– Why really can’t a machine convert the whole part of energy given to it into work? What causes a fraction of it to be lost, even when there’s no dissipative forces?
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Your question doesn’t make sense. Under what conditions do you think there wouldn’t be any dissipation going on?
The short and simple answer is that energy wants to be as spread out as possible. To prevent losses, you need to reduce ways in which energy can escape the system.
Exactly how it escapes depends on your device, but there’s so much going on in some machines, a lot of energy can be lost.
There are always dissipative forces. Friction. Air resistance. Electrical resistance. Elastic deformation.
Physics textbooks like to do all their work on perfectly rigid bodies in frictionless vacuums at absolute zero, but your machinery is going to work on elastic bodies in a filthy factory on a humid day in Atlanta.
Things deform and grind and resist, and then they heat up. This heat radiates away into the abyss and is lost forever.
If you supply the energy as heat, there are fundamental limits on how efficiently you can convert it to useful work, due to entropy effects. For details, read about the 2nd law of thermodynamics and the concept of free energy.
It depends a lot on the type of energy you are starting with.
For example, if your energy source is electricity, then an electric motor can, under the right circumstances, convert almost all of it into work. There will always be some losses to things like electrical resistance in the motor windings, and friction in the bearings. But efficiency in the high 90% range are achievable in some situations.
https://new.abb.com/news/detail/1789/ABB-motor-sets-world-record-in-energy-efficiency-saves-half-a-million-dollars#:~:text=Tests%20carried%20out%20on%20a,record%20for%20electric%20motor%20efficiency.
But other sources of energy, most importantly heat energy, have much lower rates of conversion. All heat engines — the ICE engine in your car, the jet engine on an airplane, or the steam turbine at a nuclear power plant — have a an upper high temperature and a lower low temperature at which they operate. The difference between these temperatures puts a maximum possible efficiency on how much heat can be turned into work. For commend temperatures, this maximum efficiency is often in the 30% to 50% range.
https://energyeducation.ca/encyclopedia/Thermal_efficiency#:~:text=Heat%20engines%20often%20operate%20at,the%20Second%20law%20of%20thermodynamics.
WHY this maxim is there for thermal engines is hard (maybe impossible) to explain. It comes from the basic laws of physics. I’ve studded it. I can look up the numbers and do the math. But I don’t think I can explain it. In this way its a bit lie gravity, I can tell you what it does, but I can’t explain why gravity exists.
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