A	The cross-sectional area of circular-ring
b	The widths of circular-ring and traveling arc electrode
E	The Young's modulus of circular-ring
E̅	The uniform electrical field in between circular-ring and traveling electrode
g	The initial gap between circular-ring and traveling arc electrode
H(θ)	The unit step function of θ
I	The area inertia moment of the cross-section of circular-ring
[K]	The stiffness matrix
[Kc]	The electrostatic stiffness matrix due to traveling electrostatic force
[Ks]	The structural stiffness matrix due to circular-ring
[Kα]	The electrostatic stiffness matrix due to traveling electrostatic force
[Kβ]	The electrostatic stiffness matrix due to traveling electrostatic force
k	The circumferential wave number of circular-ring
{Q}	The generalized force vector
r	The radius of circular-ring
T	A time-scale, $T=\sqrt{\rho A{a}^4/EI}$
T̅	The period of the periodically time-varying stiffness
t	The dimensionless time, t = t̂/T
t̂	Time
u(θ,t)	The dimensionless deflection of circular-ring, u = û/g
û( θ,t̂)	The deflection of circular-ring, which is a function of position and time
V	The dimensionless electrical potential difference between circular-ring and traveling arc electrode, V2 = r4ɛbV̂2/EIg2
V̂	The electrical potential difference between circular-ring and traveling arc electrode
{X}	The generalized coordinates vector
α	The generalized coordinate
β	The generalized coordinate
δ(θ)	The Dirac delta function of θ
ε	The permittivity of the medium in between circular-ring and traveling arc electrode
θ	The position of circular-ring in polar coordinate
ρ	The density of circular-ring
ϕ	The span angle of traveling arc electrode
Ω	The dimensionless angular speed of traveling arc electrode, Ω = Ω̂T
Ω̂	The angular speed of traveling arc electrode
Ωcr	The intrinsic critical speed of a traveling constant force acting on circular ring