178 THE REALITIES OF MODERN SCIENCE 



Law, we obtain unit current in the electrostatic system 

 as 1 e.s. unit of quantity per second. Symbolically 



i=Q/t. (2) 



The system formed by the plates of the battery has 

 potential energy which may be converted into kinetic 

 energy of electrons. A difference of gravitational 

 potential, as we learned on page 108, is measured in 

 energy per unit mass. In the similar case of the cell 

 it would be logical to express the potential difference 

 of the plates in ergs per electron. In general, a dif- 

 ference in electrical potential is expressed as so many 

 units of energy per unit of electricity. Several units 

 are therefore possible, depending upon the choice of 

 units for energy and quantity. In the case of the unit 

 called the volt (after Volta), the energy unit is the 

 joule and the quantity unit is the coulomb. The lat- 

 ter, named after the scientist mentioned above, is the 

 quantity represented by three thousand millions of the 

 electrostatic units which were defined by using his law. 

 The corresponding current unit, the coulomb per second, 

 is called the ampere. 



If we represent potential difference by E, quantity 

 by Q, and energy or work done by the system as TF, 

 we have 



E = W/Q 



or W=EQ (3) 



as the defining equation for E. If the quantity Q is 

 expressed by equation (2) as it we may write 



W=Eit. (4) 



In order that a body shall fall through the space 



intervening between the points of higher and lower 



