Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Magnetic field moving towards wire coilinduces an opposing magnetic fieldand associated current flowInducedmagnetic fielde-Ii!FIGURE A-g. Lenz's Law, demonstrating mutual induction between a moving magnetic field anda coiled wire conductor.the magnetic field moves with respect to the wire position. A stationary magneticfield does not induce a voltage in a stationary wire.2. A stronger magnetic field results in a stronger induced voltage. The voltage is proportionalto the number of magnetic field lines cutting across the wire conductorper unit time. If the relative speed of the wire or the magnet is increased withrespect to the other, the induced voltage will be larger because an increased numberof magnetic field lines will be cutting across the wire conductor per unit time.3. A 90-degree angle of the wire conductor relative to the magnetic field will providethe greatest number of lines to cross per unit distance, resulting in a higherinduced voltage than for other angles.4. When a solenoid (wire coil) is placed in a magnetic field, the magnetic field linescut by each turn of the coil are additive, causing the resultant induced voltage tobe directly proportional to the number of turns of the coil.The direction of current flow caused by an induced voltage is described by Lenz'sLaw (Fig. A-9: An induced current flows in a direction such that its associated magneticfield opposes the magnetic field that induced it. Lenz's Law is an important conceptthat describes self-induction and mutual induction.A time-varying cutrent in a coil wire produces a magnetic field that varies in thesame manner. By Lenz's Law, the varying magnetic field induces a potential differenceacross the coil that opposes the source voltage. Therefore, a rising and fallingsource voltage (AC) creates an opposing falling and rising induced voltage in the coil.This phenomenon, called selfinduction, is used in autotransformers that provide avariable voltage, as discussed in Chapter 5.A primary wire coil carrying AC produces a time-varying magnetic field. When asecondary wire coil is located under the influence of this magnetic field, a time-vary-