an exemplary embodiment of the present invention provides a method for sensor nodes to generate electric power by using magnetic fields existing in the air and to drive themselves by using the generated power . table 1 as below shows the electric field strength or magnetic field strength measured 30 cm away from various exemplary electronic devices . in general , the strengths of electric fields and magnetic fields are inversely proportional to the square of distance . thus , the measured strengths of electric fields and magnetic fields rapidly decrease as the distance from electronic devices increases . in case of an electric razor , for example , if the strength of a magnetic field measured at a point 15 cm away from the electric razor is 150 mg , the strength of the magnetic field measured at a point 30 cm away from the razor is 22 mg . further , the strength of a magnetic field measured at a point 45 cm away from the electric razor is 6 . 7 mg , the strength of the magnetic field measured at a point 60 cm away from the razor is 2 . 6 mg , and the strength measured at a point 90 cm away from the razor is 1 . 5 mg . however , a very strong magnetic field exists on the surface of electronic devices with little magnetic field attenuation . the present invention can be mounted on the surface of electronic devices , so the strength of a magnetic field can be used as it is , without little attenuation thereof . since magnetic and electric fields are interchangeable , electric power can be generated by electric fields existing in the air as well as by magnetic fields existing in the air . as stated above , the home network has mixed electric and magnetic fields caused by electromagnetic waves generated from home appliances . thus , an exemplary embodiment of the present invention uses the mixed electric and magnetic fields on the home network to generate power to be used in the sensor nodes . fig2 is a view for showing a structure of a sensor node 200 according to an exemplary embodiment of the present invention . the sensor node 200 has a coil unit 210 , a rectifier ( or conversion unit ) 212 , a charger 216 , an integrated circuit ( ic ) chip 214 . the coil unit 210 generates electric power by using magnetic fields existing in the air . the induced electromotive force of the coil unit 210 is calculated in equation 1 . wherein n denotes the number of coil turns , d a rate of magnetic flux changes , and dt a rate of time change . as can be appreciated from [ equation 1 ], the induced electromotive force is proportional to the number of coil turns and the rate of magnetic flux changes per unit time . the rectifier 212 converts ac power induced by the coil unit 210 into dc power . the charger 216 sends to the ic chip 214 part of the dc power received from the rectifier 212 , and charges itself with the remaining dc power . the ic chip 214 operates by using the power charged in the charger 216 if any power is not supplied from the rectifier 212 . the charger 216 can be built separately or included in the ic chip 214 or in the rectifier 212 . the ic chip 214 carries out functions established in the sensor node 200 . that is , the ic chip 214 carries out functions of sensing ambient temperatures , detecting intruders , or other sensing functions known in the art . the detailed functions of the ic chip 214 are omitted since the functions are not related to the present invention . fig3 is a view for showing another sensor node according to an exemplary embodiment of the present invention . as shown in fig2 , the sensor node of fig3 has a coil unit 210 , a rectifier 212 , a charger ( not shown ), and ic chip 214 . fig3 shows in detail how the coil unit 210 is formed , for example . as shown in fig3 , the coil unit 210 is formed with coils wound in spirals about the rectifier 212 and the ic chip 214 . the coil unit 210 has coils wound in the two - dimensional plane rather than in the three - dimensional plane for the purposes of downsizing . as stated above , the induced electromotive force caused by the coil unit 210 is sent to the rectifier 212 , and the induced electromotive force is rectified in the rectifier 212 , and sent to the ic chip 214 or the charger 216 . the sensor node is attached to a flexible plate substrate [[,]] or a film 300 , or similar substrate or film known in the art , and the plate substrate or the film 300 is attached to home appliances that generate electromagnetic waves , so that the electric power - generating efficiency can be improved . table 1 shows such efficiencies of home appliances . further , as shown in fig3 , the sensor node has not only one coil unit 210 , but also can have two or more coil units . the plate substrates or films are stacked one on another so a coil unit of at least two substrates or films is formed . by doing so , the coil unit can generate more electric power . if at least two coil units are built together , the individual coil units are interconnected or directly connected to the rectifier . in addition , the sensor node can have a ferrite - coated magnetic substance such as iron core at the center of the plate or the film of the coil unit 210 in order to maximize the induced electromotive force . that is , the ferrite - including magnetic substance increases the induced electromotive force of the coil unit . fig4 shows another coil unit of a sensor node according to an exemplary embodiment of the present invention . in fig4 , one plate substrate or film 400 has plural coil units 210 - 1 to 210 - n formed thereon . for example , fig4 shows that one plate substrate or film 400 has n coil units 210 - 1 to 210 - n formed thereon . the individual coil units 210 - 1 to 210 - n can be built to be interconnected as shown in fig4 , or the individual coil units 210 - 1 to 210 - n each can induce and send an electromotive force to the rectifier 212 . further , at least two plate substrates or films 400 can be stacked together in order that the efficiency of electromotive force generation is improved . the coil units 210 - 1 to 210 - n forming each plate substrate or film 400 can be interconnected or independent , and send an electromotive force to the rectifier 212 , respectively . as stated above , the ferrite - including iron core may be selectively located at the centers of the coil units 210 - 1 to 210 - n in order to increase the induced electromotive force . further , as shown in fig4 , a shielding screen 410 can be formed on the rear side of the plate substrate or film 400 to prevent the offset of electromagnetic waves , thereby increasing the induced electromotive force . that is , if the shielding screen 410 is not formed , there exist , together , electromagnetic waves traveling from the front to the back of the coil units 210 - 1 to 210 - n and electromagnetic waves traveling from the back to the front of the coil units 210 - 1 to 210 - n . thus , the electromagnetic waves traveling from the front to the back of the coil units 210 - 1 to 210 - n collide with the electromagnetic waves traveling from the back to the front of the coil units 210 - 1 to 210 - n . such collision reduces an amount of flux changing per unit time , causing a decrease in the induced electromotive force . therefore , the shielding screen 410 may be formed on the back of the plate substrate or film 400 , so as to cut off the electromagnetic waves traveling from the back to the front of the coil units 210 - 1 to 210 - n . moreover the same effect can be obtained if the shielding screen 410 is formed on the front of the plate substrate or film 400 . the shielding screen 410 can be formed of a metal film known in the art . further , according to another exemplary embodiment of the present invention a method is provided that is capable of preventing the electromagnetic waves traveling from the front to the back of the coil units 210 - 1 to 210 - n from being reflected by the shielding screen 410 . the electromagnetic waves reflected by the shielding screen 410 have the same influence on the electromagnetic waves traveling from the front to the back of the coil units 210 - 1 to 210 - n as the electromagnetic waves traveling from the back to the front of the coil units 210 - 1 to 210 - n have . therefore , the shielding screen 410 has to absorb the electromagnetic waves in order to eliminate the electromagnetic waves reflected by the shielding screen 410 . two methods may be used to absorb the electromagnetic wave : a method of grounding the shielding screen 410 and a method of using impedance matching . the method of grounding the shielding screen 410 is mainly used for low frequencies , and the method of using the impedance matching is mainly used for high frequencies . there is a method of inserting intermediate impedance of ¼ - wavelength between two impedance terminals , that is , the shielding screen and the plate substrate , for the impedance - matching method , but the impedance - matching method has a drawback of taking much space . another impedance - matching method is to build an rf circuit using the smith chart and lc devices on top of the shielding screen or between the shielding screen 410 and the plate substrate or the film 400 . the method of absorbing the electromagnetic waves by using the impedance matching will not be described in detail . however , an impedance - matching rf chip can be inserted on top of the shielding screen 410 or between the shielding screen and the plate substrate or the film 400 . the present invention describes the sensor nodes constituting a home network , but is not limited thereto . that is , the present invention can be applied to any devices performing operations by using electric power . as aforementioned , the present invention generates electric power by using magnetic fields , and uses the generated electric power as a driving source of the sensor nodes . the present invention also generates electric power by using the magnetic fields , so the present invention does not need batteries to be replaced at certain time intervals to drive the sensor nodes , thereby solving the extra cost problem . further , the present invention prevents the failure of the sensor nodes in advance that is caused when users inadvertently fail to replace batteries . the foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention . the present teaching can be readily applied to other types of apparatuses . also , the description of the exemplary embodiments of the present invention is intended to be illustrative , and not to limit the scope of the claims , and many alternatives , modifications , and variations will be apparent to those skilled in the art .