الأسبوع 22 · المحور الثالث، الوحدة الثالثة: المشروع البيني «الجانب المشمس»: الطاقة الشمسية والطبخ · 3 دروس
2
Which materials heat best?
Discovery Education Science P4: Term 2, Unit 3, pp. 84–97
GoalExplain how a solar cooker reflects, absorbs and traps sunlight, and read results to choose the best material for each job.
New words
reflect · to bounce light off a surfaceabsorb · to take in light and turn it into heattrap heat · to keep heat inside so it cannot escape
Explanation
A solar cooker has three jobs. First, it must reflect sunlight, which means bounce it toward the place where we want heat. Shiny materials such as foil do this well. Second, it must absorb the light and turn it into heat. Dark, dull surfaces such as black paper absorb more light than pale ones. Third, it must trap the heat so that it cannot escape into the air. A clear cover such as plastic wrap lets sunlight in and slows the heat on its way out. To choose the best material, we run a fair test. We keep the same amount of water, the same start temperature, the same time in the sun and the same sunny spot. We change only one thing, the material, and then we measure the temperature. The rise in temperature is the end temperature minus the start temperature.
Job
Best material
Why
reflect
shiny foil
bounces light toward the cup
absorb
black paper
dark surfaces take in light
trap heat
clear plastic wrap
light gets in, heat stays
Worked examples
Panel
Water after 20 minutes
foil
36 °C
white card
27 °C
plain paper
24 °C
Test 1, reflecting. Three panels are placed beside identical cups of water that start at 20 °C. After 20 minutes in the sun the cup beside foil is at 36 °C, beside white card 27 °C and beside plain paper 24 °C. The foil gives the biggest rise: 36 − 20 = 16 °C.
Test 2, absorbing. Cups covered in black paper, light cloth or nothing (a clear cup) start at 20 °C. After 20 minutes they read 34 °C, 27 °C and 24 °C. The black paper wins.
Cup
End
Rise
plastic wrap
31 °C
11 °C
cloth lid
26 °C
6 °C
open
23 °C
3 °C
Test 3, trapping. A cup with a plastic-wrap lid reaches 31 °C, one with a cloth lid 26 °C and one left open 23 °C, all from 20 °C. The rises are 11, 6 and 3 degrees. Plastic wrap traps the most heat.
Class activity
Fair test on a sunny day (a grown-up handles the cups and the thermometer). Use three identical cups with the same amount of cool tap water. Cover one with black paper, one with white paper and leave one bare. Put them in the same sunny spot. Write the start temperature, wait 20 minutes, and measure again. Everyone wears a hat and stays in the shade. Never look at the Sun, and never drink the water.
Practice
1
Which material reflected best in Test 1?
Foil.
2
A cup of water starts at 20 °C and ends at 29 °C. What is the rise?
29 − 20 = 9 °C.
3
Why must all the cups hold the same amount of water at the start?
So the test is fair: only the material changes.
4
Which job does a clear plastic lid do for a solar cooker?
