on 3/25/2002 11:59 AM, Duncan Bath at [EMAIL PROTECTED] wrote: > W*s is bare, naked SI; joules is a tag of convenience
I don't know what "bare naked" means in this context (I DO know what "bare naked" means in other contexts!) but Duncan seems to contrast it with "a unit of convenience". Thus, I conclude that Duncan means that the watt-second (and probably the watt) is somehow better, more preferable, more fundamental, more pure SI, or something than the joule. ALL names for derived units are names of convenience. The watt is a convenient name for the derived unit joule per second. Similarly, the joule is a convenient name for the derived newton-metre. I'm not sure I understand why Duncan thinks one is more "bare naked SI" than the other. All SI units are derived units except the 7 base units: metre, kilogram, second, ampere, kelvin, candela and mole. The derived units are combinations of the base units and/or other derived units. The metre per second is a derived unit that does not have a special name. The joule per second is a derived unit that has a special name: the watt. While it doesn't really matter which is defined first, the joule is normally defined first as a newton-metre, with the watt being defined later as a joule per second. Both are pure SI. Both are convenient. I'm not sure why either would be called "bare naked". The watt is also equal to a volt-ampere but that is not it's original definition. The watt is originally defined as a joule per second. Since the volt (V) is a joule per coulomb (J/C) and the coulomb (C) is defined as an ampere-second (A�s), it is easy to show that the volt-ampere equals the watt: 1 V�A = (1 J/C)�(1 A) = (1 J/A�s)�(1 A) = 1 J�s = 1 W (In proper SI, multiplication of units is shown by the raised dot. The raised dot generated by my computer is shown here between the letters X and Y: X�Y If this does not transmit properly to your computer, please note that whatever symbol is between my X and Y in the example, should be a raised dot there and in all other parts of this message.) Regards, Bill Hooper college physics teacher (retired), USA (Florida) +-+-+-+-+-+-+-+-+-+-+-+ Do It Easy, Do It Metric! +-+-+-+-+-+-+-+-+-+-+-+
