Quote:
Originally Posted by scc
I thought we had covered the bit about kW/s being the change in wattage before, or am I wrong?
And while my terminology might be off, the point stands that a kWs isn't something delivered by a battery or other stored power source
|
The point is that change in wattage, though it is possible to define as a physical unit, is not a variable that has a lot of actual use in physics or engineering.
As for batteries, vitruvian is right. In fact, batteries characteristically have a standard voltage that they deliver (it's a function of the metals that the two electrodes are made of); and batteries are rated for their total stored capacity in units of ampere-seconds (a 10-As battery could deliver 10 A for one second, or 1 A for 10 seconds, or 10 mA for 1000 seconds). So a battery has, say, 1.5 V x 100 As = 150 Vas = 150 Ws (since a watt is a volt times an ampere) = 0.15 kWs = 0.15 kJ (since a joule is the energy you deliver if you have a power flow of a watt sustained over a second). Or, since an As is a coulomb (the unit of charge), 1.5 V x 100 As = 1.5 V x 100 C = 150 VC = 150 J = 0.15 kJ.
The unit of kWs, or kJ, isn't explicitly stated for the battery, but it follows from what is stated and from the definitions of the units.
Bill Stoddard