![]() ![]() Take a hose and point it at a waterwheel like the ones that were used to turn grinding stones in watermills. In an electrical system power ( P) is equal to the voltage multiplied by the current. This is like decreasing the resistance in an electrical system, which increases the current flow.Įlectrical power is measured in watts. ![]() You probably guessed that this also makes more water come out of the hose. Let's say you increase the diameter of the hose and all of the fittings to the tank. The same is true of an electrical system: Increasing the voltage will make more current flow. What happens if you increase the pressure in the tank? You probably can guess that this makes more water come out of the hose. Let's say you have a tank of pressurized water connected to a hose that you are using to water the garden. The amout of energy is measured this way.Let's see how this relation applies to the plumbing system. 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. 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).Ī utility bill is measured in kilowatt hours, usually in 1,000 watt increments. Then you should use I = P/E to find current, or E = P/I to find power. Sometimes you need to use the power equation to find currents or voltages. This table gives the most commonly used prefix multipliers in electricity and electronics, and the fractions that they represent. But in case you haven't gotten the idea yet, you can refer to Table 2- 2. You should, by now, be able to tell from the prefixes what these units represent. You will often hear about milliwatts (mW), microwatts (uW), kilowatts (kW) and megawatts (MW). To calculate the power, we must convert the current into amperes 855 mA = 855/1000 = 0.855 A. Suppose the voltage is 117 V, and the current is 855 mA. ![]() 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: If it were a motor, some of the power would exist in the form of mechanical work. This would be the state of affairs if the resistor were an incandescent light bulb, for example. Or it might exist in several forms, such as heat, light and infrared. Then the power in watts dissipated by the resistance, call it P, is the product E X I. Suppose we call the voltage E and the current I, in volts and amperes, respectively. There's also electricity flowing through the resistance, not quantified in the diagram, either. There is a certain voltage across the resistor, not specifically given in the diagram. Some power always goes to waste, and this waste is almost all in the form of heat. This is because no equipment is 100-percent efficient. But heat is always present, in addition to any other form of power in an electrical or electronic device. In fact, there are dozens of different ways that power can be dissipated. Power can be manifested in many other ways, such as in the form of mechanical motion, or radio waves, or visible light, or noise. The heat can be measured in watts, abbreviated W, and represents electrical power. Whenever current flows through a resistance, heat results. 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). A watt is a unit of power, named after engineer James Watt, which measures the rate of energy transfer. ![]()
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