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The first and second law of thermodynamics

  1. Sep 8, 2010 #1
    In my thermodynamics book it states that the first law is,

    " When a system undergoes a complete cycle the net heat supplied plus the net work input is zero"

    And then it states that for the second law,

    "In any complete cycle the gross heat supplied plus the net work input must be greater than zero"

    Are these not contradicting each other or am I miss understanding the gross and net aspects? Could someone please help me try to understand?

    Thanks in advance
  2. jcsd
  3. Sep 8, 2010 #2


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    The first law, as you have quoted, is pretty much saying "the energy you put in must be equal to the energy you get back out again".

    The second law says "while the first law still applies, to get something useful out, you need to put the same amount in, plus a bit more".

    The Wikipedia article is pretty good.
  4. Sep 8, 2010 #3
    Thats great thanks for your input, So although the energy is conserved, to gain practical useful work extra heat is needed?
  5. Sep 9, 2010 #4
    That is correct. In other words, you can never have an energy conversion device or process which is 100% efficient.
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