Energy: The Essence of Life — Long Answer Questions (Class 9 Physics)
Medium Level (Application & Explanation)
Q1. Explain how many common energy sources (wind, biomass, fossil fuels) are ultimately related to the Sun, and identify which sources are not solar in origin. Give clear examples.
Answer:
The Sun is the primary natural source of energy for Earth. Solar radiation heats the atmosphere and surface, driving many secondary sources:
Wind energy comes from air movement caused by uneven heating of the Earth by the Sun. Thus, wind is solar-driven.
Biomass (wood, crops, dung) stores chemical energy produced by photosynthesis, which uses sunlight. So biomass is also solar-derived.
Fossil fuels (coal, oil, natural gas) are formed from ancient plants and microorganisms that captured solar energy millions of years ago; therefore, they are originally solar.
Energy sources not derived from the Sun include:
Geothermal energy, which comes from the Earth’s interior heat (radioactive decay and primordial heat).
Tidal energy, caused by gravitational pull of the Moon and Sun on Earth’s oceans (mainly gravitational, not solar).
Nuclear energy (from atomic nuclei) comes from nuclear binding energy, not sunlight.
In short, many everyday energy sources trace back to the Sun, but geothermal, tidal, and nuclear have non-solar origins.
Q2. Who was James Prescott Joule and why is the unit of energy named after him? Explain his main contributions and how they relate to the conservation of energy.
Answer:
James Prescott Joule was a 19th-century British physicist known for his experimental work on heat and electricity.
He showed that electrical energy and mechanical work could be converted into heat, and measured the relationship between them. This measurement is called the mechanical equivalent of heat.
Joule’s experiments (like stirring water with a paddle wheel using falling weights) demonstrated that the amount of heat produced equals the mechanical energy lost, supporting the idea that energy is conserved and can change forms but not disappear.
Because of his clear experimental proof and importance to thermodynamics, the SI unit of energy was named the joule (J).
His work directly supports the law of conservation of energy, which states that total energy in a closed system remains constant though it changes forms.
Q3. Describe the difference between potential energy and kinetic energy. Use the slingshot and wound toy car examples to explain energy transformation step by step.
Answer:
Potential energy is the stored energy of an object due to its position or configuration. Kinetic energy is the energy of motion.
In a slingshot:
When the rubber band is pulled back, elastic potential energy is stored in the stretched band.
On release, that potential energy converts into kinetic energy of the projectile, making it fly forward.
Some energy may become sound or heat due to air resistance and friction.
In a wound toy car:
Winding stores elastic potential energy in a spring (or chemical/electrical potential in batteries).
When released, stored energy transforms into kinetic energy of the moving car.
Friction with the surface and internal losses convert some energy to thermal energy.
Both examples show the essential idea: energy changes form (potential → kinetic), and total energy is conserved when you account for all forms including heat and sound.
Q4. Differentiate renewable and non-renewable energy sources with examples. Explain why renewable sources are considered more sustainable and mention one main limitation for each renewable type.
Answer:
Renewable energy sources are those that replenish naturally in a short time. Examples: solar, wind, hydropower, biomass, geothermal.
They are sustainable because they rely on natural processes that continue indefinitely or renew quickly.
Limitations:
Solar is intermittent (no power at night or cloudy days).
Wind varies with weather and location; turbines need steady wind.
Hydropower can affect ecosystems and depends on river flow.
Biomass can compete with food production if not managed well.
Non-renewable energy sources are finite and take millions of years to form. Examples: coal, oil, natural gas, nuclear (uranium).
They can provide continuous power but will deplete and often cause pollution and greenhouse gases.
Renewables are considered more sustainable because they produce less long-term pollution and their sources do not run out on human timescales. The main challenge is reliability and storage, which must be overcome for full replacement.
Q5. Explain mechanical energy in detail. Give formulas for gravitational potential energy and kinetic energy, and provide two practical examples where both forms appear and interchange.
Answer:
Mechanical energy is the sum of an object’s potential energy and kinetic energy. It represents the energy related to an object’s position and motion.
Important formulas:
Gravitational potential energy (PE) = m × g × h, where m is mass (kg), g is acceleration due to gravity (≈ 9.8 m/s²), and h is height (m).
Kinetic energy (KE) = 1/2 × m × v², where v is speed (m/s).
Examples:
A pendulum: At the highest point it has maximum PE and near zero KE. As it swings down, PE converts to KE; at the lowest point KE is maximum.
A roller coaster: At the top of a hill the cars have high PE. As they descend, PE becomes KE, increasing speed. Going up the next hill slows them as KE converts back to PE.
In both examples, mechanical energy is exchanged between PE and KE, and total mechanical energy remains constant if we ignore friction and air resistance.
High Complexity (Analytical & Scenario-Based)
Q6. A stone of mass 5 kg is lifted to a height of 10 m and then dropped. Calculate the potential energy at the height and the speed of the stone just before it hits the ground (ignore air resistance). Explain your steps and connect this result to the conservation of energy.
Answer:
Step 1 — Calculate gravitational potential energy (PE) at height:
PE = m × g × h = 5 kg × 9.8 m/s² × 10 m = 490 J.
Step 2 — When the stone falls (ignoring air resistance), PE converts into kinetic energy (KE). Just before impact, KE ≈ initial PE = 490 J.
Step 3 — Use KE formula to find speed v:
KE = 1/2 × m × v² → v² = 2 × KE / m = 2 × 490 / 5 = 196 → v = √196 = 14 m/s.
Explanation and connection:
The numerical result shows that the stored potential energy of 490 J becomes the kinetic energy of the same magnitude at the bottom. This demonstrates the conservation of mechanical energy: energy is not lost but transforms from one form to another. If friction or air resistance were present, some energy would convert to thermal energy, and KE at the bottom would be less than 490 J.
Q7. Design a simple school experiment to show that wind energy and biomass energy are both ultimately linked to the Sun. Describe materials, procedure, observations, and the conclusion you would draw.
Answer:
Materials: small solar lamp or heat lamp, small fan or pinwheel, potted plant or leaves, paper model of windmill, labels and notebook.
Procedure:
Place the lamp a short distance from the plant and pinwheel. Show that sunlight (lamp) warms air over the plant area.
Use the lamp to heat the air on one side so the pinwheel turns slightly (use safe setup). Explain on Earth this uneven heating produces wind.
Show a dried plant sample (biomass) and explain that living plants use sunlight for photosynthesis to build chemical energy.
Observations:
The lamp can create slight air movement (demonstrative) and plants show growth over time under light.
Dried plant matter burns or can be tested in a small controlled demo to show stored chemical energy release (only if safe and permitted).
Conclusion:
The experiment links wind to uneven heating by the Sun, and biomass chemical energy to photosynthesis driven by sunlight. Thus both are solar-origin sources. Emphasize safety and that real wind and plant growth are driven by large-scale sunlight effects.
Q8. A wind turbine extracts 500 W of electrical power while the power available in the wind passing through its swept area is 2000 W. Calculate the efficiency of the turbine. Discuss why real turbines do not reach 100% efficiency and mention the theoretical limit for wind energy extraction.
Answer:
Calculation:
Efficiency = (useful power output / power available in wind) × 100%
= (500 W / 2000 W) × 100% = 25% efficiency.
Discussion:
Real turbines cannot reach 100% because of losses such as:
Aerodynamic losses (not all wind energy is captured because some air must pass through),
Mechanical friction in bearings and gearbox,
Electrical losses in generators and cables,
Blade design limits and turbulence.
Theoretical limit:
The Betz limit states that the maximum fraction of wind energy a turbine can capture is 59.3% of the kinetic energy in the wind stream. This limit arises because extracting all energy would stop the airflow.
Conclusion:
A real value of 25% is reasonable for many turbines, showing that part of the wind’s energy is converted to electricity while the rest is lost to heat, noise, wake turbulence, and mechanical inefficiencies. Improving design and reducing losses increases efficiency but cannot surpass the Betz limit.
Q9. A rechargeable battery rated 12 V and 5 Ah is used to power a small toy. Ca...
