FEATURESwww.iop.org/journals/physedMicrowave oven experiments withmetals and light sourcesMichael Vollmer, Klaus-Peter Möllmann and Detlef KarstädtPhysikalische Technik, Fachhochschule Brandenburg, Magdeburger Straße 50,14770 Brandenburg, GermanyE-mail: [email protected]‘Don’t put metal objects in the microwave’ is common safety advice.But why? Here we describe demonstration experiments involving placinghousehold metallic objects in a microwave oven. These allow a betterunderstanding of the interaction of microwaves with materials. Light bulbsand discharge lamps can also be used in instructive demonstrations.M This article features online multimedia enhancementsIntroductionThe physics of microwave ovens and cooking withmicrowaves has been the topic of three recentarticles [1–3]. A selection of demonstrations hasalready been included in the second and thirdof these; however, microwave ovens (abbreviatedhereafter as microwaves) allow a huge varietyof other experiments which may serve as lecturedemonstrations in the classroom [4]. Thereare also many related websites with images andsometimes video clips (e.g. [5–7]), each givingfurther links. Since microwaves are part ofmany students’ everyday lives, they are a suitabletopic to raise and/or stimulate interest in physics.Many of the experiments can be performed withlittle additional equipment. Some, however,use a thermal infrared camera [8] to visualizetemperature differences in extended objects.In order to visualize the processes within amicrowave oven, we also modified a commercialmicrowave by replacing part of the door with aninfrared-transparent window. Since part of themetal grid is absent some radiation will leak out.For safety reasons a large distance is requiredbetween the observers and the camera/microwave.We have still included some of these experimentsin order to give an insight into the physics. Weemphasize that this modified oven should not berebuilt and experiments should not be repeatedunless all necessary safety regulations are obeyed.Most experiments were performed with justthe objects mentioned being placed in themicrowave. The experiments should also work ifyou place an additional small glass with water inthe microwave, although they may take a little bitlonger. This ensures that no radiation is coupledback into the magnetron, thus extending itslifetime. Some of the easy hands-on experimentsmay be dangerous, for example the explosion oflight bulbs or the sudden ignition of cigarettesor CD-ROMs. We will attempt to indicate thedangers while describing the experiments, but allrepetition of the experiments described here isdone at your own risk.Metals in microwave ovens?One often hears the statement that metals orobjects with metallic parts should never be putinto the microwave (see also [2]). Physicistsknow about the origin of this ‘wisdom’, but theyalso know about its limited range of validity.500 P HYSICS E DUCATION 39 (6) 0031-9120/04/060500+09$30.00 © 2004 IOP Publishing LtdMicrowave oven experiments with metals and light sourcesWhen microwaves interact with metals, theyare effectively absorbed, but the metals alsoreradiate most of the energy [1]. Since metalshave good thermal conductivity (according tothe Wiedemann–Franz law, it is proportional tothe electrical conductivity) the fraction of theenergy that is absorbed is rapidly distributed overthe whole metallic body. If this body is verymassive—like the walls of the microwave oven—then the new equilibrium state, which dependson the absorbed power, the heat capacity andheat losses, corresponds to a very small warming.The behaviour of smaller metal parts, however,depends strongly on their shape and mass. Verythin metal sheets or similar bodies have only avery small heat capacity and can warm up quickly.This can even lead to glowing and evaporation, e.g.from plates with golden edges. One should neverput such plates in a microwave, unless you wish toremove the golden edges.Thin metal wires: miracle candlesThe rapid warming up of thin pieces of metalby several hundred degrees within seconds caneasily be demonstrated with miracle candles. Werecommend using cork as the base for the candle.Depending on their length, the candles may needto be bent in order to fit in the microwave (figure 1).The metal wire in the microwave rapidly heatsup and this leads to self-ignition. The candlemay not ignite at the top and ignition may evenstart at two different points simultaneously (seevideo 1 accompanying the electronic version ofthis article).Figure 1. Miracle candle before treatment in themicrowave oven.If thin metal wires are used in an otherwiseempty microwave, the electric fields at the end ofthe wires can become larger than the breakdownpotential in air of about 106Vm−1. In this case,sparks may be formed and discharges occur (seealso below with cigarettes).Thin metal films: coatings on CD-ROMsand coated glass spheresUnwanted advertising CD-ROMs are perfectcheap samples for showing the rapid warming upof thin metal films. Alternatively, one could useChristmas tree decorations that are coated metalspheres. Figure 2 depicts a CD-ROM before andafter treatment in a microwave. The thin metalcoating warms up very rapidly. Depending on thelocal distribution of the electric fields, structuresare burnt into the CD while part of the metalevaporates (and condenses somewhere else in themicrowave). Safety note! If the CD-ROM staystoo long in the oven, it may start to burn! WeFigure 2. CD-ROM before and after heating in themicrowave.November 2004 P HYSICS E DUCATION 501M Vollmer et al(a)(b)(c)(d)Figure 3. (a) Coated open glass sphere, usually used as a decoration on Christmas trees. Parts (b)to(d)correspond to the situation before (b), during (c) and after (d) treatment in the microwave. In the set-up of (b) and(d) the sphere is placed in front of an operating light bulb in order to demonstrate that in (d) a large part of thecoating has evaporated, since the sphere has become transparent.usually stop the experiment after, say, 3–5 seconds.Even more convincing are the Christmas treedecorations (figure 3(a)). The metal films are verythin and may evaporate to a large extent. Thespheres are open at one spot such that a clamp maybe attached to them. Figure 3(b) shows a sphere(clamp opening pointing downward and not seen)in front of a 100 W light bulb. The coating isthick enough that only the upper rim of the lightbulb is observable. If put in the