Second Law of Thermodynamics

What is the Second Law of Thermodynamics?

The Second Law of Thermodynamics, also known as the Law of Increased Entropy, says that over time the state of disorganization or entropy in a system will always increase. 

Over time, usable energy will eventually give way to unusable energy. While energy cannot be created or destroyed according to the First Law, it can change form a useful state to a less-useful state, like thermal energy (heat).

For example, as you read this article, entropy is all around you. Cells within your body are dying and degrading, an employee or coworker is making a mistake, the floor is getting dusty, and the heat from your coffee is spreading out. Zoom out a little, and businesses are failing, crimes and revolutions are occurring, and relationships are ending. Zoom out a lot further, and we see the entire universe marching towards a collapse.

The Discovery of Entropy

The identification of entropy is attributed to Rudolf Clausius (1822–1888), a German mathematician and physicist. Clausius studied the conversion of heat into work. He recognized that heat from a body at a high temperature would flow to one at a lower temperature. 

This is how your coffee cools down the longer it’s left out — the heat from the coffee flows into the room. This happens naturally. But if you want to heat cold water to make the coffee, you need to do work — you need a power source to heat the water.

From this idea comes Clausius’s statement of the second law of thermodynamics: “heat does not pass from a body at low temperature to one at high temperature without an accompanying change elsewhere.”

Work and Energy

One thing the Second Law explains is that it is impossible to convert heat energy to mechanical energy with 100 percent efficiency. After the process of heating a gas to increase its pressure to drive a piston, there is always some leftover heat in the gas that cannot be used to do any additional work. This waste heat must be discarded by transferring it to a heat sink. In the case of a car engine, this is done by exhausting the spent fuel and air mixture to the atmosphere. Additionally, any device with movable parts produces friction that converts mechanical energy to heat that is generally unusable and must be removed from the system by transferring it to a heat sink. 

When a hot and a cold body are brought into contact with each other, heat energy will flow from the hot body to the cold body until they reach thermal equilibrium, i.e., the same temperature. However, the heat will never move back the other way; the difference in the temperatures of the two bodies will never spontaneously increase. Moving heat from a cold body to a hot body requires work to be done by an external energy source such as a heat pump.

The Arrow of Time

In theory, some interactions, such as collisions of rigid bodies or certain chemical reactions, look the same whether they are run forward or backward. In practice, however, all exchanges of energy are subject to inefficiencies, such as friction and radiative heat loss, which increase the entropy of the system being observed. 

Therefore, because there is no such thing as a perfectly reversible process, if someone asks what is the direction of time, we can answer with confidence that time always flows in the direction of increasing entropy.