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 > > >
