What is required to cause a voltage difference between two wires that are separated by insulation?

To cause a voltage difference between two wires that are separated by insulation, capacitance is essential. Capacitance refers to the ability of a system to store an electrical charge. When two conductive materials (in this case, the wires) are separated by an insulating material, they can create an electric field if a voltage is applied. This arrangement forms a capacitor, where one wire becomes positively charged and the other wire becomes negatively charged, creating a voltage difference between them.

The insulation prevents current from flowing between the wires; rather, it allows for the build-up of electric charge. This is why capacitance is critical; it allows the potential difference (voltage) to exist without direct current flow. Hence, when voltage is applied to one wire, the presence of capacitance across the insulator is what enables the establishment of a voltage difference between the two wires.