Dimensional homogeneity

[pierre_s]

Hi,

I have two very specific questions.

 

I was trying to read this paper :

http://downloads.bbc..._connection.pdf

 

and i noticed that equation 3 and equation A6 were different :

 

Eq3 : L(t)=Q(t)/£(t) (not dimensionally correct)

 

and

 

Eq A6 : L(t)=Q(t)/£(t) * delta_t

 

When you read under equation 3, you understand why this is like that ; they say :

"As seen in Eq.(A6) of Appendix A, lifetime indeed has the dimension of time, being multiplied by the sampling time interval (which is here equal to unity–1 day)."

 

Questions :

 

1/ Is it ok to have a dimensionally wrong equation if you say why in the text right next to it (i.e. numbers have a dimension and here because the time interval chosen is unity ) ?

 

2/ Is it common practice in Physics to do that ?

 

Thank you !

 

Note : i do not want a comment on the paper itself, just on this specific point.

Edited August 16, 2010 by pierre_s

[ajb]

 

1/ Is it ok to have a dimensionally wrong equation if you say why in the text right next to it (i.e. numbers have a dimension and here because the time interval chosen is unity ) ?

 

2/ Is it common practice in Physics to do that ?

 

 

It is common to pick units in which the some or all of the physical constants are set to one. At first this looks wrong on dimensional grounds, but once you realise what is going on it is ok. Sometimes such units are called natural or geometric.