Joules.
Volts = Joules/Coulomb Amperes = Coulombs/second Hours = 3600 seconds (42 Joules/Coulomb) * (20 Coulombs/second) * 3600 seconds = 3024000 Joules = 3 MJ Nat From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Bill Hooper Sent: Saturday, 2008 May 10 12:03 To: U.S. Metric Association Subject: [USMA:40881] Re: SI power questions and a lot of random thoughts :) On 2008 May 10 , at 12:45 AM, LPS wrote: I have a battery that is hooked up to a bicycle to assist in getting me up hills and such. It is represented as a 42 volt 20 ampere hour battery. I figure that a.h is really not using SI properly. The hour is not the unit of time -- it is the second that would be correct. So to properly indicate the capacity of the battery should I be using Coulombs? LPS, It is true that multiplying amperes by a time unit will tell the total electric charge that has flowed though your system in that time (and if the time is expressed in seconds the charge will be in coulombs). However, in operating you bicycle, it would seldom be of any value or any interest to know the total amount of charge that has flowed (essentially, the number of electrons). The reason that the ampere-hour (A.h) is used at all is that the time of use is of interest. If you know "the ampere-hours"* and how much current (in amperes) is flowing through your system, then you know how long it can run. That amount of time would conventionally be measured in hours. So, just because ampere-hours could be measured in coulombs does not mean it is necessary to do so. It is a bit like the measure of gasoline usage in litres per 100 kilometres. It can be shown that litres per kilometre can be expressed as an area (in square metres). It's fun to figure out what the hell area that might represent, but is certainly neither important nor useful. Thus, no one suggests that we should measure gasoline consumption in square metres. So go ahead and use your battery capacity in ampere-hours (or ampere-seconds if you want to be pure SI) but I see no value in insisting that it be expressed in coulombs just because it could be. ================== *That thing that is measured in ampere-hours should have a name but I don't know what it is. I find it objectionable to call something by its unit of measure. It could perhaps be called "the capacity of the battery", or something. Regards, Bill Hooper Fernandina Beach, Florida, USA ========================== Make It Simple; Make It Metric! ==========================
