It is used to advance the understanding of brain organization and offers a potential new standard for assessing neurological status and neurosurgical risk. One kind of specialized MRI is functional Magnetic Resonance Imaging (fMRI.) This is used to observe brain structures and determine which areas of the brain “activate” (consume more oxygen) during various cognitive tasks. However, MRI is more expensive than x-ray imaging or CT scanning. Because MRI does not use x-rays or other radiation, it is the imaging modality of choice when frequent imaging is required for diagnosis or therapy, especially in the brain. In the brain, MRI can differentiate between white matter and grey matter and can also be used to diagnose aneurysms and tumors. The brain, spinal cord and nerves, as well as muscles, ligaments, and tendons are seen much more clearly with MRI than with regular x-rays and CT for this reason MRI is often used to image knee and shoulder injuries. They differ from computed tomography (CT), in that they do not use the damaging ionizing radiation of x-rays. When equilibrium exists, the magnetic force, F=qvB, will balance the electric force, F=qE, such that a free charge in the bar will feel no net force.MRI scanners are particularly well suited to image the non-bony parts or soft tissues of the body. The separated charges will create an electric field which will tend to pull the charges back together. It will tend to move negative charge to one end, and leave the other end of the bar with a net positive charge. This force will act on free charges in the conductor. The change in the flux is thus equal to its original value,į i = B A cos q = (0.15T) p(0.12m)² = 6.8×10 -3Tm²Įmf = N ( DF / Dt) = (6.8×10 -3Tm²)/(0.20s) = 3.4×10 -2V = 34mV.Īn interesting application of Faraday's law is to produce an emf via motion of the conductor.Īs a simple example, let's consider a conducting bar moving perpendicular to a uniform magnetic field with constant velocity v.įor this first look, we have just a bar, not a complete conducting loop, and we will consider what happens using just the force on a moving charge, F = qvBsin q. When the loop is stretched so that its area is zero, the flux through the loop is zero. This is a case where the change in flux is caused by a change in the area of the loop.īoth the magnetic field and the angle q remain constant. If it takes 0.20s to close the loop, find the magnitude of the average induced emf in it during this time. The loop is grasped at points A and B and stretched until it closes. The flexible loop in Figure P20.10 has a radius of 12cm and is in a magnetic field of strength 0.15T. Magnetic flux is defined in a similar manner to electric flux.įor a loop of wire with area A, in a magnetic field, B, the magnetic flux, F is given by: We quantify the change in terms of magnetic flux. Magnetic flux will play an important role throughout this chapter.Įxperiments in the 19th century showed that a changing magnetic field can produce an emf. In this chapter, we make that connection, seeing how a magnetic field can produce a potential difference. We have seen that a magnetic field exerts a force on a wire carrying a current, and that a wire carrying a current generates a magnetic field.Ĭurrents are produced by electric fields, so there seems to be some connection between electricity and magnetism. repulsive when the currents are in opposite directions.attractive when the currents are in the same direction.Force Between Two Wires: F / l = m 0 I 1 I 2 / 2 p d.
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