Final answer:
An efficient machine is one that wastes very little energy, with efficiency measured as the ratio of useful work output to energy input. High efficiency is characterized by minimal losses, often as heat, and does not necessarily equate to rapid work or large power output. Perpetual-motion machines are theoretically impossible due to the laws of thermodynamics.
Step-by-step explanation:
For a machine to be efficient, it means that it wastes very little energy. Efficiency can be calculated as Eff = Wout / Ein, where Wout is the useful work output and Ein is the energy consumed by the machine. Contrary to some misconceptions, an efficient machine does not necessarily have to work quickly nor does it need to produce a large amount of power; rather, its hallmark is its ability to perform its intended task while minimizing energy loss, often in the form of heat or other wasteful byproducts.
The thermal efficiency of a machine demonstrates the challenge of converting energy into work, where heat Qe often represents the significant energy not converted to useful work but rather transferred to the environment. An electric motor, when coupled with a low-friction turbine, can exhibit efficiencies greater than 90%, showcasing a minimal loss of energy. In contrast, a hypothetical perpetual-motion machine would be a machine that operates indefinitely without energy loss, which is impossible according to the first law of thermodynamics.