Defining Perpetual Motion
A perpetual motion machine is a hypothetical device that, once set in motion, would continue to operate indefinitely without any external energy source or continuously produce more energy than it consumes. Such machines are classified into two main types: those that violate the first law of thermodynamics (perpetual motion of the first kind) and those that violate the second law (perpetual motion of the second kind).
Violation of the First Law of Thermodynamics
A perpetual motion machine of the first kind attempts to produce work without an input of energy, or to produce more energy than it consumes. This directly contradicts the First Law of Thermodynamics, also known as the Law of Conservation of Energy, which states that energy cannot be created or destroyed, only transformed from one form to another. Therefore, no system can generate energy out of nothing.
Violation of the Second Law of Thermodynamics
A perpetual motion machine of the second kind would extract thermal energy from a reservoir and convert it entirely into mechanical work, without any waste heat. This violates the Second Law of Thermodynamics, which dictates that in any energy transfer or transformation, some energy is always lost as unusable heat, increasing the overall entropy of a closed system. This means no process can be 100% efficient in converting heat to work.
Why They Fail in Practice
All attempts to build perpetual motion machines have failed because real-world systems are subject to inescapable energy losses, primarily due to friction, air resistance, and other dissipative forces. These forces convert useful mechanical energy into unusable thermal energy, gradually slowing down any system until it comes to a halt unless continuously supplied with external energy. The allure of such machines persists due to their promise of free energy, but they remain a scientific impossibility.