A watt is a unit of power, named after engineer James Watt, which measures the rate of energy transfer. A watt, in electrical terms, is the rate at which electrical work is done when one ampere (A) of current flows through one volt (V). Formula:
W= A* V
Whenever current flows through a resistance, heat results. This is inevitable. The heat can be measured in watts, abbreviated W, and represents electrical power. Power can be manifested in many other ways, such as in the form of mechanical motion, or radio waves, or visible light, or noise. In fact, there are dozens of different ways that power can be dissipated. 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.
There is a certain voltage across the resistor, not specifically given in the diagram. There's also electricity flowing through the resistance, not quantified in the diagram, either. Suppose we call the voltage E and the current I, in volts and amperes, respectively. Then the power in watts dissipated by the resistance, call it P, is the product E X I. That is:
P (watts) = El
This power might all be heat. Or it might exist in several forms, such as heat, light and infrared. This would be the state of affairs if the resistor were an incandescent light bulb, for example. If it were a motor, some of the power would exist in the form of mechanical work.
If the voltage across the resistance is caused by two flashlight cells in series, giving 3 V, and if the current through the resistance (a light bulb, perhaps) is 0.1 A, then E = 3 and I = 0.1, and we can calculate the power P, in watts, as:
P (watts) = El = 3 X 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 = 0.855 A. Then we have:
P (watts) = 117 X 0.855 = 100 W
You will often hear about milliwatts (mW), microwatts (uW), kilowatts (kW) and megawatts (MW). You should, by now, be able to tell from the prefixes what these units represent. But in case you haven't gotten the idea yet, you can refer to Table 2- 2. This table gives the most commonly used prefix multipliers in electricity and electronics, and the fractions that; they represent. Thus, 1 mW = 0.001 W; 1 uW = 0.001 mW = 0.000001 W; 1 kW = 1-flOO W; and 1 MW = 1,000 kW = 1,000, 000 W.
Sometimes you need to use the power equation to find currents or voltages. Then you should use I = P/E to find current, or E = P/I to find power. It's easiest to remember that P = El (watts equal volt-amperes), and derive the other equations from this by dividing through either by E (to get I) or by I (to get E).
A utility bill is measured in kilowatt hours, usually in 1,000 watt increments. A watt is a unit of electrical energy in which the units of measurement (watts and watt hours) are agreed to by an international system of units si called watts. The amout of energy is measured this way.
This 100+ page e-book is a great guide for those who have a basic interest in the field of electricity. This well-illustrated e-book, coupled with some basic knowledge of electricity, will give you a broad theoretical background in this fundamental subject.
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