Chapter 20. -SHIPBOARD ELECTRICAL SYSTEMS 



rubbed together. If the materials used are both 

 good conductors, it is difficult to obtain a de- 

 tectable charge because equalizing currents will 

 flow easily in and between the conducting mate- 

 rials. However, if the materials used are poor 

 conductors (insulators), little equalizing current 

 can flow and an electrostatic charge is built up. 



Charged Bodies 



One of the fundamental laws of electricity 

 is that like charges repel each other and unlike 

 charges attract each other . A positive charge 

 and a negative charge, being unlike, tend to 

 move toward each other; thus in the atom the 

 negative electrons are held in their orbital 

 shells by the positive attraction of the nucleus. 



The law of charged bodies may be demon- 

 strated by a simple experiment using two pith 

 (paper pulp) balls suspended near one another 

 by threads, as shown in figure 20-1, and a hard 

 rubber rod. If the hard rubber rod is rubbed to 

 give it a negative charge and then held against 

 the right-hand ball in part (A), the rod will im- 

 part a negative charge to the ball. The right- 

 hand ball will be charged negatively with respect 

 to the left-hand ball, and when released the two 

 balls will be drawn together. When the two balls 

 touch, they will remain in contact with each 

 other until the left-hand ball acquires a portion 

 of the negative charge, at which time they will 

 swing apart as shown in part (C) of figure 20-1. 

 Should positive charges be placed on both balls, 

 as shown in part (B) of figure 20-1, the balls 

 would also repel each other. 



Coulomb's Law of Charges 



The amount of attracting or repelling force 

 which acts between two electrically charged 

 bodies in free space depends upon the magnitude 

 of their charges and the distance between them. 

 This relationship between charged bodies was 

 first discovered by a French scientist named 

 Coulomb. Coulomb's law of charges states that 

 charged bodies attract or repel each other with 

 a force that is directly proportional to the 

 product of their charges and inversely propor- 

 tional to the square of the distance between 

 them . 



The practical unit of charge a body has is 

 expressed in coulombs . One coulomb is the 

 charge carried by approximately 6 x lO^^ elec- 

 trons. 



147.121 

 Figure 20-1.— Reaction between charged bodies. 



Electric Current Flow 



A difference of potential exists between two 

 bodies having opposite electrostatic charges. If 

 a path is provided between the two bodies, elec- 

 trons will flow from the negatively charged body 

 to the positively charged body until the charges 

 have equalized and the difference of potential 

 no longer exists. This movement of electrons 

 is called electric current . The rate of flow is 

 measured in amperes . One ampere may be de- 

 fined as the flow of one coulomb per second 

 past a fixed point in a conductor. 



The force or difference in potential which 

 causes electrons to flow from one charged body 

 to another is called electromotive force (emf). 

 Electromotive force is measured in volts . One 

 volt may be defined as the potential difference 

 between two points when one joule of work is 

 required to move a one-coulomb charge between 

 these points. 



MAGNETISM 



The relationship between magnetism and 

 electricity was first shown in 1819 when the 

 Danish scientist Oersted observed that a small 

 compass needle was deflected when it was passed 

 near a wire carrying a current. About 12 years 

 later, Michael Faraday discovered that moving a 

 conductor in a magnetic field would produce an 

 electric current. 



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