別稱錯了空氣的質量

小學科學課堂，在充分考慮學生的可接受性的同時，必須確保傳授知識的科學性。教師要掌握相對嚴謹的科學概念，并深刻地領會它們，才能夠對這些科學概念在學生可理解的基礎上進行通俗化、粗略化的處理，并確保這個處理的過程不出現科學性的錯誤，不對學生的后續學習造成負面影響。

案例：稱空氣的質量

課堂中，為進一步加深學生對空氣有質量的認識，一位教師出示一個裝有空氣的塑料袋（如下圖），提出問題：如何測量這袋空氣的質量？

為了啟發學生的思維，教師參考了上節課的活動，即：

取一個充好氣的皮球和若干豆子，分別放在天平兩端，使其平衡;

把皮球取下，用打氣筒打入8筒空氣，放回去后發現天平不平衡了;

這時，在放豆子的一端加入一些豆子，能讓天平再次平衡。

在前一課的教學中，如果不考慮皮球繼續打入8筒空氣后產生的膨脹，用這個方法確實可以大致測出新打入的8筒空氣的質量。但要注意的是，本節課中用這個方法卻不能測出這袋空氣的質量，道理很簡單：

這袋空氣是浸沒在空氣中的物體，周圍的空氣會給它一個向上的浮力，這個浮力的大小就等于它排開空氣的重量;

如果這袋空氣是常壓的，或者說其密度和周圍空氣的密度一樣，它排開空氣的重量就是這袋空氣的重量，這樣稱出來這袋空氣的“重量”會等于零，當然“質量”也等于零了;

如果這個袋子里的空氣壓強大于一個大氣壓，則這袋空氣的重量要大于它所受的浮力，這樣稱出來的“重量”實際是這袋空氣的重量減去一袋常壓空氣的重量;

如果袋子內空氣的壓強不是遠大于一個大氣壓，這樣稱出來“重量”就會明顯小于這袋空氣的實際重量，這樣稱出來的這袋空氣質量就會遠小于它的實際質量。

案例中教師是用塑料袋裝空氣的，袋內氣壓不可能太高，因此用這種方法完全不能夠稱量這袋空氣的質量。

為了證明這一點大家可以做一個類似的實驗，將一個沒有蓋的空礦泉水瓶放在自制的土天平上稱（也可以用托盤天平甚至電子天平稱），不需要稱出具體的質量，只要讓天平保持平衡就行，然后把這個礦泉水瓶壓癟，擠出里面的空氣再稱，會發現天平依然保持平衡，這瓶空氣的“質量”為零！

這個錯誤，并不是每個小學科學教師都能夠發現的，類似在此基礎上的違背科學的“創新”實驗也層出不窮，例如有教師運用現代技術手段，設計了一個“精確”測量空氣質量的實驗：用電子天平先測量一只沒有充氣的氣球的質量，然后給這個氣球充氣，再放在電子天平上稱量，認為兩次稱量的差值就是氣球內空氣的質量。

三年級的學生還沒有學習空氣浮力相關的知識，在這里用浮力的知識去解釋實驗現象不符合學生的認知能力。那么怎樣處理這個問題呢？

很簡單，果斷刪除測量一袋空氣質量的活動，前一節課中，學生看到向皮球中打入8筒空氣以后質量有變化，就足夠建立空氣有質量這一概念了。也可以把真空罩放在電子天平上，比較抽氣前后的質量變化，就能得知空氣有質量。

在上節課中，如果將皮球換為一個打入8筒空氣后也不會形變的容器，就可以用天平比較準確地稱量出這8筒空氣的質量。

張平柯

湖南第一師范學院物理學教授

長沙師范學院特聘教授

湖南省青少年科技教育協會常務副理事長

Don't Misname the Quality of the Air

ZHANG Pingke

中圖分類號：G424??????????????? 文獻標識碼：A???????????????? DOI：10.16400/j.cnki.kjdkx.2021.02.003

ZHANG Pingke

Professor of Physics， Hunan First Normal University

Distinguished Professor of Changsha Normal University

Executive Vice President of Hunan Youth Science and Technology Education Association

In the primary school science classroom， we must ensure the scientificity of imparting knowledge while fully considering the acceptability of students. Teachers should master relatively rigorous scientific concepts and deeply understand them， so that they can popularize and roughly deal with these scientific concepts on the basis of students' understanding， and ensure that there are no scientific mistakes in the process of dealing with them， and that they will not have a negative impact on students' follow-up study.

Case： Weigh the quality of the air

In class， in order to further deepen students' understanding of air quality， a teacher showed a plastic bag containing air （as shown in the figure below） and asked the question： how to measure the air quality of this bag？

In order to inspire students' thinking， the teacher referred to the activities of last class， namely：

Take an inflated ball and some beans and put them on both ends of the balance to balance them;

Take off the ball， use the air pump to inject 8 cylinders of air， put it back and find that the balance is unbalanced;

At this time， add some beans at the end of the bean to balance the balance again.

In the previous lesson， if the expansion of the ball after the ball continues to drive into 8 air tubes， the quality of the newly driven 8 air can be roughly measured by this method. But it should be noted that this method can not measure the air quality of this bag in this lesson. The reason is simple：

This bag of air is an object immersed in the air. The surrounding air will give it an upward buoyancy. The buoyancy is equal to the weight of the air;

If this bag of air is normal pressure， or its density is the same as that of the surrounding air， the weight of the air it displaces is the weight of this bag of air. In this way， the "weight" of this bag of air will be equal to zero， of course， the "mass" will be equal to zero;

If the pressure of the air in the bag is greater than one atmospheric pressure， the weight of the bag of air is greater than the buoyancy it bears. The "weight" is actually the weight of the bag of air minus the weight of a bag of atmospheric air;

If the pressure of air in the bag is not much greater than a atmospheric pressure， the weight will be significantly less than the actual weight of the air in the bag， so the air quality of the bag will be far less than its actual mass.

In the case， the teacher packed the air in a plastic bag， and the air pressure in the bag could not be too high， so this method could not be used to weigh the air quality.

In order to prove this， we can do a similar experiment. Put an empty mineral water bottle without a cover on a homemade earth balance （or use a tray balance or even an electronic balance）. There is no need to weigh out the specific mass. Just let the balance keep balance. Then press the mineral water bottle， squeeze out the air and weigh it again. You will find that the balance is still in balance，the "mass" of this bottle of air is zero！

This error is not found by every primary school science teacher. Similar to the "innovation" experiments against science on this basis， there are endless experiments. For example， some teachers have designed an experiment to measure air quality accurately by using modern technology： measuring the mass of a balloon without inflation with electronic level， then filling the balloon， and then putting it on the electricity The difference between the two weighing is the air mass in the balloon.

The third grade students have not yet learned the knowledge about air buoyancy， so it is not in line with the students' cognitive ability to explain the experimental phenomenon with the knowledge of buoyancy. So how to deal with this problem？

It's very simple to decisively delete the activity of measuring the air quality of a bag. In the previous lesson， the students saw that the air quality changed after 8 cylinders were injected into the ball， which is enough to establish the concept of air quality. You can also put the vacuum cover on the electronic balance and compare the mass changes before and after the air pumping， then you can know the mass of the air.

In the last lesson， if the ball is replaced by a container which will not deform even after being driven into eight cylinders of air， the mass of the eight cylinders of air can be accurately weighed by a balance.