On 2009-09-16, Zero Tolerance wrote:
On Tue, 15 Sep 2009 04:21:51 -0700 (PDT), "Man at B&Q"
wrote:

By your logic, if I leave a Sky+ box on standby, then the 20 watts it
spends on spinning the hard disc is converted into 20 watts of heat.

It is.

No. It's converted into quite a lot of 'work', quite a lot of 'motion'
(or what your earthling mind may know as "force"), and a small amount
- much less than 20 watts net worth - of heat.

No. Energy cannot be converted into "motion", it can be converted
in to kinetic energy - which would make something accelerate.

Ek = 1/2mv^2

The more energy you supply the faster it goes. Supplying 20J of
kinetic energy per second to a spinning disk would result in it
spinning very quickly indeed. The disk, instead, spins at a
constant(ish) rate because, once it reaches the operating speed,
it is also losing energy as friction (converting it in to heat)
at a rate of 20J per second. The energy initially used to
accelerate the disk to its steady state "stays" in the disk
until power is removed and it spins down as the remaining
energy is converted to heat by friction.


If that were true, it would turn Sky+ into a free energy machine -

It's taken 20W from the mains supply. How is that "free"?

You can't take 20 watts, then get 20 watts worth of use (e.g. CPUs,
processors, spinning discs, etc) out of it, then still have 20 watts
left which is magically converted into heat. That's not how it works.
There are losses at every stage of energy conversion.

The losses _are as heat_. A CPU "uses" power because the movement
of electrons required to switch a transistor on or off is an
electric current, flowing through a resistance (as CPUs are not
superconducting). This results in resistive heating:

P = I^2R

An inefficient PSU takes in energy, loses some -- as heat, and
outputs less energy. Your disk then takes energy in, and turns
some of it temporarily into movement (of the spinning platters,
of the read/write heads, of the vibrating disk), which is all
quickly turned in to heat through friction. The CPU turns
all its input power nearly instantaneously into heat (hence
the massive heatsinks and fans on powerful processors).
Even a monitor produces heat - the light will be absorbed
by the walls, ceiling, your eyes, the carpet, the window.
(Some might escape through the window and heat the atmosphere,
some might even escape the atmosphere and fly through the
universe for years -- most wont).

--
David Taylor