Let's continue with Ohm's law and electric power. We just discussed the basic properties of electric circuits such as voltage, current, and resistance. To be able to make meaningful statements about these quantities in circuits, we need to be able to describe their quantities in the same way that we might quantify mass, temperature, volume, length, or any other kind of physical quantity. For mass, we might use the unit of kilogram or gram. For temperature, we might use degrees Fahrenheit or degrees Celsius. Here are the standard units of measurement for electrical current, voltage, and resistance. Each unit of measurement is named after a famous experimenter in the electricity. The Amp After Frenchmen Andrea Ampeer, the Volt after the Italian Alessandro Volta, and the Ohm after the German George Simon Ohm. The symbols E and V are interchangeable for the most part, although some texts reserve E to represent voltage across the source such as a battery or generator, and V to represent voltage across anything else. If you wish to determine mathematically what is happening in electrical terms in simple or even complex circuits, you have to know that the current I is dependent on the two factories, the voltage V and the resistance R. This dependency was described in Ohm's law named after the German physicist George Simon Ohm as we discussed before. Ohm's law is a very simple and useful tool for analyzing electric circuits. It is used so often in the study of electricity and electronics, that it needs to be committed to memory by the serious students. For those who are not yet comfortable with algebra, there was a trick to remembering how to solve for any quantity given the other two. First, arrange the letters E, I, and R in the triangle like this. If you know E and I, and wish to determine R, just eliminate R from the picture and see what is left. R equals E divided by I. If you know E and R, and wish to determine I, eliminate I and see what's left. I equals E divided by R. Lastly, if you know I and R, and wish to determine E, eliminate E and see what's left. E equals I times R. Let's talk about electrical power. The power generated by a turbine at a hydroelectric power plant, depends on the amount of energy stored per kilogram or gallon of falling water, such as height that the waterfalls, and the quantity of water flowing per second through the turbine. The energy stored in one kilogram of water is analogous in electrical consumer such as mortar M in the circuit diagram Illustrated. It's analogous to the energy stored per unit of charge, such as 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, P equals V times I. The unit of electrical power is named Watt after the English inventor. One Watt is the power generated by one Amp at a DC voltage of one Volt. The power consumed by a device can be measured indirectly with a voltmeter and an ammeter. Power is measurable directly by means of a wattmeter, which possesses two terminals each for the voltage and current, or total of four terminals. The part of a wattmeter distribute the voltage to be measured is applied, is termed the voltage path. The path 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 mortars. This value indicates the power which the component can handle under regular operating conditions. Substituting the product I times R, for the voltage V according to Ohm's law in the power equations that we saw before, results in the following relationship. Substituting the quotient V over R for the current I, results in the following equation, P equals v square divided by R. Let's take a look at the following example. A heater consumes a current of ten amps at a voltage of 120 Volts. Its power consumption is P equals V times I, or 120 Volts times 10 Amps is 1200 Watts or 1.2 kiloWatts.
