Lorentz forces
In powerful electromagnets, the magnetic field exerts a force on each turn of the windings, due to the Lorentz force In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields: acting on the moving charges within the wire. The Lorentz force is perpendicular to both the axis of the wire and the magnetic field. It can be visualized as a pressure between the magnetic field lines, pushing them apart. It has two effects on an electromagnet's windings:
- The field lines within the axis of the coil exert a radial force on each turn of the windings, tending to push them outward in all directions. This causes a tensile stress In continuum mechanics, the concept of stress, introduced by Cauchy around 1822, is a measure of the average amount of force exerted per unit area of the surface on which internal forces act within a deformable body . In other words, it is a measure of the intensity, or internal distribution of the total internal forces acting within a deformable in the wire.
- The leakage field lines between each turn of the coil exert a repulsive force between adjacent turns, tending to push them apart.
The Lorentz forces increase with B2. In large electromagnets the windings must be firmly clamped in place, to prevent motion on power-up and power-down from causing metal fatigue In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material. In the Bitter A Bitter electromagnet or Bitter solenoid is a type of electromagnet made of circular metal plates and insulating spacers stacked in a helical configuration, rather than coils of wire. This design was invented and built in 1933 by American physicist Francis Bitter. In his honor the plates are known as Bitter plates design, below, used in very high field research magnets, the windings are constructed as flat disks to resist the radial forces, and clamped in an axial direction to resist the axial ones.
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