The first and second law of thermodynamics

[jamesd2008]

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
James

 

[brewnog]

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.

 

[jamesd2008]

Thats great thanks for your input, So although the energy is conserved, to gain practical useful work extra heat is needed?

 

[Topher925]

Thats great thanks for your input, So although the energy is conserved, to gain practical useful work extra heat is needed?

That is correct. In other words, you can never have an energy conversion device or process which is 100% efficient.

 

[alphy]

I can clarify the meaning of the first and second laws of thermodynamics. The first law states that in a closed system (where no matter can enter or leave), the total energy remains constant. This means that the total amount of energy in a system cannot be created or destroyed, only transferred or converted from one form to another. So, in a complete cycle, the net heat supplied (energy added to the system in the form of heat) plus the net work input (energy added to the system in the form of work) must equal zero, as all the energy added to the system must be accounted for. This is known as the law of conservation of energy.

On the other hand, the second law of thermodynamics deals with the direction of energy transfer and conversion. It states that in any process, the total entropy (a measure of disorder or randomness) of a closed system will either remain constant or increase, but it can never decrease. This means that in a complete cycle, the total energy added to the system (gross heat supplied plus net work input) must be greater than zero, as some energy will always be lost in the form of heat due to the increase in entropy.

So, these two laws are not contradicting each other, but rather they complement each other in explaining different aspects of energy in a closed system. The first law deals with the conservation of energy, while the second law deals with the direction of energy transfer and conversion. I hope this helps clarify any misunderstanding.