The driving force for the fluid flow in the arc is the electromagnetic force or Lorentz force.
F = J x B
Where J is the current density with its vector in the direction of the electric current flows. B is the magnetic flux vector.
This Lorentz force affects the shape of the arc (depending on the electrode tip geometry) and hence influences in the shape of the weld pool.
The electric current tends to be perpendicular to the electrode tip surface and the work piece surface and induces a magnetic field. Both electric current and magnetic field produce downward and inward forces, which push the ionic gas to impinge on the work piece surface and turn outward along the work piece surface. More of downward movement in the sharp electrode tip, producing a bell shaped arc, and less downward movement in the flat electrode tip, producing a more constricted arc.
Downward and inward momentums due to electric current and magnetic field cause different fluid flow in sharp and flat electrode tips. Downward momentum is stronger in the sharp electrode tip than in the flat electrode tip.
Reference: Suranaree University of Technology- literature
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