Warning about cheap TVs currently on sale

In article ,
Colin Stamp wrote:

Is 150C enough to cook a sausage?

60C is enough to cook a sausage to make it edible, but it won't be very
brown. 150C is plenty.

-- Richard
--
Please remember to mention me / in tapes you leave behind.

In article ,
DM 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.

If you put an uncontrolled 1kW heater in an insulated box the result would
be a fire at worst - at best the element would simply burn out in sort
order.

Any device that produces meaningful waste heat will have a method of
dissipating this - even small audio amps have heat sinks for their output
transistors which operate by dumping that excess heat into the room by
convection. So in principle the same as a TV.

--
*When did my wild oats turn to prunes and all bran?

Dave Plowman London SW
To e-mail, change noise into sound.

On 25 Jan 2009 13:10:43 GMT, (Richard Tobin)
wrote:

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.

Ah yes. I neglected to mention that the warmup/cooldown
characteristics would be altered...

Cheers,

Colin.

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

"Roderick Stewart" wrote in
message .myzen.co.uk...
In article , Dm wrote:
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,

What about a machine that does mechanical work, such as a device used to
convey heavy items to the top of scaffolding? Some of the electrical input
will be converted into potential energy.

Is there such a thing as a 100W dildo by the way? If so, will the EEC ban
them?

Bill

DM wrote:
John Rumm wrote:

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?

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.

There *is* quite a substantial change in ground here if you read some of
your earlier contributions to this thread. I could quote them for you,
but I am sure you can find them as easily. This is not in itself a bad
thing in that it suggests that your understanding is changing (and
perhaps understandably you would rather not admit some of your earlier
statements were somewhat ungrounded). I get the feeling there are/were a
few low level misunderstandings that influenced some of your earlier
statements. I am not trying to score points here, just fill some of
those gaps if I can.

(I don't intend that to sound condescending - accept my apologies if it
does)

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.

All sources dissipating at a given power *will* heat a room to the same
extent and to the same temperature once steady state is reached.

[This assumes the heat is being spread about a bit by convection etc]

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

ok, getting to the crux of it here... how would you work out T1?

(answer at the end)

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.

Ah, ok, I there is another slight gap here. Insulation will indeed
change the rate of flow of heat - it increases the "thermal gradient" or
the temperature drop across the boundary. This is why it works well the
in walls of your house.

However, rate of heat loss is dictated not only by the level of
insulation (or the thermal resistance), but also by the temperature
differential.

If both sides of a wall are at 20 deg c then there will be no nett flow
of heat from one side to the other. If there was a 10 degree
differential then there will be, and raising it to a 20 degrees
difference would double the rate.

So, if your electric fire had a thermostatic control such that it turned
off once the room temperature reached 25 degrees, then sticking it in an
insulated box would change the power consumed and passed to the room
since the internal box temp would very quickly reach 25 and turn the
heater off. Hence reducing the average power consumed from 1kW to a few
hundred watts probably.

Now, if the heater was like the one in your experiment and not
thermostatically controlled, what happens is different. The temperature
in the box will rise very quickly to a much higher level - say 150 deg
C. This will increases the temperature difference across the insulation,
and hence the rate of heat loss through it. So the heat loss from the
insulated box will remain at 1kW. In reality the insulation has made no
difference at all.

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.

Indeed it does, and as described above it makes no difference to the
heating effect.

Where it does make a difference is to the designer of the electronics in
the box. Care needs to be taken to ensure that the temperature rise of
each component is controlled, and over heating damage does not result
(or more likely; cause serious reductions in component lifetimes).
Ideally, with electronic items like a TV this would be achieved using
convected air flow (the so called chimney effect), and heatsinks on
vulnerable components to aid heat loss. In more extreme cases forced air
cooling or some other mechanism will be required.

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.

I hope you can see from the above that this is not true. A TV will get
to operational temperature and start dissipating heat at a rate equal to
its electrical power consumption within in a few minutes. In this
respect it is easily as good as most central heating systems in terms of
lag.

The issue of lag is also not that important - think of an electric
storage heater. These are designed to introduce a massive lag between
energy going in, and it being delivered to the room. However they are
still intended to be effective heaters.



Answer to question posed above:

You need to know three things (and a fourth in real world cases - but we
can ignore that for now):

The thermal conductivity of the rooms construction materials (the "u
values" as they are called in the building regs)
The surface area of each material used.
and the rate of power input.

So, we know the power input is 1kW

lets assume the rooms is starting out at the same temperature as its
surroundings.

Lets assume it is made from materials all with a u value of 2 w/m/K, and
surface area of walls, ceiling, and floor totalling 100m^2.

So equilibrium will be reached when heat out = heat in, so:

1000 = T1 x 100 x 2

therefore T1 = 5 degrees

(which sounds pretty low, but in real life you don't normally have 6
outside walls to a room!)


--
Cheers,

John.

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On Sun, 25 Jan 2009 12:53:54 +0000
Ian Jackson wrote:

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?

Yes, but given the other replies it was probably just as well that
I switched from Natural to Social Science (more girls).

R.

In article , Bill Wright wrote:
In article , Dm wrote:
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,

What about a machine that does mechanical work, such as a device used to
convey heavy items to the top of scaffolding? Some of the electrical
input
will be converted into potential energy.

A fair point, but I did say that power in and power out would be equal
after everything has stabilised. You can't go on hoisting stuff to the top
of scaffolding forever, any more than you could delay the heating effect of
an electrical appliance forever by putting it in an insulated box. Both are
forms of energy storage, neither of infinite capacity, so you'll eventually
get all the energy back, either when the insulating box becomes warm enough
on its outside to lose energy at the same rate as it is going in through
the mains cable, or when the scaffolding falls down.

Is there such a thing as a 100W dildo by the way? If so, will the EEC ban
them?

No idea, but if there is such a thing and it turns out to be completely
harmless and people have been using them for years, then they're certain to
be banned. The reason given will be something to do with terrorists,
paedophiles, racism, religious sensitivity or the environment, or any
combination of the above.

Rod.
--
Virtual Access V6.3 free usenet/email software from
http://sourceforge.net/projects/virtual-access/

"TheOldFellow" wrote in message
...
On Sun, 25 Jan 2009 12:53:54 +0000
Yes, but given the other replies it was probably just as well that
I switched from Natural to Social Science (more girls).

And less rigour.

Bill