Pick one before you read on. It is fine to be wrong.
By the end you can
Sort energy resources into renewable and non-renewable
Explain how burning fuels adds carbon dioxide to the air
Compare cooking with wood, gas and sunlight
Amina's family cooks on a wood fire. Her cousin cooks on gas. At Ravi's school the cooks use a box that catches the Sun. All of them are using energy to heat food.
A fuel is a chemical store that we burn to get heat. Wood, charcoal, coal, oil and gas are fuels. An energy resource is anything we get useful energy from. That includes fuels, sunlight, wind and flowing water.
Some resources are renewable. They keep coming, or are replaced as fast as we use them. Sunlight, wind and flowing water are renewable.
Others are non-renewable. Coal, oil and gas took millions of years to form. Once we burn them, they are gone for any time we can imagine.
Draw it first
Draw a sort of energy resources
1 Draw a line down a page. Write renewable on the left and non-renewable on the right.
2 List the resources that keep coming: sunlight, wind and flowing water.
3 List the fuels that took millions of years to form: coal, oil and gas.
Wood sits in the middle. It is renewable only if trees are planted as fast as they are cut down. If not, forests shrink and the wood runs out.
Wood has a row of its own. It is renewable only when trees are replanted as fast as they are cut.
What happens when a fuel burns? Wood, coal, oil and gas all contain carbon. The carbon joins with oxygen from the air. This makes carbon dioxide, a gas, which goes into the air.
A burning fuel takes oxygen from the air. It gives out heat and carbon dioxide.
Carbon dioxide in the air traps some of the heat that rises from the ground. More carbon dioxide traps more heat. That warms the whole Earth a little.
The carbon dioxide works like a blanket. Some heat still escapes, but a thicker blanket sends more back down.
Trees take carbon dioxide from the air as they grow. So wood burned from trees that are replanted can balance out. Wood from forests that are not replanted cannot.
Now cooking. Every fire wastes energy. Only some of the energy in the fuel goes into the pot. An open wood fire puts about 15 J in the pot for every 100 J of wood. A gas stove puts about 60 J.
An open wood fire. Most of the energy warms the air.A gas stove. A lot more of the energy goes into the pot.
Worked example
A pot of beans needs 4800 kJ. An open wood fire is about 15% efficient. How many kJ must the wood hold, and how many kg of wood is that? Dry wood holds about 16000 kJ in each kg.
The same pot needs 4800 kJ. A gas stove is about 60% efficient. How many kJ must the gas hold? How many times as much fuel energy did the wood fire need?
4800 ÷ 60 = 80.
80 × 100 = 8000 kJ.
The wood fire needed 32000 kJ. 32000 ÷ 8000 = 4 times as much.
Answer: 8000 kJ, and the wood fire needs 4 times as much
A solar cooker uses no fuel at all. A shiny surface sends sunlight onto a dark pot. The dark pot takes in the radiation and heats up. It makes no smoke and no carbon dioxide. But it needs strong sunshine, and it cooks slowly.
See it three ways
As a picture
In words
Wood and gas both make carbon dioxide when they burn. Gas is non-renewable. Wood is renewable only when trees are replanted. Sunlight makes no carbon dioxide while you cook.
Try it
Catch the Sun with a dark bottle
You need: two clear plastic bottles of the same size, with caps, water from the same jug, black paper or cloth, and shiny foil or a white cloth, a sunny spot on the ground.
Be safe: Never look at the Sun. If you use foil, never look at sunlight bouncing off it. Keep it turned away from anyone's eyes.
Fill both bottles with the same cool water and put on the caps.
Wrap one bottle in black paper. Wrap the other in shiny foil, or in a white cloth.
Stand them side by side in the sunshine for one hour. Turn the foil away from faces.
Unwrap both. Hold the back of your hand against the outside of each bottle. Feel which one is warmer.
What you will see: The black bottle is warmer. Dark surfaces take in more of the Sun's radiation. Shiny foil or white cloth sends most of it away. Your bottles may differ a little.
If you cannot try it, this is what happens.
A wrong idea many people have
Wood is natural, so burning it adds no carbon dioxide.
Why it feels right: Trees are alive and part of nature, so their smoke seems harmless.
The test that breaks it: Think of a wood fire. It gives off heat, light and smoke. Wood contains carbon, so burning it makes carbon dioxide, like any other fuel.
What is true: Burning wood adds carbon dioxide to the air. Growing trees take it back in, but only if trees are replanted as fast as they are cut.
Key words
fuel
a chemical store we burn to get heat.
energy resource
anything we get useful energy from, such as fuels, sunlight and wind.
renewable
an energy resource that keeps coming or is replaced as fast as we use it.
non-renewable
an energy resource that took millions of years to form and will run out.
carbon dioxide
a gas made when fuels with carbon burn. It traps heat in the air.
Back to your guess
The answer is B: Sunlight and wind will not run out, but coal, oil and gas will.
Coal, oil and gas took millions of years to form. We use them far faster than they form, so they will run out. Sunlight and wind keep coming.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Which of these energy resources is renewable?
See the answer
Sunlight
Sunlight keeps coming every day, so it is renewable. Coal, oil and gas are non-renewable.
If you chose A: Coal took millions of years to form. It cannot be replaced as fast as we burn it. Go to S3.8.8.
If you chose B: Gas took millions of years to form. Once we burn it, it is gone. Go to S3.8.8.
If you chose C: Oil took millions of years to form, so it is non-renewable. Go to S3.8.8.
A family burns gas to cook. What gas does burning add to the air, and what does more of it do?
See the answer
Carbon dioxide, which traps more heat and warms the Earth.
Gas contains carbon. It burns to make carbon dioxide. More carbon dioxide traps more heat.
If you chose A: Burning uses up oxygen from the air. It does not add it. Go to S3.8.8.
If you chose B: Carbon dioxide traps heat, so it warms the Earth. It does not cool it. Go to S3.8.8.
On a sunny day a family wants to cook with no smoke and no carbon dioxide. Which is best?
See the answer
A solar cooker
A solar cooker uses sunlight and burns no fuel. It makes no smoke and no carbon dioxide. It needs strong sun.
If you chose B: A wood fire makes smoke and carbon dioxide, and it is renewable only if the trees are replanted. Go to S3.8.8.
If you chose C: A gas stove burns gas, which makes carbon dioxide and is non-renewable. Go to S3.8.8.
A pot needs 1800 kJ. An open fire is about 15% efficient. How many kJ must the fuel hold?
See the answer
12000 kJ
Fuel energy = 1800 ÷ 15 × 100 = 12000 kJ.
If you chose A: You multiplied by the efficiency. A fire wastes energy, so the fuel must hold more than the pot gets. Divide. Go to S3.8.3.
If you chose B: You added a percentage to kJ, which cannot be added. Divide the useful energy by the efficiency, then multiply by 100. Go to S3.8.3.
If you chose C: That would be a fire that wastes nothing. This fire wastes most of its energy, so it needs much more. Go to S3.8.3.
Is wood a renewable energy resource?
See the answer
Yes, but only if trees are replanted as fast as they are cut.
Trees grow back, so wood can be renewable. If we cut more than we replant, it runs out.
If you chose A: Trees grow back. Wood can be renewable if new trees are planted as fast as old ones are cut. Go to S3.8.8.
If you chose C: Natural does not mean renewable. If trees are cut faster than they grow, forests shrink and the wood runs out. Go to S3.8.8.
From an earlier chapter
Review: a food has about 800 kJ in every 100 g. How much food energy is in 300 g of it?
See the answer
2400 kJ
300 g is 3 lots of 100 g. 3 × 800 = 2400 kJ.
If you chose A: You multiplied the grams by the kJ. First find how many lots of 100 g there are. Go to S3.8.7.
If you chose B: You added. The kJ are for each 100 g, so multiply by the number of lots of 100 g. Go to S3.8.7.
Keep it
Copy this into your exercise book.
Renewable: sunlight, wind, water, and wood if replanted. Non-renewable: coal, oil, gas. Burning fuels makes carbon dioxide, which traps heat. A solar cooker burns nothing.
Draw: A page split in two: renewable and non-renewable. List three resources under each.
