Electrostatic force microscopy (EFM) is a type of dynamic non-contact atomic force microscopy where the electrostatic force is probed. ("Dynamic" here means that the cantilever is oscillating and does not make contact with the sample). This force arises due to the attraction or repulsion of separated charges. It is a long-range force and can be detected 100 nm or more from the sample. For example, consider a conductive cantilever tip and sample which are separated a distance z usually by a vacuum. A bias voltage between tip and sample is applied by an external battery forming a capacitor, C, between the two. The capacitance of the system depends on the geometry of the tip and sample. The total energy stored in that capacitor is U = ½ C⋅ΔV. The work done by the battery to maintain a constant voltage, ΔV, between the capacitor plates (tip and sample) is -2U. By definition, taking the negative gradient of the total energy U_total = -U gives the force. The z component of the force (the force along the axis connecting the tip and sample) is thus: