A type of conservation of energy is also associated with emf. The electrical energy obtainable from a battery is limited by the energy stored in chemical molecular bonds. Both emf and electric potential are measured in volts, and, unfortunately, the terms voltage, potential, and emf are used rather loosely. For example, the term battery potential is often used instead of emf. Whether as an emf or an electric potential, voltage is a measure of the ability of a system to do work on a unit amount of charge by electrical means.
Voltage is a better-known quantity than electric field.
The potential between a cloud and the ground just before a typical lightning bolt is a minimum of 10,000 volts. Sometimes high voltages are needed. For instance, the electron beams in television tubes require more than 30,000 volts. Such high potentials may be developed from lower alternating potentials by using a transformer. By scuffing shoes on a carpet on a dry day, an electric potential of more than 20,000 volts can be developed, resulting in a spark.