When voltage is applied over the capacitance, electrostatic force will work to reduce the plate separation d −x. At small voltages, the electrostatic voltage is countered by the spring force Fk = kx but as voltage is increased the plates will eventually snap together. Estimating this pull-in voltage UP and the plate travel distance x before pull-in effect is required for the successful design of electrostatic actuators, switches, varactors, and sensors.