Question

This is at Molecular Biophysics Level. Please explain as simple as possible (My English is terrible, and I need help to understand this).

Why do you suppose your body is full of molecular-size motors, taking tiny steps?

Why do you think the electric company only succeeds in capturing a third of the energy content in coal, wasting the rest as heat?

Answer 1

1. Here I am considering that you are assuming a system which consist of small machines taking tiny steps:

If we take a look at a system which is performing some task. It must be using its internal-energy. This can be understood easily via car and fuel example. When we put fuel in car it runs otherwise it stops.

So, system consist of small size working entities "motors". Which work in a fashion, what we macroscopically see through our eyes. Reason we are assuming them on molecular level is because when we observe system microscopically, Whole system can be approximated using a very helpfully tool in Quantum Physics (Physics of Small Particles.) i.e.. Quantum Harmonic Oscillators or just Harmonic Oscillators. Which I assume that you were asking.

Now, Question comes why they perform task so slowly?

The reason in simplicity. As you might have studied that there are two kinds of processes one Reversible(Internal Energy do not change) and another one is Irreversible(Internal Energy changes). In nature irreversible processes take place but they are difficult to understand and work upon. So, to make things simpler and easy to understand we take reversible processes in consideration. So, there are some points to be noted to make reversible process happen (approximately).

• Changes should be so slow that it don't add up to the extra energy or Decrease the internal energy.
• Sum of energies must be conserved for a body (Though energy of whole system is conserved).

Therefore, those "motors" take tiny steps as possible.

2. Electric power plant can't use 100% of coal's energy and convert it into electricity.

Its answer lies in Third Law of thermodynamics which says

"It is not possible to reduce any assembly to the absolute zero of Temperature by any process, however idealized, in a finite number of operations"

Simply : Absolute 0 or 0 Kelvin can't be achieved.

You might heard about Carnot Cycle (If not then do revise it.). Which tells us about ideal engine and gives us a efficiency equation. Which says:

T1 is the Temperature raised by coal to boil water and run turbines and T2 is Temperature of surrounding of power station say ~250 C (~298 K). Power plant can achieve ~6000 C (~873 K). So, its very unlikely we can achieve 100% with these temperatures (also we can't use 0 K). Therefore, even if we assume that power plant works on carnot cycle (processes considered here are reversible) we can't achieve 100%.