Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy quality

  1. Jan 9, 2007 #1
    My idea is : electricity can be transfered 100% into heat. Mechanical energy can be as well. But heat can not be transfered completely into electricity or mechanical energy. From that point of view, I think that electricity and heat and... are all some forms of energy, but the quality of each kind is different?
  2. jcsd
  3. Jan 10, 2007 #2


    User Avatar
    Science Advisor

    They CAN be transformed back to electricity, but it is very difficult to do it, the "quality" have sunk, but the energy is still there and have not lessened!

    We think the only way to create matter from heat is to make the heated atoms send out their energy at a such high frequenzy that if two photons collided they would create a tiny amount of anitmatter and matter. This is of course much more difficult than making the matter send out photons.
  4. Jan 10, 2007 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor


    To the original question: Just to make sure you don't get led down the wrong way, there is another consideration here that you missed - entropy.

    You can't just account for energy content when doing conversion from one form to another. You also have to consider the entropy at every stage of the conversion. Converting one form that has a higher degree of entropy into something that has a lower degree of entropy is difficult (i.e. you won't get ALL of it back), or will require other forms of energy (such as work done) to facilitate the conversion.

    So yes, while energy is energy, the forms of energy have other factors that have to be considered when converting from one to the other.

  5. Jan 11, 2007 #4
    I agree with you at some point. But people still use different units to measure those forms of energy: calorie, joule, watt and newton metre and the conversion fators from one to another (and vice versa) is fixed. Should there be certain equation describing the 'difficulty' of converting from high entropy into low entropy form of energy (let say from heat into electricity)?
  6. Mar 6, 2011 #5
    The measure of the quality of an electric current is voltage, and the measure of the quality of a thermodynamic substance is temperature. ie, the higher the voltage, the higher the temperature of the heat which could be produced from it.
  7. Mar 6, 2011 #6
    Yes, there should be. And with regard to photon distributions, there is. The quality of a photon is measured by its frequency, and the quality of a thermodynamic substance by its temperature. There's a simple formula which allows one to convert between photon frequency and photon 'temperature'. For instance, the radiation we receive from the Sun is very nearly the same as the radiation which would be emitted by a black body at 6000 K. Hence, we can say that visible light (which is at the peak of the photon distribution emitted by the Sun) has a temperature of around 6000 K. This is what makes life on earth possible. We receive 6000 K energy from the Sun and radiate ~290 K energy back into outer space.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?