MAGNETS Opposites attract

Transcription

1 MAGNETS Opposites attract This guide offers exercises and experiments at a variety of eves on magnetism. It s an opportunity to revisit the subject matter covered in the Magnetism chapter of the Expor CYT curricuum (former teaching manua for the Canton Vaud). Even though magnetism is not deveoped as a theme in the PER, it coud be a good subject to use as the basis for a OCOM project. It s aso a good foundation from which students can then do experiments for further exporation and to discover some of its technoogica appications. Pease not that the MOTORS worksheet provides an interesting appication on eectromagnets, aowing students to buid a sma motor in a singe period with minima materias required. In this document, you aso find an interdiscipinary activity that can be undertaken in the OCOM context to buid an aternator and thus generate eectricity. Project: EPFL dgeo Soar Impuse Writing: Marie-Noëe Kaempf Graphic design: Anne-Syvie Borter, Repro EPFL Print Center Project foow-up: Yoande Berga 1/8

2 Concepts covered Physique Magnetism Eectromagnets and permanent magnets Current and votage Baance of forces Sciences Magnetic fied and migration Activity duration Introduction to theoretica concepts and exercices : 4 periods Practica activity : 3 periods

3 TERRESTRIAL MAGNETISM Magnetism is first introduced by its most famiiar concrete appication, the compass. If students have never used a compass, they wi have an opportunity to do so. As is pointed out on Figure 1 in the student guide, the Earth s magnetic South Poe (confusingy caed the North Poe) is very cose to its geographica North Poe. An artice on the history and current research on magnetic poes: nationageographic.fr/actuaite/antarctique-a-traque-du-poe-sud-magnetiquecontinue/ Nationa Geographic, Antarctique : La traque du Pôe Sud magnétique continue, Marie Dias-Aves Since these poes are generated by the iquid iron core of the panet which is in constant movement, the magnetic poes can move 40 km per year, and even inverse over much onger periods: fr.wikipedia.org/wiki/inversion_du_champ_magnetique_terrestre Wikipédia, Inversion du champ magnétique terrestre To observe the fied ines around a magnet, a magnetic rod can be dusted with iron fiings. Pace a transparency sheet or a piece of Pexigas over the magnet so the fiings can be easiy recuperated. Extension : construct a compass using a magnetized brad fastener mounted on a pointer. A magnet can aso be paced in a receptace foating on the water, ike a cork equipped with a magnetized pointer. Here s another compass project using a geometric construction of the wind rose: sciencejunior.fr/experiences/construire-une-boussoe Science Junior, Expérience : construire une boussoe, Aoïs, décembre 2010 PERMANENT MAGNETS Get two nais to stick together Students wi magnetize nais using the infuence of the natura magnet. They wi observe the transitory effect of the magnetization of iron or other metas. Making the magnetization disappear coud be an extension of the exercise. Friction or taps acceerate the process. The intensity of the nai s magnetization can aso be tested depending on its proximity to the magnet or time passed in its vicinity. Extension: measure the force of different magnets by ifting items of different weights. MAGNETS- GUIDE 3/8

4 THE REVOLUTION: THE ARTIFICIAL OR ELECTROMAGNET Experiment: Verify the reationship between eectric current and magnetic fied In this setup, to avoid overheating the circuit, incude an additiona resistant eement such as a ightbub, particuary if using a generator. This is an opportunity to refer to the Joue effect, if this concept has been previousy covered with students. If a battery is used, an eectric wire can be briefy connected to one termina or the other, but it heats up rapidy. In a these experiments, students must be reminded to respect warnings on devices or bubs and not exceed specified imits. With this experiment, students wi observe that when eectricity is fowing through the wire, the neede of the compass moves. The magnetic fied is weak around a wire with a sma current. It s thus best to insta the wire in a north-south orientation. In this way they wi observe a symmetrica deviation in the neede when the direction of the current is changed. eectric current I Ask the students to change the direction the current is fowing in the wire, to pace the compass to one side, over, and under the wire if they don t think of it themseves. Before or after the quiz, make sure to emphasize the orientation of the magnetic fied caused by the current fowing in the wire. You can give students this tip: the right thumb is positioned aong the wire in the direction of the eectric current. The tips of the other fingers indicate the direction of the circuar magnetic fied around the wire. magnetic fied B Quiz Sketch the orientation of the compass neede. The red neede indicates north. In this exercise, we assume that the eectric current in the wire is arge enough that the terrestria magnetic fied is negigibe compared to the fied created by the current. Pay carefu attention to the position of the compass in reation to the wire. (cf. remark on the experiment) Remind students to see if the wire is above or beow the compass. A) C) G G B) D) G G 4/8 MAGNETS - GUIDE

5 Figure 6 in the student guide shows how to add together magnetic fieds generated by each turn of the wire in the core of the soenoid. The individua oops cance each other out. In practice, the cois are tighty wound, as in Figure 7. The fied at the center of the coi is more homogeneous. When the coi is onger than it is wide, it is caed a soenoid. In addition to being coied cosey together, the individua cois are aid on top of each other in severa ayers. An od mobie phone charger can be taken apart as an iustration of these soenoids. To ampify the fied of an eectromagnet, a soft iron core can be paced in the soenoid. Under the effect of the magnetic fied of the eectromagnet, it wi itsef aso become a magnet. This effect is deveoped in the permanent magnets section of the student guide. It wi be interesting to see if students use this concept when they tacke the foowing probem. Quiz Using the right hand rue and making comparisons with the iustrations of the section on artificia or eectromagnets, students shoud be abe to indicate the direction of the current and ines of the fied. I S N N S I Who can pick up the most meta with his / her eectromagnet? A bade wi enabe students to strip the eectric wire so it can be hooked up to a battery. The wire can be wrapped around a screw, nai, penci and toothpick- Using a ferromagnetic core for the eectromagnet increases the overa magnetic fied generated. The more times the wire is wrapped around the core, the more stronger the eectromagnet. There is a ink to a short video on how eectromagnets work on that page of guide. MAGNETS- GUIDE 5/8

6 VÉRIFIONS... the intensity B of the magnetic fied in a soenoid is determined using the foowing reationship estabished by Ampere : B = µ 0 B the intensity of the magnetic fied in tesas [T] N the number of times the wire is wrapped around the soenoid (coi) ength of the soenoid in meters [m] (Fig. 7) I intensity of the current in amperes [A] µ 0 magnetic permeabiity of a vaccuum : µ 0 = 4π 10-7 [Tm/A] The ength of the soenoid must be arger than its diameter for the formua to be vaid. For short cois the function can be adapted as foows: r the radius of the coi B = µ 0 2r C) In this experiment, the function adapted to the short coi is: B = µ 0 2r As we wi experimentay verify that the magnetic fied is proportiona to the number of cois on the soenoid and the intensity of the eectric current running in the wire, it s not usefu to give the students two formuas. To be abe to quantify the fied, we wi try to measure the attraction of a magnet to the soenoid. Measurements in part D) and E) require patience. Ask students to think of other ways to test the force of the magnet move a tin can, ift a sma object, etc. It s hard to get accurate measurements. Even so, it s possibe to ceary show that the intensity of the magnetic fied increases with the current and the number of cois. Here are some exampes of measurements obtained. D) For 5 cois: I [A] d [cm] E) For 5 A: N d [cm] d 6/8 MAGNETS - GUIDE

7 ALL THIS IN NUMBERS... The students earn to cacuate the intensity of the magnetic fied in the interior of a soenoid as a numerica appication of a vaue to a function (Exercise 1) then are asked to tacke proportiona or inversey proportiona situations (Exercise 2). As an extension, you coud sketch for the students the intensity of the fied as a function of distance from the wire. Since the intensity of the fied in Tesas is very weak for sma currents, it s aso an opportunity to use scientific notation. Exercice 1 µ B = 0 4π 10 = T = T 0,1 Exercice 2 The intensity of the fied is True Fase 3 times greater if I tripe the number of cois 3 times greater if I space out the cois so it s 15 cm ong twice as arge if I use a 4 A current arger if the cois are smaer in diameter identica if the current is two times ess intense and there are twice as many cois identica if there are twice as many cois but the soenoid is the same identica if there are twice as many cois but the soenoid is two times onger Exercice 3 We appreciate the permanent magnet s abiity to stick to the fridge for a ong time without using eectricity. On the other hand, to pick up a car or can of soup or to drop them again, we appreciate being abe to start or stop the current running in an eectromagnet. For Soar Impuse s eectric motors, we need two kinds of magnets. This exercise can be done at the same time as the one in MOTORS. Eectromagnets are used in oudspeakers, MP3 headphones, etc. MAGNETS- GUIDE 7/8

8 TO EXPLORE FURTHER These three exercises are geared for students who know how to sove an equation and who have aready done vector addition in the context of a chapter on forces in physics. Exercice 4 Exercice 6 = B coi = µ = 4π 10-7 I B resuting = 4π 10-7 I 80 I A = 28 ma : (4π ) soenoid wire B soenoid soenoid axis Exercice 5 You may need to expain to students that the axis of the soenoid is the axis of revoution for the cyinder that it forms. coi axis If needed, show students who have difficuty that the earth s magnetic fied and the magnetic fied of the soenoid are perpendicuar to one another and must be equa in intensity because the ange formed by the neede is 45. tan 25 = B coi B coi = tan 25 µ 0 = tan 25 I = tan 25 µ 0 N 0,05 I = tan π A = 8.7 ma µ 0 N = B coi = µ = 4π ,08 N 0, = 3,2π 10-7 N : (3,2π 10-7 ) N 47 cois 8/8 MAGNETS - GUIDE