Chapter 6 · ApplicationsEnergy at work in KeralaExplore online

Applications & Explore Online പ്രവൃത്തിയും ഊർജവും

Water falling from the Idukki reservoir lights homes all over Kerala; the KSEB meter counts our electrical energy in units; a swing and a roller coaster trade potential energy for kinetic energy; a pile driver drives the foundations of bridges and metro pillars; crumple zones and helmets save lives; electric trains give energy back when they brake; and rooftop panels turn sunlight into electricity. Every lab below uses only the ideas of this chapter – and the computer works out every number from W = F s, Ep = mgh, Ek = ½mv2 and P = W/t.

📖 The ideas behind every lab

Numbers used: g = 9.8 m/s2, density of water 1000 kg/m3. Values marked “approx.” are typical real values and values marked “example” are made-up but sensible numbers – neither comes from the textbook.

1. Hydroelectric power – Idukki and Moolamattom

Kerala gets much of its own electricity from hydroelectric power stations (ജലവൈദ്യുതനിലയം) run by KSEB. The biggest is the Idukki project: the arch dam (അണക്കെട്ട്) on the Periyar holds a huge reservoir (റിസർവോയർ) high in the hills, and the water is led through tunnels and steep penstocks (പെൻസ്റ്റോക്ക്) down to the underground power house at Moolamattom, roughly 660 m lower (approx.). There six turbines (ടർബൈൻ) turn six generators (ജനറേറ്റർ) of 130 MW each – 780 MW in all (approx.). Two things decide the power: the head h (how far the water falls) and the flow Q (how many cubic metres of water pass each second). Every second, a mass ρQ of water falls through h and gives up potential energy (ρQ) g h – that is the power of the water, P = ρ Q g h. Turbines and generators are not perfect, so the electrical power is a little less: Pelectrical = efficiency × ρ Q g h.

⚡ Hydro lab: set the head and the flow, then open the gates

Example values: the flow of 130 m³/s and the efficiency of 90 % are chosen so that the power comes close to Idukki’s 780 MW; the real flow changes with the demand for electricity and with the amount of water in the reservoir. Water to fill the reservoir comes from the rain on the Western Ghats – and the rain comes from water evaporated by the Sun, so this is really solar energy (Fig 6.11).

📝 In the exam this looks like: “Which form of energy does water stored in a dam have?” → potential energy (due to its position). “Write the energy changes in a hydroelectric power station” → PE (water in the dam) → KE (water in the penstock) → mechanical energy (turbine) → electrical energy (generator) – Table 6.5. A numerical: “1000 kg of water falls through 100 m every second. Find the power of the water.” → W = mgh = 1000 × 9.8 × 100 = 980 000 J every second → P = 980 000 W = 980 kW.

2. Reading the KSEB bill – units are kilowatt-hours

The meter in your home counts electrical energy (വൈദ്യുതോർജം) in units. One unit is the energy used by a 1 kW (1000 W) appliance working for 1 hour: 1 unit = 1 kWh = 1000 W × 3600 s = 3.6 × 106 J. So the energy an appliance uses = its power (in kW) × the time it works (in hours). A 1000 W iron box used for 15 minutes uses 1 × 0.25 = 0.25 unit; a 9 W LED bulb would have to glow for 111 hours to use one unit! KSEB reads the meter once every two months, so a bill covers about 60 days.

🧾 Bill lab: how many units does a Kerala home use?

Powers are typical values (approx.); read the label on your own appliances. A fridge is switched on all day, but its motor runs only part of the time. The cost uses an example rate of ₹6 a unit – the real KSEB tariff has slabs and a fixed charge, so read your own bill.

⚠ Never touch switches or plugs with wet hands, and never open an appliance or the meter box – ask an adult or an electrician.

📝 In the exam this looks like: “A 1000 W iron box is used for 30 minutes a day. How many units does it use in 30 days?” → 1 kW × 0.5 h × 30 = 15 kWh = 15 units. “Express 1 kWh in joule” → 1000 W × 3600 s = 3.6 × 106 J. “Do all appliances use electrical energy at the same rate?” → No – they have different powers (an iron box uses about 100 times as much per second as an LED bulb).

3. Swings and roller coasters – PE ⇄ KE

On a swing or a roller coaster, energy keeps changing form. At the highest point the rider has the most potential energy and is slowest; at the lowest point the potential energy is least and the kinetic energy – and so the speed – is greatest. Without friction and air resistance the total mechanical energy (യാന്ത്രികോർജം) PE + KE never changes (conservation of energy), so a coaster released from 30 m can climb any hill lower than 30 m. With friction, a little energy turns into heat (താപോർജം) on every metre – the swing slowly dies down and the coaster may fail to climb a hill it would otherwise clear. The total, PE + KE + heat, still stays the same.

🎢 Energy lab: release the swing or the roller coaster

Simplified model (example): friction is a constant force of 0.03 × the weight for the coaster and 0.01 × the weight for the swing, so the heat = friction force × distance travelled. Notice that the mass does not change the speed – a heavier rider has more PE and more KE in the same proportion. The heights of the coaster are drawn to scale, but the track is squeezed sideways to fit.

⚠ On a swing, hold the ropes with both hands and never stand in front of a moving swing.

📝 In the exam this looks like: Let us assess Q12 – a ball released from rest at A: at the top KE = 0 and PE = total; just above the ground PE = 0 and KE = total; at every point PE + KE = total (ignoring air). Q8 / Graph 6.1 – for an object thrown up, KE falls in a straight line as the height increases. “At which point of a swing is the speed greatest?” → at the lowest point, where all the PE has become KE.

4. Pile drivers at construction sites

Bridges, flyovers and metro lines such as the Kochi Metro stand on deep foundations called piles – long columns of concrete or steel reaching down to firm ground. One way of putting a pile in is a pile driver: a crane lifts a heavy hammer (the crane does work mgh, stored as potential energy) and lets it fall. Just before it strikes, all that potential energy has become kinetic energy (Fig 6.10). On impact the hammer pushes the pile into the ground against the resisting force of the soil: work done on the soil = force × distance, so the pile sinks a distance s = energy ÷ resisting force with each blow.

🏗️ Pile-driver lab: lift, drop, repeat

The resisting forces of the soils are example values; real soil resists more the deeper the pile goes. The model also ignores the small extra fall of the hammer while the pile moves, and the energy that becomes sound and heat at each blow (you can hear it across the site!). The pile is 12 m long (example).

⚠ Never stand under a load that is being lifted – a crane hook, a pile-driver hammer or a tree branch being cut. Stay outside the barricades of a construction site.

📝 In the exam this looks like: the textbook pile driver – hammer 1000 kg, 10 m above the pile, g = 9.8 m/s2. At the top: Ep = 1000 × 9.8 × 10 = 98 000 J, Ek = 0. Just before it hits: v2 = 2gh = 196, so v = 14 m/s and Ek = ½ × 1000 × 196 = 98 000 J, Ep = 0. The total is the same – the law of conservation of energy. Extra step: if the pile sinks 0.1 m, the average resisting force = 98 000 ÷ 0.1 = 980 000 N.

5. Crumple zones and helmets

A moving car has kinetic energy ½mv2. To stop it in a crash, something must do that much (negative) work on it: F × s = Ek, so the average force is F = Ek ÷ s. The kinetic energy is fixed by the speed – but the stopping distance s is not! Modern cars have a crumple zone in front that crumples on purpose, so the car stops over half a metre instead of a few centimetres: the force on the car and on the belted passengers becomes several times smaller. A rigid steel “crash guard” fixed in front does the opposite – it makes the stop shorter and the force larger (extended activity 4). A helmet works the same way: its foam liner crushes and stops the head over a few centimetres instead of a few millimetres.

🚗 Crash-test lab: same energy, different stopping distance

Example stopping distances: rigid crash guard 0.1 m, crumple zone 0.5 m; head without a helmet 5 mm, with a helmet 30 mm. A belted passenger (60 kg, example) stops over the same distance as the car. The force is an average: the real force rises and falls during the crash.

⚠ Always wear a helmet on a two-wheeler (and a seat belt in a car), even for a short ride – and fasten the strap.

📝 In the exam this looks like: extended activity 4 – “Car bumpers are made to crumple. Why are rigid crash guards dangerous?” → in a crash the car’s kinetic energy must be removed by work, F × s = Ek. Crumpling makes s larger, so the force on the car and the passengers is smaller; a rigid guard stops the car over a tiny distance, so the force is huge. A numerical: “A car with 50 000 J of kinetic energy stops in 0.5 m. Find the average force.” → F = 50 000 ÷ 0.5 = 100 000 N.

6. Regenerative braking – getting the energy back

Ordinary brakes stop a vehicle by friction: all its kinetic energy becomes heat in the brake pads and is wasted. Electric vehicles – e-scooters, electric cars and electric trains such as those of the Kochi Metro – can also brake regeneratively: the electric motor is run as a generator, so the wheels turn the generator and the kinetic energy becomes electrical energy, which charges the battery or is fed back to the power line for other trains. The rest still becomes heat. Energy is not created – it is only changed from one form to another and put to use again.

🚆 Braking lab: where does the kinetic energy go?

Masses and the share of energy recovered are examples (real systems recover very roughly half of the braking energy, depending on the speed, the battery and the line). The braking is drawn with a steady deceleration.

📝 In the exam this looks like: “What energy change takes place in regenerative braking?” → kinetic energy → electrical energy (the motor works as a generator). A numerical: “An electric car of mass 1500 kg moving at 20 m/s (72 km/h) is stopped and 60 % of its kinetic energy is recovered. How much energy is recovered?” → Ek = ½ × 1500 × 202 = 300 000 J; 60 % of it = 180 000 J (= 0.05 kWh).

7. Rooftop solar panels

Many Kerala homes now have solar panels on the roof. A solar cell changes light energy (പ്രകാശോർജം) from the Sun into electrical energy – solar energy (സൗരോർജം) used directly. A panel’s power rating (for example 500 W) is what it gives in full, strong sunshine. Through a day the sunlight is weak in the morning, strongest at noon and weak again in the evening; the energy of a whole day equals that of a few hours of full sunshine – the sun hours (about 4–5 hours a day in Kerala on average, approx.). Some energy is also lost in the wiring, the heat of the panels and the inverter, so: energy per day ≈ number of panels × panel power × sun hours × 0.8.

☀️ Solar lab: how many units does the roof make?

Panel ratings are typical values (approx.); 0.8 is an example allowance for losses. The power–time graph is drawn as a smooth hump whose area – power × time – is the energy of the day, just as the area under a force–displacement graph is the work (Let us assess Q15).

📝 In the exam this looks like: “Name the energy change in a solar cell” → light energy → electrical energy. “A 3 kW solar plant works at full power for 5 hours. How much energy does it give, in kWh and in joule?” → 3 kW × 5 h = 15 kWh = 15 × 3.6 × 106 = 5.4 × 107 J. “Why is the Sun called the main source of energy on the Earth?” → plants, food, fossil fuels, wind, rain (hydroelectricity) and solar cells all get their energy from it (Fig 6.11).

✅ Check yourself

🌐 Explore online – resources hub

Everything in this guide works offline. The links below are extras – websites and simulations from trusted sources (PhET and others), grouped by topic. They need internet and open in a new tab, so you will not lose your place here. Ask a parent or teacher before you go online. The PhET simulations such as Energy Skate Park (named in the textbook) run directly in your browser – nothing to install.

Work, and positive and negative work

🌐 Work, and positive and negative work needs internet · opens in a new tab

Energy, its forms and units

🌐 Energy, its forms and units needs internet · opens in a new tab

Potential energy

🌐 Potential energy needs internet · opens in a new tab

Kinetic energy

🌐 Kinetic energy needs internet · opens in a new tab

Energy transformation and conservation

🌐 Energy transformation and conservation needs internet · opens in a new tab

Hydroelectric power and energy from the Sun

🌐 Hydroelectric power and energy from the Sun needs internet · opens in a new tab

Power, watt and horsepower

🌐 Power, watt and horsepower needs internet · opens in a new tab

James Prescott Joule and James Watt

🌐 James Prescott Joule and James Watt needs internet · opens in a new tab

Saving energy and safer cars

🌐 Saving energy and safer cars needs internet · opens in a new tab

More to explore

🌐 More to explore needs internet · opens in a new tab
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Now pull the whole chapter together: Summary & Quick Quiz ▶