ELEMENTARY LAWS OF CONTINUOUS CURRENTS 13 



of work necessary to carry a unit quantity of electricity 

 from the point of lower electrical pressure to the point of 

 higher pressure. 



Unit of Work. The unit of work in the electrical system 

 is the Joule and is the amount of work done in one second 

 when a current of one ampere flows under a pressure of 

 one volt. 



In the form of an equation, 



J = EIt, (6) 



where J = electrical work in joules. 



Power. Now the rate at which work is being done is evi- 

 dently equal to J/t, but J/t = EI, so that the product El 

 is a measure of the rate of energy, or power, of the electric 

 circuit. The unit of power is the watt, and is the power 

 represented by a current of one ampere flowing under a 

 pressure of one volt. A more common unit in which elec- 

 trical power is measured is the kilo- watt, which is equal 

 to one thousand watts. 



We have therefore the relation that watts El. But 

 when E = IR we also have Watts = (IR)Xl = PR, which 

 is the equation ordinarily used in calculating the power 

 used in a circuit in the form of heat. 



7. Potential and Difference of Potential. Much con- 

 fusion between the terms " electromotive force " and 

 " difference of potential " often exists and many times 

 the term " potential." is used in cases where it is really 

 inapplicable. In the previous section we used the idea 

 of electrical level and showed how the voltage, E, of the 

 electrical circuit corresponded to the difference in level 

 in the hydraulic problem. When a quantity of water at a 

 high level moves to a lower level it does work. When it 

 is at the high level it has the potentiality for doing work; 

 in other words, it possesses potential energy. Of course, 

 the water will possess some potential energy when at the 



