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Overview of Magnetic and Electric Fields: Properties, Forces, and Applications in MRI

This document provides a comprehensive overview of electric and magnetic field properties, including comparisons, field lines, and dipoles. Fundamental concepts such as Biot-Savart Law, Ampere's Law, and the behavior of magnetic forces on moving charges are explored. It details how current-carrying wires, solenoids, and electronic devices function in magnetic environments. The text also discusses the magnetic properties of materials, particularly their applications in MRI technology. Understanding these principles helps to clarify the role of magnetic fields in medical diagnostics and fusion technology.

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Overview of Magnetic and Electric Fields: Properties, Forces, and Applications in MRI

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  1. PHYS 270 – SUPPL. #4 DENNIS PAPADOPOULOS FEBRUARY 2, 2009

  2. OVERVIEW • COMPARE E-FIELD TO B-FIELD PROPERTIES • FIELDS, FIELD LINES, DIPOLES • COMPUTE B-FIELDS DUE TO CURRENTS • BIOT-SAVART, LONG WIRES, CURRENT LOOPS, SOLENOIDS • AMPERE’s LAW – EQUIVALENCE TO GAUSS’s LAW • MAGNETIC FORCES ON MOVING CHARGES • MAGNETIC PROPERTIES OF MATTER - MRI

  3. Magnetic Forces

  4. Work due to magnetic force on a charged particle

  5. MAGNETIC FORCES ON CHARGES Do the B forces do work? e.g. qF=1/2 (mv2) Magnetic force acts to change the direction of v but not the magnitude or the value of the force Centripetal acceleration

  6. PARTICLE ORBITS VAN ALLEN BELTS

  7. FORCES ON WIRES a

  8. FORCES ON WIRES a

  9. Attraction Repulsion MUTUAL LOOP FORCES m

  10. Summary Forces on Charges and Currents Attraction Repulsion

  11. SOLENOIDS What is a solenoid – A device that creates a uniform magnetic field inside and zero outside (in both cases almost uniform and almost zero) Who needs it . Electronic devices, MRI machines, Fusion machines etc

  12. How to make a solenoid

  13. B FIELD INSIDE A SOLENOID n is turns per unit length (e.g. per meter)

  14. MAGNETIC TORQUE

  15. TORQUE ON CURRENT LOOPS

  16. ELECTRIC MOTORS Armature is a coil

  17. MAGNETIC PROPERTIES OF MATTER How does a magnet picks up metal paper clips ? Why not plastic ? How is it different from a charged comb picking up pieces of paper ?

  18. Paired (diamagnetic) repelled by B Mutual cancellation No net m Unpaired (paramagnetic) attracted by B Ferromagnetic

  19. 1. Electrons are microscopic magnets due to their spin 2. Ferromagnetic materials are organized in spin aligned domains 3. In external B produce induced magnetic dipole moment Hysteresis

  20. MRI BODY CONTAINS MAINLY WATER MOLECULES, EACH CONTAINING TWO HYDROGEN ATOMS(TWO PROTONS). IN A STRONG UNIFORM MAGNETIC FIELD THE MAGNETIC MOMENTS OF THE PROTONS ALIGN WITH THE B-FIELD. WHEN A RADIO-FREQUENCY EM FIELD IS TURNED ON THEY ABSORB SOME ENERGY AND GIVE IT BACK WHEN IT IS TURNED-OFF. DESEASED TISSUE, SUCH AS TUMORS, IS DETECTED BECAUSE THE RELAXATION RATE ( RETURN TO EQUILIBRIUM) DEPENDS ON THE TISSUE.

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