Two Bottle Heaters and a Passing Cloud
Two teams of Grade 4 students each built a solar water heater from a clear plastic bottle painted black inside and filled with 500 mL of water. Team A wrapped its bottle in a clear plastic sleeve; Team B left its bottle open to the air. Both bottles started at 18 °C and were placed on the same concrete pad in full sun at 10:00. At 10:30, Team A read 31 °C and Team B read 27 °C. At 11:00 the wind picked up and a cloud passed over. At 11:30, Team A read 34 °C and Team B read 33 °C. Team A claims its sleeve "traps heat" and is clearly the better design. During the test, the pad's surface temperature rose from 20 °C to 38 °C.
1. Using the numbers, evaluate Team A's claim that the sleeve traps heat and is clearly better.
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The claim is partly supported at best.
- At 10:30 the sleeve is 4 °C ahead (31 vs 27). That fits trapped warm air slowing heat loss.
- But by 11:30 the gap has shrunk to 1 °C (34 vs 33), after wind and cloud. If the sleeve truly trapped heat, the advantage should not nearly vanish.
- One trial each, no repeats, and no measure of how fast each bottle cooled afterwards.
A fairer statement: "The sleeved bottle warmed faster in the first half hour, but the two bottles ended close together, so we cannot yet say the sleeve is clearly better." The class should repeat the test two or three times, swap bottle positions, and record temperatures every 5 minutes so the gap's rise and fall is visible.
2. Identify two variables that changed during the test and explain how each could produce the pattern the class saw.
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1. Wind speed. It rose at 11:00. Moving air carries heat away from a surface, and it does so faster from an open bottle than from one behind a plastic sleeve. Wind alone could close a gap that began as a sleeve effect.
2. Cloud cover. A passing cloud cuts the sunlight arriving at both bottles, so warming slows for both. Because the sleeve bottle was already warmer, its slower warming looks like the sleeve "stopped working," even if nothing about the sleeve changed.
A third, quieter variable: the concrete pad heated from 20 °C to 38 °C, so both bottles gained heat from below as well as above, and contact with the pad may differ slightly. Any of these could produce the pattern without the sleeve being the cause.
3. Write a three-sentence plan for a fair test that would settle whether the sleeve helps.
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- Run four bottles at once - two sleeved and two open - all filled with 500 mL of 18 °C water and placed on the same pad in the same sun, spaced so they do not shade each other.
- Record each bottle's temperature every 5 minutes for 60 minutes, note wind and cloud at each reading, and swap the positions of sleeved and open bottles halfway through so no spot on the pad is favoured.
- Compare the average temperature rise of the sleeved pair with the open pair; if the sleeved bottles are still warmer by more than a few degrees, the sleeve is doing real work.