ELECTRIC FIELD. 153 



91. Energy and tension of the electric field in air. It is a 



great help in the development of the theory of the electric field 

 if one appreciates as fully as possible the various properties of 

 the electric field. Thus equation (2) of Art. 84 expresses an 

 important property of the electric field. Another interesting 

 property of the electric field is that the lines of force tend to 

 shorten, that is, the electric field is in a state of tension along the 

 lines of force ; and another interesting property is that the electric 

 field stores energy. These two properties are here discussed for 

 the electric field in air. 



Energy of the electric field in air. The energy of a charged 

 condenser is : 



(i) 



according to Art. 81 . Substituting the value of C from equation 

 (2) of Art. 79, remembering that k = I for air, we get : 



i 

 W = - -- = ^ ax (2)* 



2B X 2.B X 2 



p> 



but ax is the volume v of the region between the plates, and - 



is the intensity e of the electric field between the plates. There- 

 fore equation (2) may be written: 



W = ^-v<* . (3) 



or, dividing both members by v, we get : 



energy per unit volume = e 2 (4) 



Now the energy of a charged condenser resides in the electric 

 field, and equation (4) shows that the energy of an electric field 



in air in joules per cubic centimeter is e 2 , where e is the 

 intensity of the field in volts per centimeter. 



* The factor is written for 884 X 



