Magnetic resonance imaging (MRI) is a medical technique for producing pictures o

Question

Magnetic resonance imaging (MRI) is a medical technique for producing pictures of the interior of the body. The patient is placed within a strong magnetic field. One safety concern is what would happen to the positively and negatively charged particles in the body fluids if an equipment failure caused the magnetic field to be shut off suddenly. An induced emf could cause these particles to flow, producing an electric current within the body. Suppose the largest surface of the body through which flux passes has an area of0.026 m2 and a normal that is parallel to a magnetic field of 3.0 T. Determine the smallest time period during which the field can be allowed to vanish if the magnitude of the average induced emf is to be kept less than 0.010 V.

Explanation / Answer

induced emf is defiend as the rate of change of magnetic flux

in mathematical form,

induced emf e = NAdB/dt    or NABW

where A = area

N = no. of tunrs

W = angular frequency

dB/dt = rate of change of amgnetic flux

so

dB/dt = emf/NA)

dB/dt = 0.01/(1 * 0.026)

dB/dt = 0.3846

dt = 3/0.3846

dt = 78 secs

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