Laws of Thermodynamics The 4 Laws of Thermodynamics

Laws of Thermodynamics

The 4 Laws of Thermodynamics • The four laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems. The laws describe how these quantities behave under various circumstances, and forbid certain phenomena (such as perpetual motion).

The Zeroth Law • (Yes, the ‘zeroth law’) • Zeroth law of thermodynamics – If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other. • If A = B and B = C, then A = C

The First Law • First law of thermodynamics – Energy can neither be created nor destroyed. It can only change forms. In any process, the total energy of the universe remains the same. For a thermodynamic cycle the net heat supplied to the system equals the net work done by the system. • Qnet = Wnet

The Second Law • The second law of thermodynamics - Heat and work are both forms of energy in transit, but they are not qualitatively equal forms of energy because work can always be converted entirely into heat, but heat can never be converted entirely into work. • Entropy plays a role, as heat being converted into work produces some waste heat. • Perpetual motion machines are impossible.

Entropy • Entropy - the measure of disorder and randomness in a system. • Example: – Let’s say you have a container of gas molecules. If all the molecules are in one corner then this would be a low entropy state (highly organized). As the particle move out and fill up the rest of the container then the entropy (disorder) increases.

Entropy • Example 2: – If you have a ball flying through the air then it will start off with its energy organized i. e. the kinetic energy of motion. As it moves through the air however, some of the kinetic energy is distributed to the air particles so the total entropy of system has increased (the total energy is conserved however, due to the first law) • Entropy is the amount of energy that is made unavailable for work (wasted, dispersed).

The Third Law • Third law of thermodynamics – As temperature approaches absolute zero, the entropy of a system approaches a constant minimum. • Essentially the closer a system gets to absolute zero, the less entropy it has. At absolute zero, the entropy would also be zero.