📖 Chapter 1 of 11 · Green Energy - Advance 1 · Class 8
Chapter 1: How Do Clean Energy Sources Really Make Electricity?
Quick Recap of Basic 2. This chapter revisits ideas from Green Energy — Basic 2 and takes them one step further.
In this chapter
- A fast recap of solar cells, turbines and generators from Basic 2.
- One idea behind almost all power stations: spin a magnet inside a coil.
- Why solar cells are the big exception.
- Meet Meher from Uttar Pradesh and Lalrinpuii from Mizoram.
The Big Idea
Almost every power station on Earth — wind, hydro, coal, gas, geothermal and even nuclear — makes electricity the same way: something spins a turbine, and the turbine spins a generator where magnets move past copper coils. The only thing that changes is what pushes the turbine: wind, falling water or steam. Solar cells are the special exception: they turn light straight into electricity with no moving parts at all.
Did You Know?
India connected all of its regional grids into one synchronised national grid on 31 December 2013. Since then, power made by a wind farm in Tamil Nadu and a solar park in Rajasthan can flow through the same network.
Story Time — The Model That Asked a Question
Meher, age 12 · Lucknow, Uttar Pradesh
Meher’s school in Lucknow announced a science fair on the theme “Powering India”. While her classmates built volcanoes and water filters, Meher decided to build three tiny power stations on one table: a toy solar panel, a cardboard wind turbine and a mini “dam” made from a plastic bottle.
Each model had to light the same small LED. The solar panel worked at once under a table lamp. The wind turbine needed a hair-dryer. The bottle dam needed water poured from high up so it could spin a little plastic wheel.
The judge, a physics professor, studied the turbine and the water wheel. “Both of these have a small motor inside, being used backwards as a generator. Do you know why that works?”
Meher took a breath. “A motor uses electricity to spin a magnet. If I spin the magnet myself, the motor gives electricity back. So wind and water just replace my hand!”
“And the solar panel?” asked the professor. “That is the odd one out,” Meher said. “Nothing spins. Light pushes electrons in the silicon directly.” The professor smiled and wrote something on his clipboard. Meher’s table won the prize for the best question, printed on her poster: “What pushes YOUR turbine?”
Story Time — Behind the Wall at Tuirial
Lalrinpuii, age 13 · Kolasib district, Mizoram
Lalrinpuii, whom everyone calls Puii, lives in the green hills of Mizoram. Her mother is an engineer with the state power department. One Saturday, she took Puii to see the Tuirial hydroelectric project, which began supplying power in 2017.
From the viewpoint, Puii saw a long lake held back by a wall across the Tuirial river. “Mamma, in Basic 2 we learnt that the water spins a turbine. But why does the height of the lake matter so much?”
“Think of a stone,” said her mother. “A stone dropped from your knee barely makes a dent. The same stone dropped from the roof hits hard. Water held high has more potential energy. When it falls down the big pipe to the powerhouse, that energy becomes speed, and the speed spins the turbines.”
Inside the visitor room, a screen showed the output in megawatts. “One megawatt is a million watts,” her mother explained. “On a rainy monsoon day, when the river is full, the number goes up. In the dry months it falls.”
On the drive home, Puii watched the rain begin to fall on the hills. “So every raindrop on these hills is a tiny bit of stored power,” she said. Her mother laughed. “Now you are thinking like an engineer.”
Recap: How a Solar Cell Works
A solar cell is a thin sandwich of specially treated silicon. When sunlight strikes it, the energy of the light frees electrons, and the cell’s built-in electric field pushes them in one direction. Metal contacts collect them, and they flow through an outside circuit as direct current (DC). In Chapter 3 you will look inside that sandwich.
Recap: Turbine + Generator
In 1831, Michael Faraday showed that moving a magnet near a coil of wire makes a current flow. A generator is simply this idea done on a giant scale: huge magnets spin inside thousands of turns of copper wire. Because the magnet keeps turning, the current keeps changing direction — this is alternating current (AC), the kind in our wall sockets.
- Wind power: moving air spins the blades.
- Hydropower: falling water spins the turbine.
- Thermal power (coal, gas, geothermal, nuclear): heat boils water, and high-pressure steam spins the turbine.
- Solar PV: no turbine — light makes current directly.
🔢 Numbers Corner
Energy change in a dam: potential energy = mass × g × height, where g ≈ 9.8 m/s². 1 kg of water falling 100 m releases about 1 × 9.8 × 100 = 980 joules.
Myth or Fact?
Myth: Solar panels and wind turbines make the same kind of electricity as a coal plant.
Fact: Partly true. Wind turbines and coal plants both make AC with generators. Solar panels make DC, which an inverter must change into AC before it can join the grid.
Solved Examples
Work it out first, then check the answer.
1. What do wind, hydro, coal and nuclear power stations have in common?
Show answer
Answer: All of them use a turbine to spin a generator. Only the thing that pushes the turbine is different: wind, water or steam.
2. Why is solar PV called the “odd one out”?
Show answer
Answer: Because it has no turbine or generator. Light frees electrons in the silicon, and the cell’s electric field pushes them into a circuit directly.
3. How much energy does 1,000 kg of water release when it falls 50 m?
Show answer
Answer: Energy = m × g × h = 1,000 × 9.8 × 50 = 4,90,000 joules (490 kJ).
4. What is the difference between AC and DC?
Show answer
Answer: DC flows in one direction only (as from a battery or a solar cell). AC keeps reversing direction many times a second (as from a generator); India’s grid uses 50 reversals per second, called 50 hertz.
5. Why does a hydro plant make more power in the monsoon?
Show answer
Answer: More rain means more water flowing into the reservoir and through the turbines, and more flow means more power.
Check Your Understanding
A. Fill in the blanks
- A generator works by moving a near a coil of copper wire.
- The current from a solar cell is current.
- India’s grid uses alternating current at hertz.
- Water held high up has energy.
- The Tuirial hydroelectric project is in the state of .
B. True or False
- A solar panel contains a turbine. ( ___ )
- Geothermal and nuclear plants use steam to spin turbines. ( ___ )
- One megawatt equals one million watts. ( ___ )
- A hydro plant makes the same power in every season. ( ___ )
C. Choose the correct option
- Who discovered that a moving magnet can make current? (a) Newton (b) Faraday (c) Edison
- Which source does NOT use a turbine? (a) Wind (b) Hydro (c) Solar PV
- The electricity in our wall sockets is (a) DC (b) AC (c) static
D. Answer in detail
- Explain, in your own words, what Meher meant by “What pushes YOUR turbine?”
- Why does the height of a dam’s lake matter?
- Draw a flow chart showing how steam makes electricity in a thermal power plant.
Brain Buster
Find a small toy motor. Connect it to an LED and spin the shaft fast with your fingers. What happens? (Try reversing the LED if nothing lights.)
Check your understanding
Tap an answer to see if it is correct. This is just for practice — nothing is saved.
1. What spins inside almost every power station?
- A turbine connected to a generator
- A solar cell
- A battery
Show answer
A. A turbine connected to a generator
2. Which source makes DC directly?
- A wind turbine
- A solar cell
- A hydro plant
Show answer
B. A solar cell
3. What is India’s grid frequency?
- 25 hertz
- 50 hertz
- 100 hertz
Show answer
B. 50 hertz
4. 1 kg of water falls 10 m. About how much energy is released?
- 9.8 joules
- 98 joules
- 980 joules
Show answer
B. 98 joules
5. Why does water stored high up matter for hydropower?
- It is cleaner
- It has more potential energy
- It is colder
Show answer
B. It has more potential energy