Senior 4 practice book · Unit 6 of 10

Sources of Energy in the World

50 questions that complete the Senior 4 quiz for this unit: 23 core and 27 challenge. Easy and hard questions are mixed. Try each question, then tap “Show answer”.

Common misconceptions

Electricity is a source of energy that we take from nature, like wood or sunlight.Electricity is a secondary source (an energy carrier): it must be generated from a primary source such as falling water, methane, peat or sunlight.
Solar and hydro power are "green", so they cost the environment nothing at all.They burn no fuel while running, but dams flood valleys and block fish, and panels and batteries need mining, factory energy and safe disposal.
When we use energy it is used up and disappears.Energy is never destroyed; it is transferred, and most of it ends up as heat spread out in the surroundings, where it is hard to use again.
Sending electricity at a higher voltage pushes more energy out of the cables, so it wastes more.For the same power a higher voltage means a smaller current, and the heating loss in the cables (I²R) falls with the square of the current.

What this unit covers

Topics marked new are not tested much in the quiz, so this book gives them extra questions.

Primary and secondary energy sources; electricity and fuels as energy carriers
Renewable and non-renewable energy sources
Rwanda's energy resources and energy mix (hydro, Lake Kivu methane, peat, solar, biomass)
Solar energy: photovoltaic panels and solar water heaters
Hydropower: power from falling water
Wind, geothermal and biomass (biogas) energy
Fossil fuels and nuclear fission
Energy flow, wasted energy and Sankey diagrams
Environmental effects: greenhouse effect, acid rain, deforestation, erosion
Carbon emissions: comparing sources by CO₂ released per kWh
Transmission of electricity: transformers and the grid
Saving energy at home: kWh, electricity bills and efficient appliances
Storing energy and matching supply to demand (batteries, pumped storage, base and peak load)
Sustainable energy development and energy security

Go to the questions

Questions (1–50)

Easier and harder questions are mixed, just like in a real exam. The 27 harder ones are marked ★ Challenge.

1True or false
Electricity is a primary energy source because it can be taken directly from nature in a usable form.
Show answer
Answer: False
Electricity must be generated from primary sources such as falling water, methane or sunlight; lightning cannot be collected and used.
Common mistakeLearners often think electricity is primary because it comes out of the wall socket ready to use, but that socket is the end of a long conversion chain.
2True or false · ★ Challenge
Methane released into the air without being burned is a stronger greenhouse gas than the carbon dioxide formed when it is burned.
Show answer
Answer: True
Kilogram for kilogram methane traps many times more heat than CO₂, so burning methane (in a stove or at Lake Kivu) is better for the climate than letting it escape.
Common mistakeLearners often think burning gas is always worse than leaving it, but escaped methane warms the climate much more than the CO₂ from burning it.
3True or false
In a Sankey diagram the width of the input arrow equals the total width of all the output arrows, because energy is conserved.
Show answer
Answer: True
Energy is not created or destroyed, so useful output plus wasted output must equal the input.
Common mistakeLearners sometimes leave out an output arrow (such as sound), so their outputs do not add up to the input.
4True or false · ★ Challenge
Geothermal heat is counted as renewable because heat keeps flowing out of the hot interior of the Earth, even though a single well can cool down if it is used too fast.
Show answer
Answer: True
The Earth's interior will stay hot for billions of years, so the resource as a whole is renewed, but each well must be managed.
Common mistakeSome learners mark this false because a well can cool, but the classification is about the resource, not one badly managed well.
5Multiple choice · ★ Challenge
Which change at a coal-fired power station would do most to reduce acid rain downwind?
1. AFitting scrubbers that remove sulfur dioxide from the flue gases
2. BBuilding much taller chimneys so that the smoke is carried higher
3. CPainting the cooling towers white to reflect sunlight
4. DRunning the boilers at night when the air is cooler
Show answer
Answer: A. Fitting scrubbers that remove sulfur dioxide from the flue gases
Acid rain comes mainly from sulfur dioxide (and nitrogen oxides); scrubbers remove SO₂ before the gases leave the chimney.
Common mistakeTaller chimneys are tempting, but they only spread the SO₂ farther away; the acid rain still falls somewhere else.
6True or false
A pumped-storage station produces more electrical energy than it uses, because the water falls back down through the turbines.
Show answer
Answer: False
Pumping uses more energy than the falling water gives back (losses in pumps, turbines and pipes); pumped storage moves energy from times of low demand to the peak.
Common mistakeThinking the falling water "adds" energy forgets that the water only returns the energy put in to lift it, minus losses.
7Multiple choice · ★ Challenge
A farmer says: 'Firewood is always renewable because trees grow back.' When is this statement NOT true?
1. AWhen trees are cut faster than new trees grow to replace them
2. BWhen the wood is burned slowly in an improved, low-smoke cookstove
3. CWhen the trees grow on hills rather than in valleys
4. DWhen the firewood is dried in the sun before use
Show answer
Answer: A. When trees are cut faster than new trees grow to replace them
Biomass is renewable only if it is replaced as fast as it is used; cutting faster than regrowth uses the forest up.
Common mistakeThe cookstove option is tempting because stoves save wood, but the stove changes how much wood is needed, not whether the wood is replaced.
8Fill in the blank
The smallest demand that the grid must supply at every hour of the day and night is called the ______ load.
Show answer
Answer: base
Base load is supplied by stations that run all the time; extra stations are switched on to meet the peak load.
Common mistakeWriting "peak" reverses the idea: peak load is the highest demand, which lasts only a few hours.
9Multiple choice · ★ Challenge
Charcoal sold in Kigali markets is made by heating wood in a kiln with little air. How is charcoal best classified?
1. AA primary source, because it is found ready-made in forests
2. BA secondary source, because it is made from wood, a primary source
3. CA non-renewable primary source, just like coal from a mine
4. DNot an energy source at all, because the wood has already been burned
Show answer
Answer: B. A secondary source, because it is made from wood, a primary source
Charcoal does not occur in nature; it is produced from wood by partial burning, so it is a secondary source (an energy carrier).
Common mistakeCalling charcoal primary comes from its similar look to coal; coal is mined from the ground, but charcoal is manufactured from wood.
10Fill in the blank
A battery charged by a solar panel and the hydrogen made by splitting water are both energy ______: they store and move energy that came from a primary source.
Show answer
Answer: carriers
An energy carrier holds energy made from another source so that it can be moved or used later; it is not itself found in nature.
Common mistakeWriting "sources" is wrong here: a battery only stores energy put into it, it does not supply new energy.
11Multiple choice · ★ Challenge
Coal, crude oil and natural gas formed from the remains of plants and animals. Where did the energy stored in them originally come from?
1. AHeat from the Earth's molten core
2. BRadioactive decay in the rocks around them
3. CThe Sun, through photosynthesis long ago
4. DThe pressure of the rock layers above them
Show answer
Answer: C. The Sun, through photosynthesis long ago
Ancient plants stored sunlight as chemical energy by photosynthesis; animals ate the plants; heat and pressure only changed the remains into fuels.
Common mistakePressure is tempting because it helped form the fuels, but pressure did not supply the stored chemical energy; sunlight did.
12Multiple choice
How does extra carbon dioxide in the atmosphere make the Earth warmer?
1. AIt makes a hole in the ozone layer that lets in much more sunlight
2. BIt absorbs infrared from the ground and re-emits some back down
3. CIt reflects more of the incoming sunlight down to the ground
4. DIt reacts with water to release heat in the clouds every day
Show answer
Answer: B. It absorbs infrared from the ground and re-emits some back down
The warm ground radiates infrared; CO₂ absorbs some of it and re-emits part back towards the surface, so less heat escapes to space.
Common mistakeThe ozone-hole option is a common mix-up: ozone loss is a different problem (more ultraviolet), not the cause of global warming.
13Fill in the blank · ★ Challenge
A thermal power station turns 36% of its fuel energy into electricity. In its Sankey diagram the arrows for wasted energy together are ______% of the width of the input arrow.
Show answer
Answer: 64
Wasted = 100% − 36% = 64% of the input.
Common mistakeWriting 36% repeats the useful share; the question asks for everything that is not electricity.
14Multiple choice
In a nuclear power station, what is the heat released by fission used for?
1. ACharging large batteries that later supply the grid
2. BHeating the uranium until it gives out light for panels
3. CBoiling water to make steam that turns the turbines
4. DMelting the waste so that it becomes less radioactive
Show answer
Answer: C. Boiling water to make steam that turns the turbines
A nuclear station is a thermal power station: fission heat makes steam, the steam spins turbines, and the turbines drive generators.
Common mistakeSome learners imagine nuclear energy becomes electricity directly; in fact it goes through heat, steam and a turbine like a coal station.
15Short answer · ★ Challenge
Besides gas for cooking, give two benefits of a biogas digester for a family farm in Rwanda.
Show answer
Model answer: The slurry left after digestion is a good fertiliser for crops. It reduces the need for firewood, saving trees and time spent collecting wood. It also cuts indoor smoke and manages dung and waste hygienically. (Any two.)
Common mistakeSaying biogas "produces no CO₂" is wrong: burning methane gives CO₂, but the carbon came from plants recently, so the net addition is small.
16Multiple choice
What does a biogas digester at a school in Huye mainly produce for cooking?
1. AHydrogen gas, made by splitting water with sunlight
2. BCarbon dioxide gas, which burns with a hot blue flame
3. CSteam, made by burning cow dung inside the sealed digester tank
4. DMethane gas, made by bacteria breaking down dung and food waste
Show answer
Answer: D. Methane gas, made by bacteria breaking down dung and food waste
In a digester, bacteria break down organic waste without air and give off a gas that is mostly methane, which burns well.
Common mistakeCarbon dioxide is tempting because it is in biogas too, but CO₂ does not burn; it is the methane that is the fuel.
17Multiple choice · ★ Challenge
Why is geothermal exploration in Rwanda focused on the area near the Virunga volcanoes and Lake Kivu?
1. AHot rocks lie close to the surface in this volcanic region
2. BThe rocks there contain large amounts of coal, oil and natural gas
3. CThe rainfall there is high, so rivers are fast-flowing
4. DThe air is cold there, so the steam condenses quickly
Show answer
Answer: A. Hot rocks lie close to the surface in this volcanic region
Geothermal power needs very hot rock or steam at a depth that can be drilled; volcanic areas have this heat close to the surface.
Common mistakeThe rainfall option mixes up geothermal with hydropower; geothermal energy comes from heat in the rocks, not from rivers.
18Multiple choice
In a Sankey diagram of a power station, what does the width of each arrow show?
1. AThe amount of energy (or power) that the arrow represents
2. BThe speed at which the energy moves through the station
3. CThe temperature of the part of the station that it comes from
4. DThe order in which the energy changes take place
Show answer
Answer: A. The amount of energy (or power) that the arrow represents
Sankey diagrams are drawn to scale: a wide arrow carries a large share of the energy, a thin arrow a small share.
Common mistakeThe order option is tempting because arrows run left to right, but order is shown by position; the width shows quantity.
19True or false · ★ Challenge
A 2000 W kettle used for 3 minutes uses more electrical energy than a 10 W LED bulb left on for 8 hours.
Show answer
Answer: True
Kettle: 2000 W × 0.05 h = 100 Wh. LED: 10 W × 8 h = 80 Wh. The kettle uses more.
Common mistakeMany learners judge by the time switched on alone; energy depends on power × time, and a high-power device uses a lot even briefly.
20Multiple choice · ★ Challenge
Why does using several different energy sources (an energy mix) make a country's electricity supply more secure?
1. AMixing sources adds their efficiencies, so each kWh costs less to make
2. BA mix of sources means the grid never needs any transformers
3. CIf one source fails, through drought or high fuel prices, the others still supply
4. DSeveral sources make the electricity flow faster along the grid
Show answer
Answer: C. If one source fails, through drought or high fuel prices, the others still supply
Relying on one source is risky: a drought cuts hydro, a price rise makes imported fuel dear; a mix spreads the risk.
Common mistakeThe "efficiencies add" option is wrong: each station keeps its own efficiency, which never adds up to more than 100%.
21Multiple choice
Which of these is a secondary energy source, that is, one produced from another source?
1. ASunlight falling on a roof
2. BWind blowing over a hilltop
3. CCrude oil still underground in an oil well
4. DElectricity from a power station
Show answer
Answer: D. Electricity from a power station
Electricity has to be generated from a primary source (water, fuel, sunlight); the other three are found in nature.
Common mistakeSunlight is tempting because solar panels make electricity from it, but the sunlight itself is the primary source and the electricity is secondary.
22True or false · ★ Challenge
A wind turbine can convert all the kinetic energy of the wind passing through it into electrical energy, because it burns no fuel.
Show answer
Answer: False
The air must keep moving away behind the blades, so only part of its kinetic energy can be taken (at most about 59%); friction and the generator waste more.
Common mistakeThinking "no fuel means no waste" confuses running cost with efficiency; every machine wastes some energy.
23Fill in the blank
In a nuclear reactor, a uranium-235 nucleus absorbs a ______ and splits into two smaller nuclei, releasing energy.
Show answer
Answer: neutron
Fission is started by a slow neutron; each split releases more neutrons, which can keep a chain reaction going.
Common mistakeWriting "proton" or "electron" is wrong: a neutron has no charge, so it is not repelled by the nucleus and can enter it.
24Short answer · ★ Challenge
A student lists: firewood, charcoal, petrol, electricity, sunlight, uranium ore. Sort them into primary and secondary sources and state the rule you used.
Show answer
Model answer: Primary: firewood, sunlight, uranium ore (used as found in nature). Secondary: charcoal (made from wood), petrol (refined from crude oil), electricity (generated from other sources). Rule: a secondary source is produced from a primary one by processing or conversion.
Common mistakePetrol is often put with primary sources because it is a fossil fuel, but it is refined from crude oil in a factory, so it is secondary.
25Multiple choice
What energy change happens in the battery of a solar home system while it is being charged during the day?
1. AElectrical energy to chemical energy
2. BLight energy to electrical energy
3. CChemical energy to electrical energy
4. DElectrical energy to light energy
Show answer
Answer: A. Electrical energy to chemical energy
While charging, the battery stores the electrical energy from the panel as chemical energy; at night it changes it back to electrical energy.
Common mistakeChoosing chemical → electrical describes the battery at night (discharging), not while it is charging.
26Multiple choice · ★ Challenge
Why is very high voltage used on long transmission lines while homes are supplied at only 230 V?
1. AHigh voltage travels faster along wires, but homes cannot use fast energy
2. BHigh voltage cuts current and losses, but it is too dangerous indoors
3. CHome appliances need a larger current than the lines can carry
4. DHigh voltage is cheaper to make, and 230 V costs more to make
Show answer
Answer: B. High voltage cuts current and losses, but it is too dangerous indoors
For a given power, high voltage means small current and small I²R losses on long lines; it is stepped down near users because high voltage is unsafe.
Common mistakeThe "travels faster" option is a misconception: the voltage changes the current, not the speed at which energy travels.
27Multiple choice
Taking the whole life of the station into account, which way of producing electricity releases the least CO₂ per kWh?
1. AA coal-fired power station
2. BA wind farm on a windy ridge
3. CA diesel generator in a town
4. DA peat-fired power station
Show answer
Answer: B. A wind farm on a windy ridge
Wind turbines burn no fuel, so their only emissions come from making and building them, which are small per kWh over their life.
Common mistakePeat is tempting because it comes from plants, but burning peat releases a lot of CO₂ per kWh, close to coal.
28Short answer · ★ Challenge
A methane power station at Lake Kivu releases CO₂ when it burns the gas. Argue whether taking the methane out of the lake and burning it is better or worse for the climate than leaving it there.
Show answer
Model answer: Better overall: burning turns methane into CO₂, which traps much less heat than methane would if it escaped. It also lowers the chance of a sudden release of the dissolved gases, and the electricity can replace diesel generators. It is still worse than solar or hydro, which release almost no CO₂ while running.
Common mistakeA one-sided answer ("it makes CO₂, so it is bad") ignores the comparison: the question is what happens compared with the other choices.
29Short answer · ★ Challenge
Suggest three practical ways a secondary school in Rwanda can cut its electricity bill without reducing teaching time, and explain why each works.
Show answer
Model answer: Replace filament or fluorescent tubes with LED lamps (same light for less power). Switch off lights, projectors and computers at break times and at night, or fit motion sensors (fewer hours of use). Use daylight from large windows and light-coloured walls; use a solar water heater for the kitchen instead of electric heating (less electrical energy needed).
Common mistakeSuggesting "use appliances at night" does not save energy on a flat tariff; it is the power and the hours of use that set the bill.
30Fill in the blank
One kilowatt-hour (kWh) is equal to ______ MJ.
Show answer
Answer: 3.6
1 kWh = 1000 W × 3600 s = 3 600 000 J = 3.6 MJ.
Common mistakeWriting 3600 forgets that 3 600 000 J is 3.6 MJ, not 3600 MJ.
31Multiple choice
At a substation near Kigali, the voltage of the grid is reduced before electricity is sent along the streets to homes. Which device does this?
1. AA step-up transformer
2. BA generator
3. CA large rheostat
4. DA step-down transformer
Show answer
Answer: D. A step-down transformer
A step-down transformer has fewer turns on its secondary coil, so it gives a lower output voltage.
Common mistakeChoosing a rheostat is wrong: reducing voltage with a resistor wastes energy as heat, while a transformer changes voltage with very little loss.
32Multiple choice · ★ Challenge
Learners measure the output of a dusty solar panel at noon as 120 W. After they clean it, it gives 150 W in the same sunshine. By what percentage did the dust reduce the output?
1. A25%
2. B30%
3. C80%
4. D20%
Show answer
Answer: D. 20%
Loss = 150 − 120 = 30 W; percentage = 30 ÷ 150 × 100 = 20% of the clean output.
Common mistakeChoosing 25% comes from dividing 30 W by the dusty 120 W; the reduction must be compared with the original clean output, 150 W.
33Multiple choice · ★ Challenge
A diesel generator's Sankey diagram shows 100 kW of fuel energy going in, 32 kW of electrical output and 45 kW in the hot exhaust gases. The rest goes into the cooling water and noise. What is that remaining power, and what is the efficiency?
1. A77 kW; 32%
2. B23 kW; 68%
3. C55 kW; 45%
4. D23 kW; 32%
Show answer
Answer: D. 23 kW; 32%
Remaining = 100 − 32 − 45 = 23 kW; efficiency = 32 ÷ 100 × 100 = 32%.
Common mistakeChoosing 68% comes from treating the wasted power (45 + 23 = 68 kW) as the useful part; efficiency uses only the useful electrical output.
34Multiple choice
A hotel in Rubavu has a solar water heater and a photovoltaic panel side by side on its roof. What does the solar water heater do that the panel does not?
1. AIt turns light into electricity and then runs a heater coil
2. BIt absorbs sunlight and heats water directly, with no electricity
3. CIt keeps working all through the night using heat stored in the roof tiles
4. DIt changes the chemical energy of the water into heat
Show answer
Answer: B. It absorbs sunlight and heats water directly, with no electricity
A solar thermal collector turns light into internal energy of water directly; a photovoltaic panel turns light into electrical energy.
Common mistakeMany learners think every solar device makes electricity first; solar water heaters have no electrical stage at all.
35Short answer · ★ Challenge
A family in Kayonza finds that its solar panel gives much less energy in April than in July. Suggest two reasons.
Show answer
Model answer: April is in the long rainy season, so clouds cut the sunlight reaching the panel and there are fewer hours of strong sunshine. Rain and humidity also mean more cloudy mornings; in July (dry season) skies are mostly clear. (Dust can lower July output, so cleaning matters too.)
Common mistakeSaying "the Sun is farther away in April" is wrong: Rwanda is near the equator and the change is caused by cloud, not by distance from the Sun.
36Short answer
Name three energy resources used in Rwanda to generate electricity and, for each, give a place where it is used.
Show answer
Model answer: Hydropower: Nyabarongo, Rusumo or Rusizi river stations. Methane gas: Lake Kivu (KivuWatt plant near Karongi/Rubavu). Peat: Gishoma (Rusizi) or the Akanyaru marshes (Gisagara). Solar: the Rwamagana solar field and rooftop or mini-grid systems.
Common mistakeWriting "Lake Kivu gas = natural gas from wells" is a common slip; the methane is dissolved in the deep lake water and is pumped up with it.
37Short answer · ★ Challenge
Describe the Sankey diagram for a 60 W filament lamp that gives out 3 W of light, and state its efficiency.
Show answer
Model answer: One input arrow of width 60 W splits into a thin arrow of 3 W (light, useful) going straight on and a wide arrow of 57 W (heat, wasted) bending down. The widths of the outputs add up to the input. Efficiency = 3 ÷ 60 × 100 = 5%.
Common mistakeDrawing the light arrow wider than the heat arrow is a common error; a filament lamp wastes about 95% of its input as heat.
38Multiple choice
Why does a long dry season worry Rwanda's electricity planners more than it would worry planners in a country that relies on coal?
1. ADry air stops solar panels from producing any electricity
2. BHot weather makes Lake Kivu methane burn much less well in the engines
3. CLower river flows reduce the output of hydropower stations
4. DDust in the dry season blocks the high-voltage lines
Show answer
Answer: C. Lower river flows reduce the output of hydropower stations
A large share of Rwanda's electricity comes from hydropower, and the power available depends on the flow of water in the rivers.
Common mistakeThe solar option is wrong: the dry season is sunnier, so panels give more energy, not less.
39Multiple choice · ★ Challenge
Demand on Rwanda's grid is highest at about 7 pm, when people get home and switch on lights and televisions. Which type of station is best for meeting this short evening peak?
1. AA solar farm without batteries, whose output is greatest at noon
2. BA nuclear station, which takes many hours to change its output
3. CA hydro station with a reservoir, which can start up within minutes
4. DA peat station, which needs hours to heat up its cold boilers
Show answer
Answer: C. A hydro station with a reservoir, which can start up within minutes
A peak lasts only a few hours, so the station must start and stop quickly; opening the gates of a reservoir does this in minutes.
Common mistakeSolar is tempting as the cheapest source, but at 7 pm the Sun has set, so a solar farm with no storage gives nothing at the peak.
40Fill in the blank
A power station releases 1.0 kg of CO₂ for each kWh it produces. A town that uses 5000 kWh per day from it is responsible for ______ tonnes of CO₂ per day.
Show answer
Answer: 5
CO₂ = 5000 kWh × 1.0 kg/kWh = 5000 kg = 5 tonnes.
Common mistakeWriting 5000 forgets to change kilograms to tonnes (1 tonne = 1000 kg).
41Multiple choice · ★ Challenge
A diesel generator at a health centre releases 2.7 kg of CO₂ for every litre burned and uses 0.30 L of diesel per kWh. The centre uses 40 kWh a day. How much CO₂ does it release each day?
1. A108 kg
2. B12 kg
3. C32 kg
4. D0.81 kg
Show answer
Answer: C. 32 kg
Diesel used = 0.30 × 40 = 12 L; CO₂ = 12 × 2.7 = 32.4 kg ≈ 32 kg per day.
Common mistakeChoosing 12 kg stops at the litres of diesel; each litre releases 2.7 kg of CO₂, so you must multiply again.
42Short answer · ★ Challenge
A solar home battery stores 1.8 kWh, but only half of this should be used before recharging. Each night the household uses three 5 W LED bulbs for 5 h and a 30 W television for 3 h. Can the battery cover 4 cloudy days without any recharging? Show your working.
Show answer
Model answer: Usable energy = 0.5 × 1.8 kWh = 0.9 kWh = 900 Wh. Use per night = 3 × 5 × 5 + 30 × 3 = 75 + 90 = 165 Wh. Nights covered = 900 ÷ 165 ≈ 5.5, so yes, it can cover 4 nights.
Common mistakeUsing the full 1.8 kWh ignores the limit on how far the battery may be emptied; deep discharge shortens battery life.
43Short answer
Explain two ways in which improved cookstoves help sustainable development in rural Rwanda.
Show answer
Model answer: They need much less firewood or charcoal for the same cooking, so fewer trees are cut and forests can regrow (resources last for future generations). They produce less smoke indoors, improving the health of women and children, and families spend less time or money collecting fuel.
Common mistakeSaying improved stoves "use no fuel" is wrong; they still burn wood or charcoal, but transfer more of its energy to the pot.
44Multiple choice · ★ Challenge
Two hydropower sites have the same flow of water each second. Site X has a head (height of fall) of 30 m and site Y a head of 120 m. With equal efficiency, how does the power of Y compare with that of X?
1. A2 times as much
2. B4 times as much
3. C16 times as much
4. Dthe same, as the flow is equal
Show answer
Answer: B. 4 times as much
P = η(m/t)gh, so with the same flow P ∝ h; 120 ÷ 30 = 4, so Y gives 4 times the power.
Common mistakeChoosing 16 times comes from squaring the height ratio, as if P ∝ h²; the power is directly proportional to the head.
45Short answer
Explain how cutting trees for firewood on Rwanda's steep hills can lead to soil erosion and also reduce hydroelectric output.
Show answer
Model answer: Tree roots hold the soil and leaves slow the rain. On bare slopes, heavy rain washes the soil away (erosion). The soil is carried into rivers and settles in dam reservoirs (silting), reducing the water they can store and damaging turbines, so less electricity can be produced.
Common mistakeSaying deforestation "only adds CO₂" misses the local effects: erosion and silting directly harm farms and power stations.
46Short answer · ★ Challenge
Most of the energy wasted by a peat or methane power station leaves as warm cooling water. Suggest one way this "waste" heat could be used, and say why this raises the overall usefulness of the fuel.
Show answer
Model answer: The warm water could be piped to dry tea leaves, coffee or maize, to warm fish ponds or greenhouses, or to heat nearby buildings. More of the fuel energy is then used for a purpose, so less fuel is needed overall for the same jobs.
Common mistakeSaying the waste heat can be "turned back into fuel" breaks the energy rules; low-temperature heat can be used for heating, not easily for making electricity again.
47Short answer
Explain why most of Rwanda's hydropower stations are built on rivers in the hilly west and north rather than in the flatter east.
Show answer
Model answer: The power from falling water depends on the flow and on the head (P = η(m/t)gh). Rivers in the hilly west and north drop steeply over short distances, giving large heads, while rivers in the flat east fall very little, so the same flow would give much less power.
Common mistakeAnswering "there is more water in the west" misses the main point: even with the same flow, a small drop in height gives little power.
48Short answer · ★ Challenge
A solar mini-grid sends 12 kW at 400 V to a village through cables of total resistance 0.50 Ω. Find the current, the power lost in the cables and the voltage that actually reaches the village.
Show answer
Model answer: I = P/V = 12 000 ÷ 400 = 30 A. Power lost = I²R = 30² × 0.50 = 450 W. Voltage drop = IR = 30 × 0.50 = 15 V, so the village receives 400 − 15 = 385 V.
Common mistakeUsing V²/R = 400² ÷ 0.50 gives 320 kW lost, more than is sent; the 400 V is not across the cables, only the 15 V drop is.
49Multiple choice
A home electricity meter reads 2345.6 kWh at the start of a week and 2371.6 kWh at the end. Electricity costs 200 FRW per kWh. What did the electricity cost for that week?
1. A520 FRW
2. B36 400 FRW
3. C130 FRW
4. D5200 FRW
Show answer
Answer: D. 5200 FRW
Energy used = 2371.6 − 2345.6 = 26.0 kWh; cost = 26.0 × 200 = 5200 FRW.
Common mistakeChoosing 36 400 FRW multiplies by 7 days as well, but the meter difference is already the total for the whole week.
50Short answer · ★ Challenge
A family replaces a kerosene lamp with a solar lantern. The lamp burned 0.05 L of kerosene per hour for 4 hours every night, and each litre of kerosene releases 2.5 kg of CO₂. How much CO₂ does the change save in a year (365 days)?
Show answer
Model answer: Kerosene per year = 0.05 × 4 × 365 = 73 L. CO₂ saved = 73 × 2.5 = 182.5 kg ≈ 180 kg per year.
Common mistakeForgetting to multiply by the 4 hours each night gives a value four times too small; build the yearly use step by step.