Question

You are evaluating the feasibility of a device that will monitor the breathing of patients in a hospital. The device is an elastic band, containing many loops of a conductive material, that goes around a patient’s torso. As the patient breathes, the size of the band changes. You decide to calculate the number of loops necessary to generate a detectable emf in the band (which must be at least 1 mV). You know that the average value of the Earth’s magnetic field is 5 x 10E-5 T and decide to take the situation where this magnetic field is at a 45° angle to the band. You also decide to use in your model a patient who has a torso with a cross sectional area of 700 cm2 and which increases by 10% when inhaling. An average respiratory rate is 20 breaths each minute.

As a second step in testing the feasibiity of the device from the previous problem, you decide to evaluate possible sources of interference. The magnetic field one meter away from a cell phone is about 10E-10 T when averaged over an area such as the torso band for this device. The magnetic field oscillates at the cell phone's operating frequency (1900 MHz is a typical value), so we can write the magnetic field as a function of time using the equation B(t) = B0sin(2?πft) where B0 = 10E-10 T and f = 1900 MHz. You now need both to estimate the maximum emf that might be induced in the band when it is one meter from an operating cell phone and determine if it is large enough to interfere with the emf induced from the patient’s breathing (10E-3 V). The specifications of the band are given in the previous problem. If you were not able to calculate the number of loops needed in the band, use 6000 as an approximate number.

Answer 1