Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGsource (fig. 20-4). The resistance^ of a circuit(opposition to current flow) controls the amountof current flow through the circuit. The unit ofelectrical resistance, the ohm (symbol fi), isnamed after the German physicist Georg SimonOhm, who in the 19th century proved by experimentthe constant proportionality between currentand voltage in the simple electric circuit.^-=- 12 VOLT=:LOADSOURCE1Figure 20-4.— Simple electric circuit.OHM'S LAW4.125Ohm's law is fundamentally linear and thereforesimple. It is exact and applies to d-c circuitsand devices in its basic form; in a modifiedform it may also be applied to a-c circuits.Ohm's law may be stated in words as: theintensity of the current (in amperes) in anyelectric circuit is equal to the difference inpotential (in volts) across the circuit dividedby the resistance (in ohms) of the circuit . Expressedas an equation, Ohm's law becomes1=^*Rcircuit, power is equal to the product of thevoltage and the current. Expressing the powerin watts (P), the current in amperes (I), and theemf in volts (E), the equation isThe various implications of Ohm's law maybe derived from the algebraic transposition ofthe units I, E, R, and P. A summary of the 12basic formulas which may be derived fromtransposing these units is given in figure 20-5.in each quadrant of the smaller circleThe unitis equivalent to the quantities in the same quadrantof the larger circle.Series CircuitsThe analysis of a series circuit to determinevalues for voltage, current, resistance, andpower is relatively simple. It is necessary onlyto draw or to visualize the circuit, to list theknown values, and to determine the unknownvalues by means of Ohm's law and Kirchhoff'slaw of voltages.Kirchhoff's law of voltages states that thealgebraic sum of all the voltages in any completeelectric circuit is equal to zero . In otherwords, the sum of all positive voltages must beequal to the sum of all negative voltages. Forany given voltage rise there must be an equalvoltage drop somewhere in the circuit. The voltagerise (potential source) is usually regardedIEwhereIERintensity of current (in amperes)difference in potential (in volts)resistance (in ohms)If any two of these quantities are known, thethird may be found by applying the equation.In addition to the volt, the ampere, and theohm, the unit of power frequently appears inelectric circuit calculations. In a d-c electricAll conductors have some resistance, and thereforea circuit made up of nothlngbut conductors would havesome resistance, however small it might be. In circuitscontaining long conductors, through which anappreciable amount of currentlsdrawn, the resistanceof the conductors becomes important. For the purposesof this chapter, however, the resistance of theconducting wires is neglected.Figure 20-5.—Summary of basicformulas.27.236Ohm's law496