PRINCIPLES OF NAVAL ENGINEERING 



source (fig. 20-4). The resistance^ of a circuit 

 (opposition to current flow) controls the amount 

 of current flow through the circuit. The unit of 

 electrical resistance, the ohm (symbol fi), is 

 named after the German physicist Georg Simon 

 Ohm, who in the 19th century proved by experi- 

 ment the constant proportionality between cur- 

 rent and voltage in the simple electric circuit. 



^ 



1 



-=- 12 VOLT 

 ■=: SOURCE 



LOAD 



4.125 

 Figure 20-4.— Simple electric circuit. 



OHM'S LAW 



Ohm's law is fundamentally linear and there- 

 fore simple. It is exact and applies to d-c cir- 

 cuits and devices in its basic form; in a modified 

 form it may also be applied to a-c circuits. 



Ohm's law may be stated in words as: the 

 intensity of the current (in amperes) in any 

 electric circuit is equal to the difference in 

 potential (in volts) across the circuit divided 

 by the resistance (in ohms) of the circuit . Ex- 

 pressed as an equation, Ohm's law becomes 



1=^ 

 * R 



where 



I 



E 



R 



intensity of current (in amperes) 

 difference in potential (in volts) 

 resistance (in ohms) 



If any two of these quantities are known, the 

 third may be found by applying the equation. 



In addition to the volt, the ampere, and the 

 ohm, the unit of power frequently appears in 

 electric circuit calculations. In a d-c electric 



All conductors have some resistance, and therefore 

 a circuit made up of nothlngbut conductors would have 

 some resistance, however small it might be. In cir- 

 cuits containing long conductors, through which an 

 appreciable amount of currentlsdrawn, the resistance 

 of the conductors becomes important. For the pur- 

 poses of this chapter, however, the resistance of the 

 conducting wires is neglected. 



circuit, power is equal to the product of the 

 voltage and the current. Expressing the power 

 in watts (P), the current in amperes (I), and the 

 emf in volts (E), the equation is 



IE 



The various implications of Ohm's law may 

 be derived from the algebraic transposition of 

 the units I, E, R, and P. A summary of the 12 

 basic formulas which may be derived from 

 transposing these units is given in figure 20-5. 

 The unit in each quadrant of the smaller circle 

 is equivalent to the quantities in the same quad- 

 rant of the larger circle. 



Series Circuits 



The analysis of a series circuit to determine 

 values for voltage, current, resistance, and 

 power is relatively simple. It is necessary only 

 to draw or to visualize the circuit, to list the 

 known values, and to determine the unknown 

 values by means of Ohm's law and Kirchhoff's 

 law of voltages. 



Kirchhoff's law of voltages states that the 

 algebraic sum of all the voltages in any com- 

 plete electric circuit is equal to zero . In other 

 words, the sum of all positive voltages must be 

 equal to the sum of all negative voltages. For 

 any given voltage rise there must be an equal 

 voltage drop somewhere in the circuit. The volt- 

 age rise (potential source) is usually regarded 



Figure 20-5.— Summary of basic 

 formulas. 



27.236 

 Ohm's law 



496 



