الأسبوع 20 · المحور الثالث، الوحدة الثالثة، المفهوم 3.3 «مصادر الطاقة المتجددة» (أ): طاقة الشمس وطاقة الرياح · 3 دروس
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Mills of wind and water, then and now
Discovery Education Science P4: Term 2, Unit 3, pp. 56–69
GoalDescribe how windmills and watermills worked, compare their advantages and drawbacks, and sort energy sources as renewable or not.
New words
windmill · a machine with large sails or blades that wind turnswatermill · a machine with a wheel that moving water turnsrenewable energy · energy from a source that is not used up, such as sunlight, wind or moving water
Explanation
Long ago there was no electricity. People needed machines to do hard jobs, and one of the commonest was to crush grain into flour at a mill. In a windmill the wind pushed the big sails, which turned stones inside. In a watermill a river or a stream turned a wheel with paddles. Both used renewable energy: the wind and the water are never used up. Neither gave out smoke. They also had drawbacks: when the wind dropped or the stream dried up, the mill stopped, and a mill had to be built where the wind or water was. Today we still use wind and moving water, but with modern turbines. These look different: a tall tower, long thin blades shaped to catch the wind, and a generator that makes electricity, not only turning stones.
Feature
Windmill
Watermill
Energy used
moving air
moving water
Advantage
renewable, no smoke
renewable, no smoke
Drawback
stops when the wind drops
stops if the stream dries up
Worked examples
A miller has 60 kg of grain. A windmill crushes 12 kg of grain every hour on a windy day. After 5 hours it has crushed 12 × 5 = 60 kg. But on a calm day it crushes none.
Item
Energy source
Renewable?
solar calculator
sunlight
yes
petrol car
petrol
no
pinwheel
wind
yes
gas cooker
natural gas
no
Farida sorts the energy sources of devices. A solar calculator uses sunlight: renewable. A petrol car uses petrol: nonrenewable. A pinwheel in the breeze uses wind: renewable. A gas cooker uses natural gas: nonrenewable.
Feature
Old windmill
Modern turbine
Blades or sails
often 4 wide sails
usually 3 long thin blades
Job
turn millstones
turn a generator
Old windmills often had four wide sails. Modern wind turbines usually have three long thin blades on a tall tower. They look different because they are made to catch more wind and drive a generator.
Class activity
Then and now. With a grown-up, find a picture of an old windmill or watermill and one of a modern wind turbine. Draw both. Write two things that are the same and two that are different. Then list three advantages and two drawbacks of using the wind.
Practice
1
What did windmills and watermills do before electricity?
They used wind or water to turn machinery that crushed grain into flour.
2
Give one advantage and one drawback of a windmill.
Advantage: the wind is free and renewable. Drawback: it stops when there is no wind.
3
Which two sources are renewable? Say the letters.
A and C: the Sun and the wind.
4
A windmill crushes 8 kg of grain every hour. How many kg in 6 hours?
