38 



ELECTRICITY. 

 ELECTRICITY. 



(49) THE DIMENSIONS OF UNITS. 



If any physical quantity Q be measured in terms of a length L, 

 an interval of time T, and a mass J/, so that . 



the quantity Q is said to be of the dimension a in length, /3 in 

 time, and 7 in mass. 



The velocity (v) of a moving body is measured by the linear 

 space passed over in the unit of time. Acceleration or velocity- 

 increment (a) is measured by the increase or decrease in the velocity 

 of the moving body during the unit of time. Force (F), anything 

 which changes or tends to change the motion of a body, is measured 

 by the mass moved multiplied by the acceleration produced. Work 

 (W) is measured by the force multiplied by the distance through 

 which it acts. The energy of a system is measured by the work 

 which it can do, hence energy also is measured by force multiplied 

 by distance. The power (P) of a motor is measured by the rate at 

 which it works, that is by the work done in the unit of time. 



Length 

 Surface 

 Volume 



Time 

 Velocity 



Acceleration 



Mass 



Momentum 



Force 



Work and Energy 



Power 

 Density 



Geometry. 



L 



S = L* 



V =. L 2 



Kinematics. 



T 



_ L 

 T 

 v 



- ,. 



Kinetics. 

 M 



Mv 



F = aM 



W = LF = i, 



M 

 V 





Dimensions. 

 L 1 

 L* 



T i 



LWT-* 



L'MT-' 2 



(50) THE C.G.S. SYSTEM. (Of. 9.) 



To obtain uniformity of measures it is convenient to adopt : 

 The CENTIMETRE as the unit of length. 

 The GRAM as the unit of mass. 

 The mean solar SECOND as the unit of time. * 



