Dear Martin, Thanks for the idea of the light bulb and the jam -- I like it and I'm sure that I'll use it when an occasion arises.
Cheers, Pat Naughtin PO Box 305 Belmont 3216 Geelong, Australia 61 3 5241 2008 [EMAIL PROTECTED] http://www.metricationmatters.com On 11/03/06 9:28 PM, "Martin Vlietstra" <[EMAIL PROTECTED]> wrote: > Hi Pat, > > I recently had that problem while doing some part-time teaching of physics. > I got around it by getting a jar of jam and a light bulb. The light-bulb > had "100W" stamped on it, while the nutritional information part of the > label on the jam jar "Energy - nnn Joules /100g". It is quite simple - one > spoon of jam will contain a specific quantity of energy - if you take a long > time to eat the jam, the rate at which you absorb the energy will be slower. > On the other hand a light bulb produces 100W of light (or rather 10W of > light and 90W of heat) regardless of how long it is on. > > I did of course explain to my student that if, after eating the jam, he sat > on a bicycle, had a dynamo attached to the wheel and started pedalling, the > energy from the jam would be converted to light energy for a specific period > of time. (I also explained that this was not a 100% efficient machine). > > Does this example help at all. > > Regards > Martin. > ----- Original Message ----- > From: "Pat Naughtin" <[EMAIL PROTECTED]> > To: "U.S. Metric Association" <[email protected]> > Sent: Saturday, March 11, 2006 8:25 AM > Subject: [USMA:36244] Re: diesel electric propulsion > > >> Dear Robert and All, >> >> In your email below, I note your careful use of the word 'power' to mean > the >> 'time rate of using energy'. >> >> However, such careful use is not all that common in the public media. For >> example, we have here in the Melbourne paper, 'The Age', a journalist with >> the title, 'Energy Reporter' who regularly uses the word 'power' when he >> means energy, and the word 'energy' when he means power. >> >> As an example he recently wrote that if a particular power station did not >> produce enough kilowatt hours of electricity, there would be a power >> failure. >> >> Do you have a rule of thumb to indicate when to use 'power' and when to > use >> 'energy'? How would you advise a young journalist to distinguish between >> these concepts and their correct metric or SI units. >> >> Cheers, >> >> Pat Naughtin >> PO Box 305 Belmont 3216 >> Geelong, Australia >> 61 3 5241 2008 >> [EMAIL PROTECTED] >> http://www.metricationmatters.com >> >> On 11/03/06 6:59 AM, "Robert H. Bushnell" <[EMAIL PROTECTED]> wrote: >> >>> 2006 March 10 >>> In the matter of propulsion with diesel engines, the design was made > optimum >>> more than 60 years ago. A railroad locomotive has a diesel engine (often >>> more than one) which drives a generator which drives electric motors > which >>> drive the propulsion wheels with fixed gears (no gear shift). The > generator >>> and the motors are each compounded to allow maxumum power at any speed. > The >>> generator and the motors are a matched set. A compound motor has both a >>> shunt field winding and a series field winding. The design of the > number of >>> wire turns (the windings) in each of these fields and the number of > turns in >>> the armature (the moving part of the motor) is motor engineering at its >>> best. The design gives maximum power transfer without controls in the >>> electric circuit. The throttle (the injector stroke) on the diesel > engine >>> is the only control. Railroad diesel engines are never turned off so > there >>> is a way to stop generation. (I would open the shunt field so the > engine >>> could idle with no generation.) >>> >>> To start a train from zero speed the diesel engine can throttle up to >>> maximum power while the train is still hardly movimg. The compounding > in >>> both the generator and the motors gives high efficiency at this low > speed. >>> The compound windings give high efficiency at any speed so the throttle > can >>> be left wide open at any speed. >>> >>> For ship propulsion, the design may be different. For a screw > propellor, >>> power and efficiency change with speed of the shaft rotation and the > speed >>> of the ship. I have been told that the design of ship screws is more an > art >>> than an engineering matter. So, screw performance is found by > experiment. >>> Given this measured performance, the motor compound can be designed. >>> >>> Robert H Bushnell PhD PE >>> >> >
