College Physics Volume 1 (Chs. 1-16); Mastering Physics with Pearson eText -- ValuePack Access Card -- for College Physics (10th Edition)
10th Edition
ISBN: 9780134151779
Author: Hugh D. Young, Philip W. Adams, Raymond Joseph Chastain
Publisher: PEARSON
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Chapter 21, Problem 61PP
BIO Quenching an MRI magnet. Magnets carrying very large currents are used to produce the uniform, large-magnitude magnetic fields that are required for magnetic resonance imaging (MRI). A typical MRI magnet may be a solenoid that is 2.0 m long and 1.0 m in diameter, has a self-inductance of 4.4 H, and carries a current of 750 A. A normal wire carrying that much current would dissipate a great deal of electric power as heat, so most MRI magnets are made with coils of superconducting wire cooled by liquid helium at a temperature just under its boiling point (4.2 K). After a current is established in the wire, the power supply is disconnected and the magnet leads are shorted together through a piece of superconductor so that the current flows without resistance as long as the liquid helium keeps the magnet cold.
Under rare circumstances, a small segment of the magnet’s wire may lose its superconducting properties and develop resistance. In this segment, electrical energy is converted to thermal energy, which can boil off some of the liquid helium. More of the wire then warms up and loses its superconducting properties, thus dissipating even more energy as heat. Because the latent heat of vaporization of liquid helium Is quite low (20.9 kJ/kg), once the wire begins to warm up, all of the liquid helium may boil off rapidly. This event, called a quench, can damage the magnet. Also, a large volume of helium gas is generated as the liquid helium boils off, causing an asphyxiation hazard, and the resulting rapid pressure buildup can lead to an explosion. You can see how important it is to keep the wire resistance in an MRI magnet at zero and to have devices that detect a quench and shut down the current immediately.
61. If a small part of this magnet loses its superconducting properties and the resistance of the magnet wire suddenly rises from 0 to a constant 0.005 Ω, how much time will it take for the current to decrease to half of its initial value?
- A. 4.7 min
- B. 10 min
- C. 15 min
- D. 30 min
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Chapter 21 Solutions
College Physics Volume 1 (Chs. 1-16); Mastering Physics with Pearson eText -- ValuePack Access Card -- for College Physics (10th Edition)
Ch. 21 - Prob. 1CQCh. 21 - Suppose you drop a cylindrical magnet down a long,...Ch. 21 - A long, straight current-carrying wire passes...Ch. 21 - Two closely wound circular coils have the same...Ch. 21 - Prob. 5CQCh. 21 - Why does a transformer not work with dc current?Ch. 21 - Does Lenzs law say that the induced current in a...Ch. 21 - Does Faradays law say that a large magnetic flux...Ch. 21 - An airplane is in level flight over Antarctica,...Ch. 21 - Prob. 10CQ
Ch. 21 - A metal ring can be moved into and out of the...Ch. 21 - Prob. 12CQCh. 21 - A square loop of wire is pulled upward out of the...Ch. 21 - The two solenoids in Figure 21.36 are coaxial and...Ch. 21 - A metal ring is oriented with the plane of its...Ch. 21 - Prob. 4MCPCh. 21 - A metal loop moves at constant velocity toward a...Ch. 21 - A steady current of 1.5 A flows through the...Ch. 21 - Suppose you continue to hold the current in the...Ch. 21 - A vertical bar moves horizontally at constant...Ch. 21 - The vertical loops A and C in Figure 21.41 e are...Ch. 21 - The vertical loops A and C in Figure 21.41 e are...Ch. 21 - After the switch S in the circuit in Figure 21.42...Ch. 21 - A metal loop is being pushed at a constant...Ch. 21 - A circular area with a radius of 6.50 cm lies in...Ch. 21 - Prob. 2PCh. 21 - An empty cylindrical food container with a lid on...Ch. 21 - A single loop of wire with an area of 0.0900 m2 is...Ch. 21 - A coil of wire with 200 circular turns of radius...Ch. 21 - In a physics laboratory experiment, a coil with...Ch. 21 - A closely wound rectangular coil of 80 turns has...Ch. 21 - Prob. 8PCh. 21 - Prob. 9PCh. 21 - A circular loop of wire a radius of 12.0 cm is...Ch. 21 - A cardboard tube is wrapped with windings of...Ch. 21 - A circular loop of wire is in a soalially uniform...Ch. 21 - Prob. 13PCh. 21 - A solenoid carrying a current i is moving toward a...Ch. 21 - A metal bar is pulled to the right perpendicular...Ch. 21 - Two closed loops A and C are close to a long wire...Ch. 21 - A bar magnet is held above a circular loop of wire...Ch. 21 - The current in Figure 21.54 obeys the equation I =...Ch. 21 - A bar magnet is close to a metal loop. When this...Ch. 21 - A very thin 15.0 cm copper bar is aligned...Ch. 21 - When a thin 12.0 cm iron rod moves with a constant...Ch. 21 - You wish to produce a potential difference of 10 V...Ch. 21 - A 1.41 m bar moves through a uniform, 1.20 T...Ch. 21 - The conducting rod ab shown in Figure 21.58 makes...Ch. 21 - BO Measuring blood flow. Blood contains positive...Ch. 21 - Prob. 26PCh. 21 - Prob. 27PCh. 21 - Prob. 28PCh. 21 - Prob. 29PCh. 21 - Prob. 30PCh. 21 - Prob. 31PCh. 21 - Prob. 32PCh. 21 - Prob. 33PCh. 21 - Prob. 34PCh. 21 - Prob. 35PCh. 21 - A transformer consists of 275 primary windings and...Ch. 21 - You need a transformer that will draw 15 W of...Ch. 21 - A step-up transformer. A transformer connected to...Ch. 21 - Prob. 39PCh. 21 - Prob. 40PCh. 21 - Prob. 41PCh. 21 - A solenoid 25.0 cm long and with a cross-sectional...Ch. 21 - Prob. 43PCh. 21 - Prob. 44PCh. 21 - Prob. 45PCh. 21 - Prob. 46PCh. 21 - Prob. 47PCh. 21 - Prob. 48PCh. 21 - Prob. 49PCh. 21 - A 12.0 F capacitor and a 5.25 mH inductor are...Ch. 21 - Prob. 51PCh. 21 - A 15.0 F capacitor is charged to 175 C and then...Ch. 21 - Prob. 53GPCh. 21 - A rectangular circuit is moved at a constant...Ch. 21 - Prob. 55GPCh. 21 - A flexible circular loop 6.50 cm in diameter lies...Ch. 21 - Prob. 57GPCh. 21 - Prob. 58GPCh. 21 - Consider the circuit in Figure 21.64 (a) Just...Ch. 21 - How many turns does this typical MRI magnet have?...Ch. 21 - BIO Quenching an MRI magnet. Magnets carrying very...Ch. 21 - If part of the magnet develops resistance and...Ch. 21 - BIO Quenching an MRI magnet. Magnets carrying very...Ch. 21 - Prob. 64PPCh. 21 - Consider the brain tissue at the level of the...Ch. 21 - Prob. 66PPCh. 21 - Which graph best represents the time t dependence...
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