Final answer:
The approximate voltage for which conduction starts, or breakdown voltage, is about 75 kV for a gap of 2.5 cm in dry air conditions. This value can vary with factors such as air humidity and surface structure.
Step-by-step explanation:
The approximate voltage for which conduction starts, often referred to as breakdown voltage, is about 75 kV. Conduction happens when the electric potential is high enough to cause a spark to jump across a gap. In air, this can occur at lower voltages under certain conditions like increased humidity or pointed surface structures. For example, with about 75 kV a spark can jump across a 2.5 cm gap, and with 150 kV it can jump a 5 cm gap. Such knowledge is critical when considering the design of power transmission lines and the prevention of unintentional sparking.
Factors that affect the breakdown voltage include the shape of the surface, the humidity of the air, and the distance between the conductors. For instance, pointed surfaces can create greater fields than smooth surfaces, resulting in a spark at lower voltages. Similarly, humid air breaks down at a lower field strength, meaning that a smaller voltage is required to initiate a spark in humid conditions compared to dry air.