Question

How was the first law of thermodynamics discovered?

Answer 1

The first law of thermodynamics is essentially the law of conservation of energy.

It was established after JP Joule discovered the equivalence of heat and work or energy and work.

Thus for a closed system, if a heat `dQ` is supplied from outside, the heat must be used by the system to either perform work or increase it's energy or in the natural case do both of them.

In that case, law if conservation of energy demands that,

`dQ = dU + dW` where symbols carry their usual meaning.

This is the first law of thermodynamics.