Electrical quantities

Voltage and Current

Voltage is considered to be a physical pressure which causes current to flow. The relationship between the voltage V across a resistor or a metal wire and the current I flowing there, is given by the well-known Ohm’s law:
V=RI
where R is a proportional constant known as resistance. Figure (a) shows the arrangement for the voltmeter and ammeter for this measurement.
The current is considered to be a flow of electric charge carried by a mass of electrons as shown in fig. (b). The units of voltage and current are volt (V) and ampere (A), respectively. The unit of resistance is ohm (Ω)

Wattage

Electric power is colloquially called wattage and this is the measurement of current×voltage and the unit is watt (W). It represents the electric energy that is supplied or consumed per second，and often denoted by P.