Warning about cheap TVs currently on sale

John Rumm wrote:
DM wrote:

Every eletronic element within a TV has mass, all mass when heated
up stores that heat as potential energy relative to teh surrounding
environment. The heat is stored, and not dissipated efficeintly.

No.

All the components in the TV will have a heat capacity - that is
dictated by the specific heat capacity of the material in question,
and the amount you have of it.

So for a simple carbon resistor that will be around 700J /kg/K if you
ignore its leads for the moment.

As a current flows through the resister it will get hotter, so as you
say its potential energy will increase and it will be storing a small
amount of energy as heat. However the rate of heat loss to its
surroundings will also increase since this is dictated by the
temperature differential to its surroundings. If the current remains
constant, the the power being developed in the resister will also
remain constant and the temperature will continue to rise. After a
time one of two things could happen. The resistor could over heat and
fail open circuit, or more likely, equilibrium will be reached. At
this point the rate of heat loss to the surroundings will exactly
match the power input to it. When you turn the set off, the resister
will then lose its stored heat to the surroundings as well.

Hence ultimately all the energy dissipated by the resistor is lost as
heat.

Ultimately yes - but the issue if power and energy conversion efficeiny

So, if you agree that all the energy that you put in will ultimately
come out as heat, you would also have to agree that if you stick 1kJ
into a TV, the amount of energy released to the room will also be 1kJ.

Now efficiency in the case of a heater is defined in terms of the ratio
of energy put in to it that is usefully dissipated where you want it. A
gas boiler will not be 100% efficient since some of the heat generated
will pass out of the flue and not into your central heating. An electric
heater however is pretty much 100% efficient (at point of use) - even
the voltage drops on its connecting leads will typically result in heat
being deposited where you want it.

You belive light output all contributes to heat, but again this is
incorrect- everything would appear black if this was true. -
evidently that is mot so.

How do you come to that conclusion?

When light strikes an object, some of it is reflected, and some will
be absorbed (and this will happen no matter how reflective the
surface, since no surface it perfectly reflective). The proportion of
the light which is absorbed will result in a rise in temperature of
the object absorbing it. Of the reflected light, it will also in
turn hit an object where some will be absorbed and some reflected.
This will continue to happen until all the light has been absorbed.
Now in your closed box, the light will bounce around for few nano
seconds - but ultimately it will all be absorbed - how many
reflections are sustainable before absorption of all the photons will
depend on the reflectivity or the surfaces and the amount of light in
the first place.

So unless the light keeps on being generated, it gets dark very
quickly. Assuming the light stays on, then steady state is also
reached very quickly, with all the light being generated, being
reabsorbed within a few reflections, but the reflections prevent
everything appearing black.

Again a simepl matter of time.

I think you be confusing yourself by thinking in terms of rate of
transfer of energy (i.e. power), when it is the total amount of energy
that matters.

If it is absobed then eth ernergy transfer is 1nS saym if it is
reflected and absorbed at a later time then the energy transfer is 2ns

So in both cases, 100% of the energy is absorbed.

say. Now in case 1 the power trasfer is doubele case 2. So it is a

Say your turn on a light for 1 second in a sealed box. The box is such
that it takes 10nS for the first photons to all be absorbed. That means
if you plot a graph of the energy absorbed over time, you will see a
ramp up starting at time = 0, that will reach maximum at t = 10ns. It
will stay at a steady state until you switch the light off, in which
case you will see a ramp down that finishes at 1 sec + 10ns.

The energy in = energy out, the rate of flow of energy in = rate of flow
of energy out once equilibrium is reached. The "storage" of the system
(i.e. the flight time of the light prior to absorption) will introduce a
delay, but not ultimately have any effect on the total energy radiated
and absorbed.

more efficeint case to heaqt up the matter. We are talking about the
effciency of differnet sources for heat- we can not wait indefiently

You seem to be attempting to shift ground a little here. The discussion
was whether the TV consuming power at 100W would also be radiating
energy to the room at 100W. I take it you now accept that it would once
it is warmed up?

So in the case of the TV, you may stick 100W into it for 10 hours. 1kWh
of energy all told. It may take 15 mins to reach full operating
temperature. During which time its apparent dissipation will appear to
ramp up from nil to 100W. However after warm up, it will continue
dissipating at 100W until you turn it off. At which point it will
continue to dissipate at a decaying rate loosing any stored heat energy.

Any time delay or lag in response does not change the efficiency,
although it lowers the responsiveness. A TV is probably comparable to a
normal radiator in terms of response time.




John there is not a shift of ground or anything - however things have
inevitably drifted.

The position remains the same- not all sources will heat up a room to
the same extent- In this context we are talking about a tmeprature
increase- there is no argument regarding energy in = energy out.

Let us take an absurb extreme situation - we take a 1kW eletric heater-
it raises the temeprature of the room by T1.

Now we stick it in insulated box in the same room- the temperature rise
in the room is obviously less. So we have less heat trasnfered from the
source to the room.

Yes this is obviously fairly extreme, but as soon as you stick a bunch
of electronics inside a plastic box then this condition exists to some
extent.

A TV is not as efficient way to heat up a room- it is not a good source
of heat in this respect., where heat is indicated by a rise in
temperature of the room.

On Sun, 25 Jan 2009 11:46:05 +0000, DM wrote:

[snip]
Let us take an absurb extreme situation - we take a 1kW eletric heater-
it raises the temeprature of the room by T1.

Now we stick it in insulated box in the same room- the temperature rise
in the room is obviously less. So we have less heat trasnfered from the
source to the room.

This is the crux of your problem. All of your understanding of the
subject is based on this incorrect belief.

As everyone else on the thread already knows, the ultimate temperature
rise of the room will be the same in both cases, assuming the heater
and the insulated box can be made to survive the experience.

What would it take to convince you of this?

Cheers,

Colin.

--
Threre's a spam-trap on my return address, just in case you hadn't noticed...

In article , Dm wrote:
A TV is not as efficient way to heat up a room- it is not a good source
of heat in this respect., where heat is indicated by a rise in
temperature of the room.

For practical purposes, energy losses due to light and sound will be
infinitesimally negligible, so in terms of heat output, a 100 Watt TV set
is exactly as efficient as a 100W bulb, or a 100 Watt heater, or a 100
Watt electric dildo. 100 Watts is 100 Watts. If there's 100 Watts of
electrical energy going in there'll be 100 Watts of heat energy coming
out, and once conditions have stabilised, whatever the device is it will
raise the temerature of its surroundings at exactly the same rate - or
have you discovered some new laws of physics that the rest of us don't
know about?

Rod.
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http://sourceforge.net/projects/virtual-access/

In message , DM
writes


John there is not a shift of ground or anything - however things have
inevitably drifted.

The position remains the same- not all sources will heat up a room to
the same extent- In this context we are talking about a tmeprature
increase- there is no argument regarding energy in = energy out.

Let us take an absurb extreme situation - we take a 1kW eletric heater-
it raises the temeprature of the room by T1.

Now we stick it in insulated box in the same room- the temperature rise
in the room is obviously less. So we have less heat trasnfered from the
source to the room.

Yes this is obviously fairly extreme, but as soon as you stick a bunch
of electronics inside a plastic box then this condition exists to some
extent.

A TV is not as efficient way to heat up a room- it is not a good source
of heat in this respect., where heat is indicated by a rise in
temperature of the room.

Have you ever thought what eventually happens to a sausage in fridge in
a completely thermally insulated room?
--
Ian

The message
from "Bill Wright" contains these words:


"Richard Tobin" wrote in message
...
In article ,
If you want to be instantly heated, probably the quickest way is to
use a hairdryer.

No, climb into the microwave.

Bill

Well, that's fine for a hamster, but us humans will have to make do
with the hairdryer ;-)

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

On Sun, 25 Jan 2009 12:37:10 +0000
Ian Jackson wrote:

In message , DM
writes

A TV is not as efficient way to heat up a room- it is not a good source
of heat in this respect., where heat is indicated by a rise in
temperature of the room.

Have you ever thought what eventually happens to a sausage in fridge in
a completely thermally insulated room?


It goes bad along with everything else in the fridge as the fridge's
fuse/cable/compressor melts long before it cooks the sausage
etc. properly. Incidentally, this was a question asked of me by an
interviewer for a place at a University in 1968. It brought back fond
memories, thanks!

R.

In message , TheOldFellow
writes
On Sun, 25 Jan 2009 12:37:10 +0000
Ian Jackson wrote:

In message , DM
writes

A TV is not as efficient way to heat up a room- it is not a good source
of heat in this respect., where heat is indicated by a rise in
temperature of the room.

Have you ever thought what eventually happens to a sausage in fridge in
a completely thermally insulated room?


It goes bad along with everything else in the fridge as the fridge's
fuse/cable/compressor melts long before it cooks the sausage
etc. properly. Incidentally, this was a question asked of me by an
interviewer for a place at a University in 1968. It brought back fond
memories, thanks!

R.
Glad to be of service! Did you get in?
--
Ian

The message
from Colin Stamp contains these words:

On Sun, 25 Jan 2009 11:46:05 +0000, DM wrote:

[snip]
Let us take an absurb extreme situation - we take a 1kW eletric heater-
it raises the temeprature of the room by T1.

Now we stick it in insulated box in the same room- the temperature rise
in the room is obviously less. So we have less heat trasnfered from the
source to the room.

This is the crux of your problem. All of your understanding of the
subject is based on this incorrect belief.

As everyone else on the thread already knows, the ultimate temperature
rise of the room will be the same in both cases, assuming the heater
and the insulated box can be made to survive the experience.

Exactly! Ignoring real world limitations of any insulating materials
used, as well as those used in the electric heater itself, the
temperature inside the insulated box would just keep on rising until the
temperature of the electric fire increases the heat flow via the thermal
resistance of the insulating box to the room. There would be a 'thermal
lag' introduced, but once a thermal equilibrium had been reached, the
heating effect would become exactly the same as without the insulating
box.

What would it take to convince you of this?

Personally, I'm guessing a _fuller_ understanding of the laws of
thermodynamics? ;-)

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

On Sun, 25 Jan 2009 12:48:37 +0000, TheOldFellow
wrote:


It goes bad along with everything else in the fridge as the fridge's
fuse/cable/compressor melts long before it cooks the sausage
etc. properly. Incidentally, this was a question asked of me by an
interviewer for a place at a University in 1968. It brought back fond
memories, thanks!


I'm not so sure about that )

I'd have thought the self-resetting thermal cut-out on the compressor
would come into play here.

I've no idea what temperature they operate at - let's say 150C. The
first time it might cut out at a room temperature of, say, 50C. Then,
as the compressor cools, it'll cut back in etc. When it cuts out for
the last time, the room temperature will be somewhere near 150C. I
reckon the motor, cable etc. will just-about survive to that point.

Is 150C enough to cook a sausage?

Cheers,

Colin.

--
Threre's a spam-trap on my return address, just in case you hadn't noticed...

In article ,
Colin Stamp wrote:

Let us take an absurb extreme situation - we take a 1kW eletric heater-
it raises the temeprature of the room by T1.

Now we stick it in insulated box in the same room- the temperature rise
in the room is obviously less. So we have less heat trasnfered from the
source to the room.

This is the crux of your problem. All of your understanding of the
subject is based on this incorrect belief.

As everyone else on the thread already knows, the ultimate temperature
rise of the room will be the same in both cases, assuming the heater
and the insulated box can be made to survive the experience.

Be careful - there is a latency issue here. If the room is constantly
losing heat, and the heaters are only on for a short time, then the
heater-in-a-box will not raise the room to the same temperature,
because it will take longer for the heat to get out. On the other
hand it will keep the room warm for longer. Obviously if they are
left on continuously the room will reach the same equilibrium
temperature.

In practice, a TV is likely to be quite quick at dissipating heat,
since most of it is generated in the screen.

-- Richard
--
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