ELECTRICAL FORCES 



39 



Positives repel; negatives repel; but positive attracts negative. Coulomb ob- 

 served that the force of repulsion of like charges increases as the size of each, 

 and decreases as the square of the distance. Thus 



F = 



ed 2 



where Fis the force in dynes, <7, and q 2 are the charges in coulombs, d is the 

 distance in centimeters, and e is the proportionality constant, called the di- 

 electric constant (Figure 2-4). Unit charge is formally defined through 

 Coulomb's facts: when two like charges are 1 cm apart and repel each other 

 with a force of 1 dyne, each carries unit charge. 



DISTANCE x 



Figure 2-4. Interaction of Electrical Charges: (a) Coulomb's case; (b) field strength. 



Bioelectric Potentials 



At the microscopic level the most important potential differences in the 

 living system arise from concentration differences (why they do will be seen 

 later), and these occur almost without exception across living membranes. 

 For example, in heart muscle cell the potential difference or voltage between 

 the inside and outside of the cell, across the cell membrane, is about 85 mv, 

 on the average, and cycles above and below this, as the heart beats. 



