Lorenz Effects

Lorentz force, the force acting on moving charged particles in a magnetic field (Figure 1), plays a crucial role in various applications ranging from electronic devices and motors, sensors, imaging to biomedical applications. It has been shown that magnetic field is able to image current and conductivity which has many biological and medical applications such as mapping electrical activity in the brain and heart, and for detecting abnormal tissues like tumors by changes in electrical properties. The Lorentz force plays an increasingly larger role in novel imaging techniques such as magneto-acoustic imaging of current, Hall effect imaging, ultrasonically-induced Lorentz force imaging of conductivity, magneto-acoustic tomography with magnetic induction, and Lorentz force imaging of action currents using magnetic resonance imaging. Our group is using the effect of Lorentz force to study the flame, electrochemical reactions, soft materials as well as Schileren technique. The effect of magnetic field on ionic currents is an interdisciplinary concept from electrochemistry, hydrodynamics and magnetism. The results are sometimes surprising, and their elucidation can lead to unexpected insights into fundamental electrochemical processes, as well as new practical applications. We are currently working on the effect of Lorentz force on the electrochemical oscillation reactions. We have shown that Lorentz force can enhance mass transport in electrochemical reactions. This effect is called the magnetohydrodynamic (MHD) effect and is caused by magnetic forces which induce convective movements in the electrolyte.

Lorenz-Fig1 Figure 1: Schematic of movement of a charged particle in magnetic field.
 

In fact, magnetic force induces a convective movement in the electrolyte due to the Lorentz force which is given by:

F= q(E + v × B)

where E is the electric field, B is the magnetic field and v is the velocity of charged particle (q). When an ion (charged particle) enters the magnetic field, experiences a force which is perpendicular to the direction of the speed of the object and the magnetic field. This force causes a centripetal acceleration and consequently a circular motion of the particle in the medium based on the equations described below. In the absence of an electric field:

 

 

Equation

These equations reveal that a charged particle with speed of v perpendicular to the magnetic field moves in a circular pathway. The radius of this circular motion inversely depends on the strengths of the magnetic field. It means that in regions with high magnetic field strength we have rotational motion with smaller radius while in higher magnetic field strength regions the radius of circular motion is larger. In fact, the component of the velocity parallel to the magnetic field lines is unaffected, since the magnetic force is zero for motion parallel to the field. This produces helical motion (i.e., spiral motion) rather than a circular motion (Figure 2). Therefore, the Lorentz force improves mass transfer in electrochemical cells because of the rotational and spiral motion.

Lorenz-Fig2 Figure 2: Schematic of a spiral motion of a charged particle rather than a circular motion.