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Magnetic Field, Properties, Lecture notes of Medical Physics

Medical University VarnaMedical Physics

An introduction to magnetic field, including its generation, magnetic induction, field lines, direction, Lorentz force, work, and applications. It also covers the interaction between current carrying wires, Ampere's Force Law, magnetic dipole moment, and torque on a magnetic dipole. the concepts with examples and formulas. It is useful for students studying physics, electrical engineering, or related fields.

Typology: Lecture notes

2020/2021

Available from 02/01/2022

daniel-raj
daniel-raj 🇧🇬

11 documents

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11. Magnetic Field:
* Magnetic Field is generated by moving electrical charges and acts on
moving charges.
* Any moving charges generates a magnetic field
* Magnetic Induction B, (magnetic flux density) is a vector quantity
which describes the magnitude and the direction of the magnetic field.
* It corresponds to intensity of the electric field.
* Magnetic Field lines: Magnetic induction vector at a point is
tangential to the field line through the point.
* Magnitude of the field is proportion to the density of the filed lines.
* Magnetic Field lines are closed loops and magnetic charges do not
exist.
* Direction of Magnetic induction: Right hand grip rule: Thumb in the
direction of the moving charge and all other fingers in the direction of
the magnetic field.
* Magnitude of the magnetic induction is given by the Biot – Savart
law.
* Lorentz force acts on the electrical charges moving in a magnetic
field. F = qvB sin ф. Direction of the force is determined by the right
hand rule.
pf3
pf4
pf5

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11. Magnetic Field:

  • Magnetic Field is generated by moving electrical charges and acts on moving charges.
  • Any moving charges generates a magnetic field
  • Magnetic Induction B, (magnetic flux density) is a vector quantity which describes the magnitude and the direction of the magnetic field.
  • It corresponds to intensity of the electric field.
  • Magnetic Field lines: Magnetic induction vector at a point is tangential to the field line through the point.
  • Magnitude of the field is proportion to the density of the filed lines.
  • Magnetic Field lines are closed loops and magnetic charges do not exist.
  • Direction of Magnetic induction: Right hand grip rule: Thumb in the direction of the moving charge and all other fingers in the direction of the magnetic field.
  • Magnitude of the magnetic induction is given by the Biot – Savart law.
  • Lorentz force acts on the electrical charges moving in a magnetic field. F = qvB sin ф. Direction of the force is determined by the right hand rule.
  • Work of the Magnetic Field: A magnetic field can do no work. It is perpendicular to the direction of movement. The magnetic field is not conservative.
  • Movement of charged particles in a magnetic field is given by the Lorentz force. The Lorentz force acts as a centripetal force. The particle performs circular motion. The radius of the circle depends on the mass, charge and velocity of the particle and the magnetic induction of the field.
  • Applications: Used to control movement of charges particles in Electron Microscopes, Mass spectrometers, Electric measuring instruments, CRTs of TVs and Monitors.
  • Forces between Current carrying wires: Current carrying wires interact with each other by a magnetic force. The Lorentz force acts on charges moving in another wire.
  • The interaction force between two current elements is given by Amperes Force Law. Two parallel wires carrying current in the same direction will attract each other. Def. of Ampere : Two infinitely long and thin parallel wires spaced apart by 1m in vacuum carry a current of 1 A if the force on each wire per unit length is exactly 2x10^-1 N/m.
  • Magnetic dipole moment of a current loop: Direction is determined the right-hand grip rule. Magnitude: Pm = IS, where I is the current and S is the area.
  • Torque on a Magnetic Dipole: In an external magnetic field, a torque is generated and the magnetic moment aligns itself to the field lines of the magnetic induction.
  • Ferromagnetic Substances: Consist of numerous magnetic domains in which the magnetic moments of the molecules/atoms are aligned. Ex. Iron, cobalt, nickel. Above a critical (Curie) temperature, thermal motions destroy the domains and the ferromagnets become ordinary paramagnetic materials. Ferromagnetic Effect – When placed in a magnetic field, the external magnetic field aligns the magnetic domains. The internal magnetic field becomes much stronger than the external field. There is large amplification of the magnetic field inside the ferromagnetic material. The material remains magnetized after the external field is removed (Permanent magnets).
  • Magnetic Hysteresis: The internal magnetic field changes take different paths when the external field is decreased or increased.
  • Applications of Ferromagnets: Used for the Generation of strong magnetic fields, Ex. extraction of ferromagnetic particles out of the eye (up to depth of 25mm).
  • Biological tissue is mostly Diamagnetic, contains some paramagnetic atoms and molecules. The amount of ferromagnetic material in the body is negligible.
  • Effect of Magnetic fields on the Human Body:
  1. Magnetohydrodynamic slowdown of blood flow
  2. Mechanical vibrations of nerve and muscle fibres
  3. Distortion of transmitted electrical pulses
  4. Orientational and conformational changes of biologically active macromolecules in solutions.
  • Magnetocardiography: Biopotentials induce weak current which in turn produce weak magnetic fields. Magnetocardiography registers the variations of the hearts magnetic field over time. It is a non- contact method for investigation of the heart.

Notes: