
Variable air capacitor consists of fixed stator plates and movable rotor. Capacitance between the stator plates depends on the position of the rotor.
Problem Type:
3D extrusion electrostatics.
Geometry:
Given:
Relative permittivity of air ε = 1,
Potential difference ΔU = 1000 V
Problem:
Find the mutual capacitance between two stators as a function of the rotor position.
Solution:
QuickField simulation gives stored electric energy value W. With known potential difference ΔU the capacitance then can be calculated as C = 2W / ΔU^{2}.
Results:
Rotating angle α 
Energy W 
Capacitance C 
0° 
1.29e6 J 
2.58e12 C 
30° 
2.25e6 J 
4.50e12 C 
60° 
2.97e6 J 
5.94e12 C 
90° 
3.22e6 J 
6.44e12 C 
See the variable_capacitor.pbm problem in the Examples folder.