張平柯

小學(xué)科學(xué)課堂里，“電磁鐵”內(nèi)容的一個主要環(huán)節(jié)是研究電磁鐵的磁性強(qiáng)弱與線圈圈數(shù)和電流強(qiáng)度的關(guān)系，實驗中常通過電磁鐵吸引大頭針的個數(shù)來進(jìn)行判斷。

在探究完“電磁鐵的磁性強(qiáng)弱與線圈圈數(shù)關(guān)系”后，教師根據(jù)教材提出新的問題：通過電池串聯(lián)，研究增加電池節(jié)數(shù)對電磁鐵磁性的影響。

但是，實際教學(xué)中往往效果很不明顯——隨著電池節(jié)數(shù)的增加，電磁鐵吸引大頭針的個數(shù)幾乎不變，這樣的結(jié)果讓學(xué)生感到迷惑，相當(dāng)一部分一線科學(xué)教師也覺得很奇怪。有好幾位一線教師向我反映了這個情況，問這是為什么，有沒有辦法改進(jìn)這個實驗。

我們來分析一下其中的道理。

取電池電動勢為1.5 V，內(nèi)阻為0.5Ω，可以算得使用1節(jié)電池時，流過線圈的電流約為2.75 A；使用2節(jié)電池時，電池電動勢變?yōu)? V，但內(nèi)阻也變?yōu)?Ω，代入數(shù)據(jù)計算，此時流過線圈的電流為2.87 A；使用3節(jié)電池時，流過線圈的電流為2.91 A。隨著串聯(lián)電池的增加，導(dǎo)致增加微小電流，自然不會出現(xiàn)教材和教師預(yù)想的“電磁鐵磁性明顯增強(qiáng)”。實際上，很多實驗過程中用到的導(dǎo)線截面積遠(yuǎn)遠(yuǎn)大于0.2 mm2，電池幾乎工作在短路狀態(tài)，這種情況下，隨著串聯(lián)電池的增加，通過線圈的電流幾乎不變，不管串聯(lián)幾節(jié)電池，流過線圈的電流都是3 A左右，也不會導(dǎo)致電磁鐵磁性的變化。

教材意識到這個實驗中電池幾乎工作在短路狀態(tài)，因此在后面的內(nèi)容中特別提醒“因為用的導(dǎo)線較短，這個電磁鐵是很耗電的，不要把它長時間接在電池上”。這個提醒很好，但還不夠，電池短路不僅是損傷電池的問題，還存在電池爆炸、起火等危險，中央電視臺專門做過一個節(jié)目，探究不同品牌的干電池短路后的表現(xiàn)，實驗中有一些存在設(shè)計缺陷的干電池在短路一段時間后發(fā)生了爆炸。如果學(xué)生養(yǎng)成了隨意短路電池的習(xí)慣，那是非常危險的，特別是大容量電池——例如汽車上的鉛酸電池短路，以及沒有保護(hù)板的鋰電池短路，會造成爆炸、起火等嚴(yán)重的安全事故。

那么這個實驗怎樣改進(jìn)呢？

很簡單，用細(xì)的漆包線代替那種聚氯乙烯外皮的粗導(dǎo)線就可以了，例如用0.08 mm直徑的漆包線（太細(xì)容易斷）取代直徑0.5 mm的導(dǎo)線，長度可以增加到4 m（如果導(dǎo)線太短，電阻太小，電流密度過高，導(dǎo)線會嚴(yán)重發(fā)熱）。這樣，可以算得導(dǎo)線的電阻約為14Ω，這時用1節(jié)電池，流過線圈的電流約為0.1 A；用2節(jié)電池，流過線圈的電流約為0.2 A，隨著電池節(jié)數(shù)的增加，電流幾乎成正比例增加，效果就非常明顯了。

這種情況下流過線圈的電流變小了，是不是會導(dǎo)致電磁鐵磁性太弱影響實驗結(jié)果？其實不會，用更長更細(xì)的導(dǎo)線可以在同一鐵芯（往往選擇大小適當(dāng)?shù)蔫F釘或螺絲充當(dāng)）上繞更多的圈數(shù)，圈數(shù)增加可以增加電磁鐵的磁性。

在實驗中必須強(qiáng)調(diào)，不要用太粗的導(dǎo)線做這個實驗，粗而短的導(dǎo)線，對電流幾乎沒有阻礙，這種情況稱之為短路，電池短路是一種既損壞電池又充滿危險的行為。

本內(nèi)容教學(xué)中，還有部分教師反映，有的電磁鐵斷電后，鋼釘還能夠吸引一些大頭針，干擾學(xué)生對電磁鐵的認(rèn)識，出現(xiàn)這一現(xiàn)象是因為選用了含碳量很高的鐵釘或螺絲做鐵芯，其通電磁化后斷電，會有剩磁。教學(xué)時可以選擇含碳量低的鐵釘或螺絲，或者選擇含碳量很低的軟鐵絲。此外，還可以開展尋找剩磁最強(qiáng)鐵釘?shù)耐卣够顒印?/p>

The Conductor of Electromagnet Coil is Particular

ZHANG Pingke

中圖分類號：G424文獻(xiàn)標(biāo)識碼：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.