Thermodynamics is the study of energy and heat. The laws of thermodynamics describe the relationship between matter and energy and how they relate to temperature and entropy. Many texts list the three laws of thermodynamics, but really there are four laws (although the 4th law is called the zeroeth law).
Here’s a list of the laws of thermodynamics and a quick summary of what each law means.
Zeroeth Law of Thermodynamics
The zeroeth law of thermodynamics establishes the concept of temperature:
If two systems are in thermal equilibrium with a third system, they must be in thermal equilibrium with each other.
Example: If your car is the same temperature as your house and your car is the same temperature as your office, then your home and your office are the same temperature as each other.
First Law of Thermodynamics
The first law of thermodynamics is also known as the Law of Conservation of Energy. It states energy of a system may change forms, but it is neither created nor destroyed. One way to state this law is “you can’t get something for nothing”.
Another way of stating the first law is that internal energy (∆E) of a system is the sum of the heat flow across its boundaries (q) and the work done on the system (W).
∆E = q + W
Example: The energy of a child on a swing is a combination of kinetic and potential energy. The total energy of the system is the same whether the child is at the top of the swing’s arc (minimum kinetic energy, maximum potential energy), the bottom of the arc (maximum kinetic energy, minimum potential energy), or anywhere in between.
Second Law of Thermodynamics
The second law of thermodynamics states the entropy of a system not in thermal equilibrium increases. Entropy is a measure of the randomness or disorder of a thermodynamic system. As entropy increases, less energy is available for useful work. If the first law states you can’t get something for nothing, you could consider the second law to mean “… and you can’t break even.”
Example: A watch driven by a spring winds down as its potential energy converts into kinetic energy. After that, the watch won’t run again until you add new energy into the system by winding the watch.
Example: Over time, your bedroom gets messier and messier. It only gets cleaner if you expend energy and pick things up.
The second law of thermodynamics also indicates the direction of heat flow in a system. Thermal energy only flows from higher energy to lower energy.
Third Law of Thermodynamics
The third law states the entropy of a system approaches as constant value as the temperature approaches absolute zero. Absolute zero is the lowest theoretically possible temperature (0K or zero Kelvin).
The entropy of a system at absolute zero is nearly zero, but not necessarily exactly zero. Entropy depends on how many ground states a system has. Pure crystalline matter attains perfect order. It has one minimum ground state and has zero entropy at absolute zero. However, most matter never quite attains zero entropy.
Perpetual Motion Machines Are Impossible
One implication of the laws of thermodynamics is that perpetual motion machines are not possible. While energy may change from one form into another, entropy increases, and a bit of usable energy is lost. Machines are powered by energy sources that eventually are depleted. The closest people can get to perpetual motion is to use an initial power supply that seems endless, such as solar power, as the initial energy input.
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