DefinitionsElectric current is the directed flow of free charge carriers (e.g., electrons or ions); this means the transmission of electrical energy. Current [I] is measured in amperes [A]. Voltage is the difference in electrical potential energy between two points and is measured in volts [V]. The electrical resistance is a measure of how much an electrical conductor opposes the current flow. The unit of the electrical resistance is Ohm [Ω].
The relationship between electric current, voltage and resistance is given by Ohm’s law:
I = U/R
This means, the higher the voltage and the smaller the resistance, the higher the current.
There are two types of electrical current: direct current and alternating current (see figure). Direct current (DC) is the unidirectional flow of electric charge (see left side of the figure). Alternating current (AC) occurs when the flow of electric charge periodically reverses direction (see right side of the figure). The frequency tells us how often this change happens per second. The unit of frequency is Hertz [Hz]. For example, an alternating current of 50 Hz has 50 periods per second.Electric voltage is subdivided into low voltage and high voltage according to the strength of voltage. Low voltage is up to 1,000 V AC and up to 1,500 V DC. Correspondingly, high voltage starts at 1,000 V AC and at 1,500 V DC. Household current is an example of low voltage AC: it is supplied with a frequency of 60 Hz and a line voltage of 110 V in the USA, Canada and other American countries whereas in Europe the frequency is 50 Hz and the line voltage is 230 V. Household current is used for electrical household appliances (e.g., electric kettle and washing machine). Several appliances (e.g., radio set, TV set or computer) have to transform AC into DC. Low voltage DC is for example used in batteries and photovoltaic installations. High voltage AC is used for transport of electric current from the power plant via high voltage power lines and underground cables with voltages up to 380 kV. High voltage DC is increasingly used for energy transport via high voltage direct current (HVDC) power lines.
An electric arc can occur close to a power line (AC or DC). High voltages between two objects of different potential that are not in contact with each other can be discharged (breakdown) and generate a current spark emitting very high temperatures.