# Watts/Heat relationship (dumb question)

Discussion in 'Electronic Basics' started by Rene, Oct 20, 2004.

1. ### ReneGuest

Suppose that I have a light bulb consuming 100 watts, a radio also consuming
100 watts and an electric heater also consuming 100 watts.

If I were to measure the heat generated by all these 3 devices, would I get
the same amount of heat? In other words, would all devices consuming the
same wattage (solid state, no moving parts) generate the same amount of
heat?

Thank you.

2. ### ReneGuest

One more thing, assume they also use the same voltage.

3. ### Tim AutonGuest

The light bulb and the radio will put out a fraction of their energy
as light or sound respectively, so it's not all heat. However, it
doesn't take long before that light and sound will hit something, not
bounce off and be converted into heat.

Tim

4. ### ReneGuest

The light bulb and the radio will put out a fraction of their energy
So, does this mean that at the end, they will all be generating the same
amount of heat?

5. ### John PopelishGuest

In a very general way, watts are heat, unless you can follow then to
some other end (like potential energy or chemical energy etc.) If you
put each of the devices you mention in calorimeters (devices that
measure heat by how much the temperature of a known thermal mass rises
in a given period of time (and if you compensated for the heat stored
in the devices, themselves) it would not matter what device consumed
the watts or what voltage this power was driven by. Watts convert
directly to calories which are measure of heat energy.

6. ### Tim AutonGuest

Yes. If you put the devices in light-proof, sound-proof boxes then you
would measure that all the boxes would be heated by the same 100W.
It'll generally all get converted to heat wherever the light and sound
goes, but it's tricky to measure if some of the light gets all the way
to Alpha Centauri!

Very strictly speaking some may not end up as heat in certain
circumstances - sound or light may end up using some energy breaking
chemical bonds for example, but for a radio and light bulb that's
insignificant. Even for devices designed to break bonds (like the
ultrasound thingies used to smash gallstones) it's a tiny fraction
that doesn't end up as heat very quickly.

Tim

7. ### rayjkingGuest

Hi,

The light bulb is resistive and is purchased for the value of the watts.
This assumes the filament is at the correct temperature for visible light
conversion.
Other items ( small motors and radios and other items that convert ac into
dc are given a rating of peak power ( watts ) so as to be easy to calculate
a fuse that should be used in the main switch box to survive repetive re
starts after power outages ).

Ray

8. ### rayjkingGuest

on the lable but uses maybe one half .

Ray

9. ### peterkenGuest

yes, if they all converted exactly the same amount of energy into something
else (movement, light, sound)
no, if they all convert different amounts of energy into something else

the bulb will convert an amount into light, the non-light-energy will be
turned mostly into heat
the heater will have losses in light-radiation (usually IR), the rest will
be dissipated as heat
the radio will convert energy into sound, the rest will be dissipated as
heat

so it all depends on the efficiency of every aparatus that will be compared

Suppose that I have a light bulb consuming 100 watts, a radio also consuming
100 watts and an electric heater also consuming 100 watts.

If I were to measure the heat generated by all these 3 devices, would I get
the same amount of heat? In other words, would all devices consuming the
same wattage (solid state, no moving parts) generate the same amount of
heat?

Thank you.

10. ### rayjkingGuest

Rene,

You are correct. If you measure watts to be equal then you are correct
except for the power factor which is ( Power = E x I x Cos of the angle
between the voltage and current ) ( theta ) which is different for loads
other than resistive. Just measuring the voltage and just measuring the
current ( E x I ) gives you apparent power which is always more than that
which is creating heat-watts.

Ray

..

11. ### peterkenGuest

Ray

Correct, but as I read the question
- a light bulb and a heater are both resistive loads

As I see the question I read "all these apparatus consuming the same
wattage..."
so I in fact read "do all these apparatus turn the applied (identical) power
into heat"

Answer is no, given the fact all of them are designed for different purposes
so some of the applied power is turned into something else as intended per
design, and the rest into heat

Proof for this can be established empirically

Or just take next situation :
I'd like to stand next to my 1kW heater on a cold day, it gets me warm
I wouldn't like to stand next to my 1kW amplifier on a cold day, it mostly
creates noise but gives me ampel heat Rene,

You are correct. If you measure watts to be equal then you are correct
except for the power factor which is ( Power = E x I x Cos of the angle
between the voltage and current ) ( theta ) which is different for loads
other than resistive. Just measuring the voltage and just measuring the
current ( E x I ) gives you apparent power which is always more than that
which is creating heat-watts.

Ray

..
something else (movement, light, sound)

12. ### rayjkingGuest

Peter,
With the rising oil/energy prices I may convert back to head phones in the
less than 5 watt range.

Ray

13. ### Lord GarthGuest

Let's hope you don't actually need 5 watts when using headphones!

14. ### Rich GriseGuest

With headphones, a few milliwatts are probably sufficient. It's amazing
how little actual sound power it takes to be really, really loud. One
watt of sound power will fill a room uncomfortably loud. Any amp. power
over that is nothing but "reserve." Well, and the inefficiencies, of
course, like speakers that need 100 watts just to get the cone up out
of bed. ;-)

Cheers!
Rich

15. ### Dr Engelbert BuxbaumGuest

There is something called the first law of thermodynamics which states
that energy can not be created or destroyed. However, different forms of
energy can be interconverted. Thus you can turn electrical energy into
heat, light or sound.

However, these energies have different "values" because of the second
law of thermodynamics. Crudely speaking, this law states that the amount
of disorder ("entropy") increases in an isolated system during each
reaction. Thus electrical energy, with its ordered flow of electrons,
can be turned into heat (random motion of molecules) with 100%
efficiency. The reverse reaction is possible only with a loss, thus the
efficiency of a power station is limited to 40% or so.

If the universe is considered an isolated system (which can not be
proven), than all forms of energy must eventually be converted to heat
because of the second law of thermodynamics, und this heat must
distribute evenly throughout the universe. Once this has happend, no
further reactions will be possible.  