world's biggest cities are near the ocean. If the sea rises, parts of these cities could end up completely underwater.

📝 Teacher's Note: Use examples like Mumbai, New York, or London to show students that global warming impacts famous places they know.

🎯 Exam Tip: Focus on the "rise in sea level" as the primary mechanism for city flooding.

Question 14S. How does global warming affect agriculture?
Answer: Global warming will cause drastic changes in the patterns of wind, rainfall etc. Thus it will result in low agricultural yield.
In simple words: Since farmers rely on regular rain and predictable weather, global warming messes up their schedules, making it harder to grow enough food.

📝 Teacher's Note: Changes in climate can turn fertile plains into deserts or cause unseasonal floods that drown crops before they can be harvested.

🎯 Exam Tip: Use the term "agricultural yield" to describe the amount of food produced.

Question 15S. Suggest two ways to minimize global warming.
Answer:
(i) Use of renewable sources of energy to generate electricity in place of generating electricity from the fossil fuels based power plants.
(ii) Controlling population through family planning, welfare reforms and the empowerment of women.
In simple words: We can help by switching to clean power like wind and solar, and by making sure the human population doesn't grow faster than the Earth can handle.

📝 Teacher's Note: Switching to renewable energy reduces the "input" of greenhouse gases, while population control reduces the overall "demand" for energy and resources.

🎯 Exam Tip: "Renewable energy sources" is the most important technical solution to mention in exams.

Question 16S. What is meant by a 'carbon tax'?
Answer: The tax calculated on the basis of carbon emission from industry, number of employee hour and turnover of the factory is called carbon tax. This tax shall be paid by industries. This will encourage the industries to use the energy efficient techniques.
In simple words: A carbon tax is like a fine that factories have to pay if they blow out too much smoke. It makes polluting expensive so that companies find cleaner ways to work.

📝 Teacher's Note: This is an economic solution to an environmental problem. It creates a financial incentive for companies to "go green."

🎯 Exam Tip: Mention that the tax is paid by "industries" and its purpose is to "encourage energy efficient techniques."

Question 1M. Which gas is primarily responsible for the greenhouse effect?
Answer: Carbon dioxide
In simple words: CO2 is the main gas that traps heat in our atmosphere.

📝 Teacher's Note: While other gases like methane are stronger per molecule, carbon dioxide is released in such huge amounts that it does the most total damage.

🎯 Exam Tip: Carbon dioxide is the correct answer for the "main" or "primary" greenhouse gas.

Question 2M. Global warming results in an:
Answer: Increase in temperature
In simple words: Global warming literally means the whole world gets warmer.

📝 Teacher's Note: This warming isn't just "hotter summers"; it's a rise in the average temperature of the entire planet's surface.

🎯 Exam Tip: The fundamental result of global warming is the "increase in average effective temperature."

Question 3M. What would be Earth's average temperature without the greenhouse effect?
Answer: Without green house effect, the average temperature of Earth’s surface would have been \( -18^\circ\text{C} \).
In simple words: Without our air's natural heating, the world would be freezing cold.

📝 Teacher's Note: This is a standard theoretical calculation based on how much energy the Earth receives from the Sun vs how much it would radiate away without an atmosphere.

🎯 Exam Tip: Remember the negative sign! \( -18^\circ\text{C} \) is much colder than freezing (\( 0^\circ\text{C} \)).

Question 4M. What has global warming resulted in regarding the oceans?
Answer: The global warming has resulted in the increase in sea levels.
In simple words: As the world heats up, ice melts and water expands, making the oceans deeper.

📝 Teacher's Note: Sea levels rise for two reasons: melting land ice (glaciers) adding water, and "thermal expansion" (water taking up more space as it warms).

🎯 Exam Tip: "Rise in sea levels" is the most direct consequence of global warming on the oceans.