Power, as defined at Dictionary.com, is "work done or energy transferred per unit of time," and is represented by the symbol P. In electronics, power (P) can be thought of as the amount of energy given off by a resistance, or load, when current is passed through it. Power is measured in watts (W). One watt is equal to one joule.
Dictionary.com defines joule as "a unit of electrical energy equal to the work done when a current of one ampere is passed through a resistance of one ohm for one second." James Prescott Joule, an English physicist, recognized this relationship and derived an equation from Ohm's Law describing power (P) as the product of voltage (V) and current (I), or P = V * I.
Using Watt's Law
Using the equation, P = V * I, and given any two of the three variables (P, V, or I), the unknown value can be solved. The "Solution Wheel," shown in Figure 4 below, can be used to find the appropriate equation for solving the unknown. For example, if the P is covered, V by I (or V * I) will be the equation needed to solve for power. Similarly, if the V is covered, P over I (or P / I) will be used to solve for voltage.
Calculating Current Using Watt's Law
Watt's Law can be used when designing a circuit to help determine wire gauge and circuit protection. It works with either alternating current (AC) or direct current (DC) applications. Current for a circuit can be calculated given the total wattage of the loads and the source voltage. Using the solution wheel, covering I, yields the equation P / V.
Determining Wattage for a Load
Wattage for most devices can be found on the UL Listing label (shown in Figures 2 and 3) or in the owner's manual and technical data accompanying the device. The wattage rating will typically be found with the voltage rating, and will be designated with a "W" or, in some cases, "VA."
The VA designation stands for Volt-Amps, which corelates to power being equal to the product of voltage and current. It should also be noted that the wattage rating reflects maximum-sustained power usage, but does not reflect any current spikes that may occur upon start-up.
The relationship between voltage, current and power is the basis of Watt's Law, and it is a useful tool for determining circuit characteristics and design specifications.
Power. Dictionary.com. Dictionary.com Unabridged (v 1.1). Random House, Inc. http://dictionary.reference.com/browse/power (accessed: February 20, 2009).
Joule. Dictionary.com. WordNet® 3.0. Princeton University. http://dictionary.reference.com/browse/joule (accessed: February 20, 2009).
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