On Apr 23, 11:25*am, Tom Anderson wrote:
On Fri, 22 Apr 2011, bob wrote:
On Apr 21, 7:18�pm, Tom Anderson wrote:
On Thu, 21 Apr 2011, Capt. Deltic wrote:
On 21 Apr, 09:58, wrote:
On Thu, 21 Apr 2011 09:33:41 +0100

Graeme Wall wrote:
Pedantically they have motors, not engines. �The latter being those
nasty infernal combustion thingies. �Motors run on nice clean electrickery.

Tell that to Arthur Daley!

To be even more pedantic, an engine generates power, while a motor
consumes power.

What? *What*?

'Generates' power? 'Consumes' power? Has that small matter called the
first law of thermodynamics passed you by?

I assume Uncle Roger means "shaft power" as used in the context of the
second law applied to a control volume (ie work rather than heat).

I'm afraid i'm not familiar with the term "shaft power"; a quick google
suggests it means the mechanical power at the drive shaft, but i don't see
how that's relevant here. Is that what you meant? In what way does an
electric motor consume it?

That is the derivation of the term, but thermodynamically things like
electricity and magnetic forces Behave in the same way. It is a
general term for energy entering a system that does not have an
associated entropy change with it.

Calling one an engine and one a motor is a matter of convention. It's
preposterous to ascribe a fundamental meaning to the distinction.

The distinction is related to the second law. *A motor converts "work"
to other "work" while an engine converts heat to work (and some left
over heat).

There's no difference between what a combustion and an electric engine do
he matter flows from a place where it has a high potential to where it
has a low potential, increasing entropy as a result, and that flow is
harnessed to turn a shaft. In a combusion engine, the matter is steam or
combustion gases, and the potential is of the heat-and-pressure kind. In
an electric engine, the matter is electrons, and the potential is of the
electrical kind.

The difference is entropy. In a control volume type analysis, shaft
work (electrical power) adds no entropy to the system, heat transfer
does. If you have a motor, you put some work in and get some work out
(in the case of hydraulics the difference in work is the pressure x
volume pumping work to get it in or out). In a heat engine (including
all combustion engines) the second law places constraints on the
engine behaviour requirig a heat rejection as well as heat input and
work input in order to do work. It all comes down to entropy.

To get another angle on it, do you think a watermill is a motor or an
engine? What about a piston engine driven by pressurised water?

Water mill: work done by a falling weight is converted to work in a
shaft. No heat.
Hydraulic motor: work done pumping water in is converted to work in
the shaft. No heat.

IMHO, the whole heat/work dichotomy has been really unhelpful since it did
its part in getting thermodynamics started. There are states with
different potentials, and various amounts of stuff in those states. Stuff
wants to flow from high-potential states to low-potential states, and you
can harness such flows. There's no value in drawing a distinction between
potentials where the states are is in the same place in space (eg chemical
potential, electron potential in an atom) and those where they aren't
(primarily heat gradients).

But to ignore the difference between these is to ignore entropy and to
therefore ignore the second law of thermodynamics. The entire basis
of the second law is the difference between heat and work, the
relation between heat and temperature, and the limitations on things
like the maximum efficiency of heat engines (motors can be100%
efficient, heat engines can not).

Robin