– 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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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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