Power and the Watt กำลังงานและวัตต์

Whenever current flows through a resistance, heat results. The heat can be measured in watts (symbolized W) and represents electrical power. (As a variable quantity in equations, power is denoted by the uppercase italic letter P.) Power can be manifested in many forms, such as mechanical motion, radio waves, visible light, or noise. But heat is always present, in addition to any other form of power, in an electrical or electronic device. This is because no equipment is 100 percent efficient. Some power always goes to waste, and this waste is almost all in the form of heat.



Fig. 1. Whenever current passes through a component having resistance, a voltage exists across that component.

Look again at Fig. 1. There is a certain voltage across the resistor, not specifically indicated.
There’s also a current flowing through the resistance, and it is not quantified in the diagram, either. Suppose we call the voltage E and the current I, in volts (V) and amperes (A), respectively. Then the power in watts dissipated by the resistance, call it P, is the product of the voltage in volts and the current in amperes:

P = EI

If the voltage E across the resistance is caused by two flashlight cells in series, giving 3 V, and if the current I through the resistance (a light bulb, perhaps) is 0.1 A, then E = 3 V and I = 0.1 A, and we can calculate the power P in watts as follows:


P = EI = 3 × 0.1 = 0.3 W

Suppose the voltage is 117 V, and the current is 855 mA. To calculate the power, we must convert the current into amperes: 855 mA = 855/1000 A = 0.855 A. Then:

P = EI = 117 × 0.855 = 100 W