Physics (Mechanics and Heat)

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n a recent article in Physics Today, Meredith and Redish emphasized the need to.make introductory physics .o,lrr., beneficial for life sciences majors.l In this study,

a lab activity is proposed to measure the intensity of electro-

magnetic waves emitted by cell phones and connect these

measurements to various standards, biological topics, and

personal health. The debate on whether or not cell phones can cause brain

tumors has been going on for years due to the lack of conclu-

sive evidence of cell phone danger; claims of chronic health

problems and fertility problems have been reporte d.2'3 The use

of cell phones, especially among students, is mounting, which

makes studying cetl phone signals exciting while also relating

physics to biology and health. Many students are not aware that

cell phones emit microwave radiation, usually called radio fre-

quency (RF) radiation. A warning in many cell phone manuals

alerts the user of a possible danger of the radiation emitted by

cell phones. Apple, for example, recommends that iPhone users

carrytheir phones at least 10 mm from the body and use hands-

free optioni such as headphones to reduce RF exposu re.4 Wavelength and frequency determine the enetgy carried

by the electromagnetic wave. Electromagnetic energy may be

thought of as being carried by photons or quanta whose energy

E is given by E - hf,where h is Plank's constant andfis the frequency. Gamma rays and x-rays have the highest frequency

of all electromagnetic waves and caffy enough energy per

quantum to break bonds between molecules inside biologi-

cal tissues -these waves are call ed ionizing radiation. Waves

such as light and microwaves are call ed non-ionizing radia-

tionbecause their quanta are insufficient to break molecular

bonds. Though microwaves cannot break bonds, they can heat

biological tissues; such is the case of microwave ovens. That

is because when these waves penetrate biological tissues, they

transfer energy to the molecules, increasing their kinetic ener-

gies and thus increasing their temperatures. This may lead to

adverse health effects either directly due to the heating of the

tissues, such as burns or hemorrhage, or due to a breakdown

of local or systemic temperature regulation.S The Food and

Drug Administration (FDA) has set safety limits for acceptable human exposure at certain frequencies.6 For the frequency

of the microwave oven,2.45 GHz,the FDA placed the human

exposure limit at 5.0 mW/cmz.Butcell phones operate at ava- riety of microwave frequencies ranging from 0.8 to 2.2A0 GHz.

Since the exposure limit depends on frequency, one expects a different allowed level of exposure for every frequency. For

simplicity, we will use 5.0 mW/cm2 as a guide for exposure limits of the cell phone signal, but note that the absorption of EM waves by water drops dramatically with frequency in this regime (a factor of 100 less absorption for every factor of 10 in frequency), so 5.0 mW/cm'^uybe considered an underesti- mate in that sense.T

Harmful biological effects are usually expressed in terms of thermal effects, which are based on the heating of the tissues

during exposure to microwave radiation. Kundi, howevet, at-

gued that other nonthermal biological effects can be caused by

RF radiation.s By looking at the research on mice conducted

by Repacholi et a1.,8 Kundi concluded that 0.1 17 pW lcmz should be the limit of exposure to RF radiation, which is more than 10,000 times less than the standards set forth by the FDA.

While no fixed limit on biological effects can be accurately concluded, we must still be aware of the possible consequences

of microwaves at levels below the FDA standards.

Procedure and results Using a celI phone of a known brand with a visible antenna

of length 3 cln, we measured the intensity of the microwaves

emitted by the phone (while calling) using a reasonably priced

device called Cellsensor purchased through the Internet and

available at Amazon.com. This device comes calibrated at 835

MHz, which is the center frequency of standard cell phones.

The phone was placed on a wooden table at least seven feet

away from any object to reduce reflections and interference.

The intensity I as a function of distance r from the antenna is shown in Fig. 1. The manual of the CellSensor points out,

based on FCC regulations, some of the complications of the RF

fietd intensity measurement when in proximity (within 20 cm) to the emitting antenna. Our measurements in Fig.

1 start at 16 crn, but we will assume that the intensities are more reasonable at distances beyond 20 cmfrom the antenna.

One can see from Fig. 1 that / is within the FDA safety limit of 5.0 mW/cm2 for microwave oven frequency for distances

greater than 20 cm. Howevet the student is alerted to the fact

that safety limits depend on the frequency of the cell phone, as mentioned above, and the 5.0 mW/cm2 limit is used here only as a guide. Note that the intensities measured were not

completely stable at the measurements, but fluctuated periodi-

calIy. The fluctuation of the readings was typically about3}o/o; in Fig. 1 we show data indicative of the maximum values that

Fig. 1. lntensity t of emitted microwaves vs distance r from the antenna of a brand-name cell phone. The solid line is the expo- nential fit produced by KaleidaGraph software.

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236 THE PHYSICS TEACHEF O VOI. 53, APNIL 2015

we saw in our experiment. Also, we detected no measurable background radiation. We alert students to the fact that these measurements do not reflect an accurate level of RF radiation as the presence of the student near the phone and the detector alters the accuracy of the reading. Proper measurements of RF radiation from mobile phones require the use of expensive protessional equipment.

The student then compares the intensity behavior of Fig. 1 to the inverse square behavior 1 lfi using two points on the figure. For example, the student calculatei the intensity ratio Irllzfor two points rL = 2l cmand 12 - 30 cm, expecting a ratio of about 2 if the inverse square law is obeyed by this data. Since Fig. 1 gives about a factor of s for I1l12, the student is led to conclude that the observed intensity falls off faster than L lP . There are many reasons for this deviation from the ll/ behav- ior, including the fact that L lfi is for a point source (versus the 3-cm antenna) and the attenuation of the RF radiation due to the air.

Using graphing software,the student can find an alternative to the LIP behavior. The following exponential equation was found using KaleidaGraph software to be one of the best fits (solid line) for the measured (dots) values in Fig. 1:

I-Ioe-br, (1) where /s is the initial intensity at r = 0 extrapolated from the KaleidaGraph fit, and b is aconstant determined by the Ka- leidaGraph software.

It should be made clear to the student that the fit by Eq. (1) has not been grounded in solid physics principles irere; it is only a representation of a real-world intensity d..uy that does not match the inverse square law behavior. For the more sophisticated student, a modeling that combines the inverse square decrease of a point source with an attenuation factor of the data can be done with the equation 7 - (plfi) e -)rr,where \ is the attenuation factor coefficient of microwaves in air and p could be compared to the total power output of the cell phone. Though there is physics behind this equation, it is still u1 up- proximation for a point source emission.

Recapitulated from their textbook,g students are presented with the theory that shows the relation between the intensity (/) of the electromagnetic wave and the strength of the mag- netic (B) field as follows:

I - (cl24il 82 using any intensity data point from Fig. 1 at distances

greater than 20 cmrEq. (2) gives a value of the magnetic field in the order of 1 0-7 T. The magnetic field is even much higher at r - 5 cm, which is approximately the distance to the brain from the cell phone when placed at the ear.For comparison, uhe static magnetic field on the Earth's surface is of order 10-sT r* hixe the human brain naturally produces its own magnetic fielCs rrith a typical amplitude of I 0-r2 T near the scatp aue to the electric currents associated with neural activity.r0 one can see that, while 100 times less than Earth's static magnetic field, the magnetic field generated by the cell phone is at least 100,000 times stronger than that created by the brain. More- over, according to volkow et a1.,3 the oscillatory frequencies of cell phones correspond to some of the frequencies recorded in neural tissues. The student is then prompted to answer the

question: Can the RF radiation emitted by cell phones influ- ence neural activity when absorbed by the brain? The student might also be challenged to estimate how much radiation might be absorbed when placing water or other materials be- tween the cell phone and the detector.

Gonclusion This activity engages beginning college students as it uti-

lizes a familiar and popular device, the ..tt phone. comparing our measurements with FDA standards and the magnetic field produced by the human brain allows students to Uoia physics, biology, and health. The science of measuring RF radiation is complex as readings are affected by many iariables such as the phone itself and the presence of nearby objects. we let students know that our intensity measurements are not precise due to the unpredictable effect of reflections within the room and off the user; therefore, the comparison to FDA limits or the brain signal is not exact. However, this comparison serves to stimulate students' thinking and inspirer rrorr-physics students-this is very much needed inlntroductory physics courses taken by life sciences majors. This activity can also be used as an opportunity to learn about the properties of waves in general and to explore the science of generutirrg magnetic fields from the electric currents of neurons.

References 1' D' C. Meredith and E. F. Redish, "Reinventing physics for life-

sciences major sl' phys. Today 66 (T), 3g (20r il. 2. M. Dobes, B. Shadbolt, v. G. Khu rana,s. /ain, s. F. Smith, R.

smee, M. Dexter, and R. cook, 'A multicenter study of primary brain tumor incidence in Austral ia 2000-2008:' Ir{e'uro-Oncolo- gy 13 (7),7s3 (201r).

3. N. D. volkow, D. Tomasi, G. /. wang, p. vaska, I.s.Folec F. Telan, D. Xoff, I. Lo, and c. wong, 'rEff.ct of ..tt phone radio- frequenry signal exposure on brain glucose metabolismi, /. Am. Med. Assoc.305, g11 (201t).

4. Apple Inc., http://www.appre.com/legar/rfexposure (2014). 5' M. Kundi, "Environmental Health Issues of Radiofrequency

an d Microwave Exp o sure," http : //www. s alzbur g. gv.at/ Proceedings_(06)_Kundi.pdf (2000)

6. FDA (Food and Drug Administration),,,2007 performance Standards for Microwave and Radio Frequency Emitting prod- ucts i' http : //www. acc e s s data. fda. gov/s cripts/cdrh/ cfdo c s I cfcfr I cFRSearch. cfm ?cFRpart= r 03 O&showFR= I .

7 . I. D. |ackson , classical Electrodynamics, 2nd,. ed. (wiley, lg7 5), p. 29L,Fig.7 .9.

8. M. H. Repacholi, A. Basten, v. Gebski, D. Noonan, I. Finnie, and A. w. Harris "Lymphomas in Ep-piml transgenic mice ex- posed to pulsed 900 MHzelectromagnetic fieldsl; Radiat.Res. 147,631 (1997).

9. R. A. Serway and c. vuill e, college physics,gth ed. (Brooks/ Cole, 2012), p.760.

10. s. I. williamson and L. Kaufman, "Magnetic fields of the hu_ man brain," l. Magn. Magn. Mater. rs (rg), 154g (1gg0).

Wathlq Ahdr"rfi-ffi ezzaq received his PhD in 1 g86 from the lJniversity of lllinois at Chicago. He compteted his post-doctorate work at Michigan State University. Currently he is a professor of physics and director of lntroductory Physics Curriculum at West Virginia llniversity. He has had research activities in diverse areas inctuding studies of magnetic and transport properties of thin fitms and multilayers, studies of magnetic nanopatticles, applied studies retated to heatth and environment, and research in education.

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