# Laws of Thermodynamics

The laws of thermodynamic describe the relationships between matter and energy under different conditions. (image credit: Brenda Clarke)

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, probably to confuse you).

Here’s a list of the laws of thermodynamics and a quick summary of what each law means.

Zeroeth Law of Thermodynamics

If two systems are in thermal equilibrium with a third system, they must be in thermal equilibrium with each other. This law establishes the concept of temperature.

Example: If your car is the same temperature as your house and your car is the same temperature and 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”.

### Second Law of Thermodynamics

The second law 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 will wind down as its potential energy is converted into kinetic energy. After that, the watch won’t run again until new energy is input into the system by winding the watch.

### 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.

### 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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