The National Electricity Supply System, Grade 7: Coal to Kettle
One page says South Africa has 50 000 million tons of coal left. Another says 53 million tons. Those differ by a factor of a thousand, and a Grade 7 can work out which one is right.
South Africa burns roughly 250 million tons of coal a year. So:
- 53 million tons ÷ 250 million a year = about a fifth of a year. Ten weeks.
- 50 000 million tons ÷ 250 million a year = 200 years.
And "another 200 years" is exactly what the smaller figure is printed next to. So the 50 000 million is the sound number, the 53 million is out by a thousand, and the sentence disproves itself on its own page.
This is the best arithmetic exercise in the Grade 7 year, because the class is not checking a sum. They are checking whether a published number can possibly be true.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 7 |
| Term | 3 |
| Strand | Energy and Change |
| Topic | The national electricity supply system |
| Status | Class activities, research and a survey. Nothing here is formally assessed |
| Marks | The topic revision is 25. Our worksheet is 40 |
From coal seam to light switch
The grid is an energy system, and the useful thing to teach is the chain of conversions rather than the hardware.
| Stage | What happens | Energy becomes |
|---|---|---|
| 1. The boiler | Coal is burnt to heat water | Chemical to thermal |
| 2. The steam | High-pressure steam is forced at the turbine blades | Thermal to kinetic |
| 3. The turbine | The blades spin a shaft | Kinetic, now rotating |
| 4. The generator | Coils turn inside a magnetic field | Kinetic to electrical |
| 5. The transformers | Voltage is stepped up for the long journey, then down again near you | Electrical, at a different voltage |
| 6. Your house | The kettle, the lights, the geyser | Electrical to thermal, light, kinetic |
South Africa has roughly 28 000 km of high-voltage cable and 33 200 km of low-voltage cable. Voltage is stepped up for transmission because thinner current through a wire wastes less as heat, which is the whole reason the pylons carry such dangerous voltages.
The dynamo case study is accurate and worth doing: Faraday, 1831, a copper disc spun between the poles of a magnet. That is still how almost all of our electricity is made, whether the thing turning the shaft is steam, falling water or wind.
One small thing to watch: a list of power station parts may name a cooling tower that does not appear in the diagram beside it, which labels a condenser instead. They are related but not the same thing: the condenser turns the used steam back into water, and the cooling tower is where that heat is dumped into the air.
Watts are not energy
You will see a graph with its axis labelled "Energy consumption (W)".
The watt is a unit of power, not energy. Power is the rate at which energy is used.
| Quantity | Unit | What it tells you |
|---|---|---|
| Power | Watt (W) or kilowatt | How fast energy is being used. A 2 kW kettle, a 100 W bulb |
| Energy | Joule (J) or kilowatt hour (kWh) | How much has been used altogether. This is what you pay for |
The way to feel the difference: a 2 kW kettle and a 2 kW heater draw power at the same rate. Run the kettle for three minutes and the heater for three hours, and the heater has used sixty times the energy. Same power, very different bill.
And this is the second axis-label error of the Grade 7 year, after a graph labelled "Acceleration (metres per second)" in Term 2. Both make the same mistake: naming a quantity and giving the unit of its rate. Worth telling a class to read the axis before they read the line.
One page argues against the next one
"Reducing use in the long term is not a practical option."
And then the very next unit is entirely about reducing use in the home, with fifteen practical ways to do it.
The next unit is right and the sentence is wrong, and the fifteen actions are all sound and locally apt: a geyser blanket, ceiling insulation, the right size pot on the right size plate, switching the geyser off at the board, shorter showers.
Worth putting the two in front of a class together and asking which one the evidence supports. The answer is on the facing page.
Three things that need a date, or have one
| Claim | Status |
|---|---|
| "Approximately 85 % of all energy in South Africa comes from burning coal" | Dated, and it measures something different from the other coal figure in this book, which says Eskom generates 95 % of our electricity. One is about all energy, the other about electricity, and independent producers have changed both. Any percentage here needs the year attached |
| The energy-saving bulb shown is a CFL | Dated. Compact fluorescents contain mercury and have largely been replaced by LED, which uses less again and has nothing hazardous in it |
| "Underground cables deliver electricity to our homes" | Not usually, in South Africa. Most residential distribution here is overhead, on poles. A learner can check it by looking up at their own street |
The provincial map is good, and it correctly uses Gqeberha. One label to update: Nelspruit was renamed Mbombela in 2009.
The energy accounting in this topic is correct, which matters
The practice test at the end does its energy budgets properly:
| Appliance | Energy in | Energy out | Balances? |
|---|---|---|---|
| Tumble dryer | 2 800 J | 1 540 + 600 + 660 = 2 800 J | Exactly |
| Hairdryer | 2 800 J | 1 000 + 1 000 + 800 = 2 800 J | Exactly |
Both balance to the joule, which is how it should be done, and it is worth praising because an activity earlier in the same term has a television taking 500 J and giving out 600. The exam honours conservation of energy and the activity breaks it. Our insulation and energy saving page covers that one.
One figure in those tables is illustrative rather than real: 660 joules of sound per second from a tumble dryer would be deafening. The arithmetic is the point, not the acoustics.
The survey, which is the best activity here
The skills page on conducting a survey is correct and well structured, and the household electricity survey is genuinely useful because the learner ends up with their own data about their own home.
- List every appliance and find its power rating, which is on a plate or sticker on the appliance itself, in W or kW
- Estimate hours of use per day, honestly rather than optimistically
- Energy in kWh = power in kW x hours. A 2 kW kettle for half an hour is 1 kWh
- Multiply by the tariff to get rands
- Rank the appliances by cost, not by how often they are used
The result is almost always the same and almost always a surprise: the geyser wins. It is not used most often, it just uses a great deal every time. That is why the geyser blanket is first on every energy-saving list.
Our Grade 9 cost of electricity project does this calculation properly with current tariffs, and it is the right place to send a class that wants the full version.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| Two coal reserve figures differ by a thousand | One of them is wrong | Divide each by 250 million tons a year. Only one gives a sensible answer |
| A learner labels an energy axis in watts | The printed graph | Joules or kilowatt hours. Watts are power |
| The survey totals look impossibly high | Power in watts multiplied as though it were kilowatts | Divide watts by 1 000 first. A 100 W bulb is 0,1 kW |
| No appliance rating can be found | The plate is on the back or underneath | Look under the kettle and behind the fridge. Online for the rest |
| The class concludes the TV costs the most | Ranked by hours rather than by energy | Rank by kWh. The geyser almost always wins |
| Two sources disagree on the coal percentage | One is all energy, the other is electricity | Different measures. Record which, and the year |
How the 40 marks are made up
| Part | Marks |
|---|---|
| The grid, and the chain of energy conversions | 10 |
| The coal arithmetic, and checking a published number | 10 |
| Power against energy, and the units | 8 |
| Conserving electricity, and the survey | 8 |
| Spotting a page that argues against itself | 4 |
Nothing in this topic is formally assessed. The topic revision is 25 marks and it adds up correctly. Our worksheet is 40 and the split above is ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, including the coal reserve calculation and the survey table
- Marking memorandum, with what to accept and the five answers that look right and score nothing
Related practicals
- The Cost of Running Household Appliances, Grade 9. The full version of the survey, with current tariffs, as a CAPS project
- Sources of Energy, Grade 7. Where the coal comes from, and the other two coal lifetimes in this book
- Insulation and Energy Saving, Grade 7. The television that makes 600 J out of 500, and the assessed Term 3 project
- Electric Circuits, Grade 8. What the electricity does once it is through your wall
What you need to run it
Nothing from us, and that is three Grade 7 topics now where the honest answer is none.
This topic is reading, arithmetic and a survey of your own home. It needs a calculator, a recent electricity bill or a tariff figure, and a willingness to look at the rating plate on the back of the fridge.
There is no apparatus and we are not going to invent any. The nearest thing to a purchase is a thermometer for the rest of the Energy and Change strand, which our insulation page explains properly, and that belongs to a different topic.
What would genuinely help a class here is a copy of a real municipal bill. Bring your own, with the account number covered.