Question

Q1: Explain under what circumstances eddy currents occur, and what is their direction.

Q2: Explain how magnetic braking works.

Q3: Define magnetic flux. What, if anything is flowing?

Q4: Accurately describe Lenz's rule and give an example.

Q5: If a MRI experiences the current breakdown over a period of 0.2 seconds, meaning, the magnetic field drops from 3 T to 0, what currents can one expect in patient's body? Cf. Example 30.9

Answer 1

1) Eddy currents occur when the object is placed inside a changing magnetic field. It also occurs if the object continuously moves in and out of the system. Such processes, which occur when an object is placed in magnetic field leads to the production of Eddy currents. It flows in planes perpendicular to the magnetic field.

2) Magnetic Braking occurs due to induced currents and Lenz's Law. If we attach a metal plate to the end of a pendulum and let it swing, its speed will greatly decrease when it passes between the poles of the magnet. When the plate enters the magnetic field, an electric field is induced in metal and circulating eddy currents are thus generated. Therefore, these currents act to oppose the change in flux through the plate, in accordance to Lenz's law. These currents in turn heat the plate, thereby reducing it's kinetic energy. This is basically the theory behind how Magnetic braking occurs.

3) Magnetic Flux is defined as the number of magnetic field lines passing through a closed surface. It gives us the measure of the amount of magnetic field that passes through a given surface area. It is a measure of the flow of the magnetic field lines that passes through a given area. So, if the magnetic field lines are flowing, it contributes to the net resultant magnetic flux.

4) Lenz's law states that the current induced in a circuit due to a change in magnetic field is directed to generally oppose the direction of the source that produces it. Lenz's Law governs a major part of magnetism. Example: Thrusting a pole of a permanent bar magnet through a coil of wire. This induces an electric current in a coil. The current sets up a magnetic field inside a coil, making it a magnet. Lenz's law provides us the direction of the induced current.

5) T = 0.2 secs. Change in magnetic field = 3T. So, around 15 A of current can be expected to be there in the patient's body. This should be avoided. It can turn fatal and the process of MRI should be stopped in case of such a breakdown.