张平柯

小學科学课堂里，“电磁铁”内容的一个主要环节是研究电磁铁的磁性强弱与线圈圈数和电流强度的关系，实验中常通过电磁铁吸引大头针的个数来进行判断。

在探究完“电磁铁的磁性强弱与线圈圈数关系”后，教师根据教材提出新的问题：通过电池串联，研究增加电池节数对电磁铁磁性的影响。

但是，实际教学中往往效果很不明显——随着电池节数的增加，电磁铁吸引大头针的个数几乎不变，这样的结果让学生感到迷惑，相当一部分一线科学教师也觉得很奇怪。有好几位一线教师向我反映了这个情况，问这是为什么，有没有办法改进这个实验。

我们来分析一下其中的道理。

取电池电动势为1.5 V，内阻为0.5Ω，可以算得使用1节电池时，流过线圈的电流约为2.75 A；使用2节电池时，电池电动势变为3 V，但内阻也变为1Ω，代入数据计算，此时流过线圈的电流为2.87 A；使用3节电池时，流过线圈的电流为2.91 A。随着串联电池的增加，导致增加微小电流，自然不会出现教材和教师预想的“电磁铁磁性明显增强”。实际上，很多实验过程中用到的导线截面积远远大于0.2 mm2，电池几乎工作在短路状态，这种情况下，随着串联电池的增加，通过线圈的电流几乎不变，不管串联几节电池，流过线圈的电流都是3 A左右，也不会导致电磁铁磁性的变化。

教材意识到这个实验中电池几乎工作在短路状态，因此在后面的内容中特别提醒“因为用的导线较短，这个电磁铁是很耗电的，不要把它长时间接在电池上”。这个提醒很好，但还不够，电池短路不仅是损伤电池的问题，还存在电池爆炸、起火等危险，中央电视台专门做过一个节目，探究不同品牌的干电池短路后的表现，实验中有一些存在设计缺陷的干电池在短路一段时间后发生了爆炸。如果学生养成了随意短路电池的习惯，那是非常危险的，特别是大容量电池——例如汽车上的铅酸电池短路，以及没有保护板的锂电池短路，会造成爆炸、起火等严重的安全事故。

那么这个实验怎样改进呢？

很简单，用细的漆包线代替那种聚氯乙烯外皮的粗导线就可以了，例如用0.08 mm直径的漆包线（太细容易断）取代直径0.5 mm的导线，长度可以增加到4 m（如果导线太短，电阻太小，电流密度过高，导线会严重发热）。这样，可以算得导线的电阻约为14Ω，这时用1节电池，流过线圈的电流约为0.1 A；用2节电池，流过线圈的电流约为0.2 A，随着电池节数的增加，电流几乎成正比例增加，效果就非常明显了。

这种情况下流过线圈的电流变小了，是不是会导致电磁铁磁性太弱影响实验结果？其实不会，用更长更细的导线可以在同一铁芯（往往选择大小适当的铁钉或螺丝充当）上绕更多的圈数，圈数增加可以增加电磁铁的磁性。

在实验中必须强调，不要用太粗的导线做这个实验，粗而短的导线，对电流几乎没有阻碍，这种情况称之为短路，电池短路是一种既损坏电池又充满危险的行为。

本内容教学中，还有部分教师反映，有的电磁铁断电后，钢钉还能够吸引一些大头针，干扰学生对电磁铁的认识，出现这一现象是因为选用了含碳量很高的铁钉或螺丝做铁芯，其通电磁化后断电，会有剩磁。教学时可以选择含碳量低的铁钉或螺丝，或者选择含碳量很低的软铁丝。此外，还可以开展寻找剩磁最强铁钉的拓展活动。

The Conductor of Electromagnet Coil is Particular

ZHANG Pingke

中图分类号：G424文献标识码：ADOI：10.16400/j.cnki.kjdk.2021.18.001

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 primary school science class， one of the main links of "electromagnet" content is to study the rela？ tionship between the magnetic strength of electromag？ net and the coil number and current intensity. In the experiment， we often judge by the number of needles attracted by electromagnet.

Afterexploring"therelationshipbetweenthe magnetic strength of electromagnet and the number of coils"， the teacher put forward a new problem accord？ ing to the textbook： through the series connection of batteries，tostudytheinfluenceofincreasingthe number of batteries on the magnet of electromagnet.

However， in actual teaching， the effect is often not obvious - with the increase of the number of bat？ teries， the number of pins attracted by electromagnets isalmostunchanged，whichpuzzlesstudentsand makes quite a number of front-line science teachers feel strange. Several front-line teachers reflected the situation to me and asked why and if there was any way to improve the experiment.

Let’s analyze the reason.

Generally，theinternalresistanceofthenew No.1 battery is about 0.5Ω（the specific internal re？ sistance is related to the quality of the battery）， the older battery is higher. The conductor used in the il？ lustrationsofthetextbookisrelativelythick，so make a very conservative estimation. If it is a single corewire，itscorediameterisatleast0.5mm， equivalent to the cross-sectional area is about 0.2 mm2， if the resistivity of copper wire is 1.75×10-8Ω？m. According to the formula of resistivity： [R]=ρLS]

It can be calculated that the wire resistance is about 0.045Ω， according to Ohm’s law of the whole circuit： [i = ER + r]

The electromotive force of the battery is 1.5 V and the internal resistance is 0.5 vΩ，It can be cal？ culated that when one battery is used， the current flowing through the coil is about 2.75 a； When two batteries are used， the EMF of the battery becomes 3 V， but the internal resistance also becomes 1 vΩ， The current flowing through the coil is 2.87 a； When using three batteries， the current through the coil is 2.91 a. With the increase of series batteries， result？ ing in the increase of small current， it will not ap？ pear that the "electromagnet magnetism is significantly enhanced" expected by textbooks and teachers. In fact， the cross-sectional area of wires used in many experiments is much larger than 0.2 mm2， and the batteryalmostworksintheshort-circuitstate.In this case， with the increase of series batteries， the current through the coil is almost constant. No mat？ terhowmanybatteriesareinseries，thecurrent through the coil is about 3 A， which will not lead to the change of electromagnet magnetism.

Thetextbookrealizesthatthebatteryalmost works in the short circuit state in this experiment， so in the following content， it is specially reminded that"because the wire used is short， this electromagnet consumes a lot of power， so don’t connect it to the battery for a long time". This is a good reminder， but it is not enough. Battery short circuit is not only the problem of damaging the battery， but also the danger of battery explosion and fire. CCTV has done a special program to explore the performance of dry batteries of different brands after short circuit. In the experiment， some dry batteries with design defects ex？ ploded after short circuit for a period of time. If stu？ dents develop the habit of short circuiting batteries at will， it is very dangerous， especially for large ca？ pacity batteries， such as lead-acid batteries in cars and lithium batteries without protective plates， which will cause serious safety accidents such as explosion and fire.

So how can this experiment be improved？

It’s very simple. It’s OK to replace the thick PVC wire with thin enameled wire. For example， if the 0.08 mm diameter enameled wire （too thin and easy to break） is used to replace the 0.5 mm diame？ ter wire， the length can be increased to 4 m （if the wire is too short， the resistance is too small， and the current density is too high， the wire will be seriously heated）. In this way， the resistance of the wire can be calculated to be about 14Ω，At this time， with one battery， the current flowing through the coil is about 0.1 a； With two batteries， the current flowing through the coil is about 0.2 A. with the increase of the number of batteries， the current increases almost in a positive proportion， and the effect is very obvi？ ous.

In this case， the current flowing through the coil becomessmaller，willtheelectromagnetmagnetism be too weak to affect the experimental results？ In fact， it will not. With longer and thinner wires， more turns can be wound on the same iron core （usually with proper size nails or screws）. The increase of turns can increase the magnetism of the electromag？ net.

In the experiment， it must be emphasized that do not use too thickwire to do this experiment， thick and short wire， almost no obstruction to the cur？ rent，thissituationiscalledshortcircuit，battery short circuit is a kind of behavior that not only dam？ ages the battery， but also full of danger.

In the teaching of this content， some teachers re？ flect that after the power failure of some electromag？ nets，thesteelnailscanalsoattractsomepins， which interferes with the students’ understanding of electromagnets. This phenomenon is due to the selec？ tion of iron nails or screws with high carbon content as the iron core， which will have residual magnetism afterthepowerfailureofelectromagnetism.When teaching， we can choose nails or screws with low car？ bon content， or soft wire with low carbon content. In addition， we can also carry out expansion activities to find the strongest remanence nails.