Is the Drinking Bird a perpetual motion machine?
Depending on what type of perpetual motion machine is being described, all perpetual motion machines violate either the First or Second Law of Thermodynamics, so the answer is no, the Drinking Bird is not a perpetual motion machine.
So what powers the Drinking Bird? If it is moving it must have kinetic energy and the Principle of the Conservation of Energy says that this kinetic energy must have come from somewhere. It does not come from the water as, despite what you might have read about “water powered cars”*, water is not a fuel and does not store or transfer energy.
The correct answer is that the Drinking Bird is powered by the ambient heat of the room it is placed in. The process that occurs is as follows:
- The head of the bird is placed in the water and the cloth material of its head soaks up some of this water.
- The water evaporates from the cloth head, lowering the temperature of the vapour trapped inside it. The movement of the head through the air as the bird oscillates back and forth helps speed up this evaporation. (The red fluid is dyed dichloromethane and the bird is evacuated during construction so that only dichloromethane and dichloromethane vapour is present in the central column.)
- As the temperature of the vapour decreases it turns back into a liquid, lowering the pressure inside the central column.
- This lowered pressure draws dichloromethane from the reservoir at the base of the bird up the central column.
- The fluid being drawn up the central column of the bird raises its centre of gravity and causes it to tip over.
- As the bird tips over, the central column becomes open to the reservoir at the base of the bird, causing pressures to equalise and the fluid to drain back out into the reservoir. At the same time the head is resubmerged into the water and the process repeats.
The Drinking Bird is a heat engine that uses the difference in temperature between the head and the base of bird to perform work, transferring thermal energy to kinetic energy. This temperature difference is created by evaporation that is powered by a room’s ambient heat. If you were able to perfectly insulate a room so that no thermal energy could enter or leave then you could cool the room down by leaving a drinking bird running.†
* “Water powered cars” are actually being powered by hydrogen fuel cells. The hydrogen comes from the water and is extracted using electricity, making “water powered cars” actually powered (indirectly) by electricity.
† Friction between the air and the bird, and between the bird and its bearings would transfer some of the bird’s kinetic energy into thermal energy, heating the room back up.