Discussion Forum

I have got a clamped-clamped beam, with a DC current flowing through.
If the beam is surrounded by a DC magnetic flux density (orthogonal to the direction of the current flow), a Lorentz Force arises and the beam deflects.
If next to the beam there is another similar structure (not moving) and if the two blocks are conductive, the two form a capacitor.
The capacitance value depends on the magnetic flux density magnitude, as it depends of the distance between the two.
I would like to implement this system in COMSOL.

I was thinking of three steps:
1) Magnetic Fields (mf)
I can define the external magnetic flux density and the current density flowing through the spring. The Lorentz force x,y,z components are available as simulation outputs (mf.FLtzx, mf.FLtzy and mf.FLtzz). This step gives almost no troubles.

2) Solid Mechanics (solid)
I define a body load on the beam, defined as force per unit volume, using the "Lorentz force contribution (mf/al1)" option (COMSOL itself suggests this), and the simulation should return the corresponding displacement. But, if I do this, the solution gives 0 displacement as a result. No displacement is shown.
On the other hand, if I apply an "user defined" body load as [B*I*L/V], where B is the magnetic flux density, I is the current, L is the length of the spring and V is the volume of the spring expressed in [N/m^3], the simulation works.
Any idea of why this happens? It's annoying because the user should "manually" input the Lorentz force and I would like to avoid this.

3) Electrostatics (es)
I define two terminals (the two structures, the beam and the fixed one) and I can calculate the capacitance.
And here's the other problem: how can I "tell COMSOL" that the beam is not "at rest" but it's deformed (due to the Lorentz force application)?

Thank you!

Paolo