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Watts/Heat relationship (dumb question)

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

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  1. Rene

    Rene Guest

    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. Rene

    Rene Guest

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

    Tim Auton Guest

    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. Rene

    Rene Guest

    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. 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 Auton

    Tim Auton Guest

    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. rayjking

    rayjking Guest

    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. rayjking

    rayjking Guest

    on the lable but uses maybe one half .

    Ray
     
  9. peterken

    peterken Guest

    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. rayjking

    rayjking Guest

    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. peterken

    peterken Guest

    Ray

    Correct, but as I read the question
    - a light bulb and a heater are both resistive loads
    - a radio is a more complex load

    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. rayjking

    rayjking Guest

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

    Ray
     
  13. Lord Garth

    Lord Garth Guest


    Let's hope you don't actually need 5 watts when using headphones!
     
  14. Rich Grise

    Rich Grise Guest

    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. 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.

    Thus, in this sense, the answer to your question is: Yes.
     
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