# Electrical Power !

The power generated by a turbine at a hydroelectric power plant (illustrated below on the left) depends on the amount of energy stored per kilogram of falling water (i.e. the height that the water falls) and the quantity of water flowing per second through the turbine. The energy stored in 1 kilogram of water is analogous in an electrical consumer (e.g. motor __M__ in the circuit diagram below) to the energy stored per unit of charge, i.e. electrical potential. Thus the current of water is analogous to an electric current.

Electrical power P is proportional to the values of the voltage and current, and is determined by the following equation:

The unit of electrical power is named the *Watt* (W) after the English inventor. 1 W is the power generated by 1 A at a DC voltage of 1 V. The power consumed by a device can therefore be measured indirectly with a voltmeter and ammeter. Power is measurable directly by means of a wattmeter possessing two terminals each for the voltage and current, i.e. a total of four terminals. The part of a wattmeter to which the voltage to be measured is applied is termed the *voltage path*, the part through which the current to be measured flows is termed the *current path*.

*Rated power or nominal power* are frequently specified for electrical devices such as incandescent lamps and motors. This value indicates the power which the component can handle under regular operating conditions.

Substituting the product *I* · *R* for the voltage *U* according to Ohm's law in the power equation above results in the following relationship:

Alternatively, substituting the quotient *U*/*R *for the current *I* results in the following relationship:

Example 1: A heater consumes a current of 15 A at a voltage of 220 V. Its power consumption is *P* = *U* · *I* = 220 V · 15 A = 3300 W = 3.3 kW.

**Example 2: A wire-wound resistor has a rating of 10 W, 4 kW. Its maximum permissible current according to the equation P = I^{2} · R is 0.05 A, and maximum permissible voltage according to the equation P = U^{2} /R is 200 V.**